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Coastal upwelling, marked by cool sea surface temperatures, modulates the wind stress and heat fluxes at the air-sea interface. However, the impact of upwelling on offshore wind power has been scarcely studied. This study uses satellite sea surface temperature data and a numerical model to examine how coastal upwelling shapes the diurnal evolution of the marine boundary layer, focusing on implications for offshore wind energy. The study region is the U.S. Mid Atlantic Bight, specifically the coast of New Jersey, known for its persistent summertime upwelling events. We run numerical experiments with upwelling, and upwelling artificially removed, to assess differences in the atmospheric response. For the wind event considered, results agree with theory where a stable, upwelling-cooled atmospheric boundary layer leads to reduced air-sea drag and turbulence intensity, higher wind speeds at hub height, and greater vertical shear relative to the scenario with upwelling removed. This response is likely caused by a sea breeze superimposed on onshore background winds. Experiments with parameterized turbines show that an 18-hour power generation at a lease area close to the shore was 6.5% higher with upwelling (4.86 GWh and 4.56 GWh, respectively). While upwelling can modulate offshore wind, the nature of the modulation is strongly dependent on the boundary layer regimes, background wind direction, and synoptic or mesoscale weather patterns. [ABSTRACT FROM AUTHOR]

Rapid formation of a deep layer of fog and clouds extending to a height of 2 km was observed over Sable Island off Nova Scotia on 13–14 July 2019. This fog and cloud event was dominantly caused by the rapid intrusion of a trough from a cyclone over NE Canada. The trough quickly moved from SW–W to E–NE and encroached on the coastal waters of Nova Scotia. Due to the warm front in the trough, the air temperature increased up to 4 km in height with increased stability, while the sea‐level pressure at Sable Island dropped by 11 hPa within 12 hours. The moderate to strong winds turned counterclockwise from the N to the SE, converging with the SE winds maintained by the ridge located to the east. There was no indication of an advection‐type fog mechanism because the surface air temperature was not adjusted to the colder sea‐surface temperature that could have resulted in saturation of the surface air. The leading portion of a massive cloud band in the trough passing over Sable Island included precipitation before fog formation. The rain that evaporated in the unsaturated and warm subcloud layer effectively reduced the air temperature and significantly increased the wet‐bulb temperature in this layer. The cloud extended into the moist subcloud layer, while fog formed at 1400 UTC 13 July and lasted until 2327 UTC 13 July (with two brief periods of mist lasting less than two hours). The development of this deep layer of fog and cloud in the NW Atlantic was caused by evolving synoptic conditions through the simultaneous effects of a rapid trough intrusion over coastal waters with a massive cloud band, induced precipitation causing cloud deepening into the subcloud layer, and surface wind convergence forcing vertical mixing in a stable marine boundary layer. [ABSTRACT FROM AUTHOR]

Numerical prediction of the interactions between wind and ocean waves is essential for climate modeling and a wide range of offshore operations. Large Eddy Simulation (LES) of the marine atmospheric boundary layer is a practical numerical predictive tool but requires parameterization of surface fluxes at the air–water interface. Current momentum flux parameterizations primarily use wave-phase adapting computational grids, incurring high computational costs, or use an equilibrium model based on Monin–Obukhov similarity theory for rough surfaces that cannot resolve wave phase information. To include wave phase-resolving physics at a cost similar to the equilibrium model, the MOving Surface Drag (MOSD) model is introduced. It assumes ideal airflow over locally piece-wise planar representations of moving water wave surfaces. Horizontally unresolved interactions are still modeled using the equilibrium model. Validation against experimental and numerical datasets with known monochromatic waves demonstrates the robustness and accuracy of the model in representing wave-induced impacts on mean velocity and Reynolds stress profiles. The model is formulated to be applicable to a broad range of wave fields and its ability to represent cross-swell and multiple wavelength cases is illustrated. Additionally, the model is applied to LES of a laboratory-scale fixed-bottom offshore wind turbine model, and the results are compared with wind tunnel experimental data. The LES with the MOSD model shows good agreement in wind–wave–wake interactions and phase-dependent physics at a low computational cost. The model's simplicity and minimal computational needs make it valuable for studying turbulent atmospheric-scale flows over the sea, particularly in offshore wind energy research. [ABSTRACT FROM AUTHOR]

Coastal upwelling, marked by cool sea surface temperatures, modulates the wind stress and heat fluxes at the air-sea interface. However, the impact of upwelling on offshore wind power has been scarcely studied. This study uses satellite sea surface temperature data and a numerical model to examine how coastal upwelling shapes the diurnal evolution of the marine boundary layer, focusing on implications for offshore wind energy. The study region is the U.S. Mid Atlantic Bight, specifically the coast of New Jersey, known for its persistent summertime upwelling events. We run numerical experiments with upwelling, and upwelling artificially removed, to assess differences in the atmospheric response. For the wind event considered, results agree with theory where a stable, upwelling-cooled atmospheric boundary layer leads to reduced air-sea drag and turbulence intensity, higher wind speeds at hub height, and greater vertical shear relative to the scenario with upwelling removed. This response is likely caused by a sea breeze superimposed on onshore background winds. Experiments with parameterized turbines show that an 18-hour power generation at a lease area close to the shore was 6.5% higher with upwelling (4.86 GWh and 4.56 GWh, respectively). While upwelling can modulate offshore wind, the nature of the modulation is strongly dependent on the boundary layer regimes, background wind direction, and synoptic or mesoscale weather patterns.

Information is provided on the results of gas geochemical and hydrometeorological studies of Amur and Ussuri bays, Sea of Japan, on cruises 84 and 85 of the R/V Professor Gagarinskiy in December 2022. For the first time for the winter period, synchronous measurements of greenhouse gases and atomic mercury were carried out in the surface marine boundary layer of Peter the Great Gulf, and new information was obtained on the seasonal characteristics of gas geochemical fields in the water column and bottom sediments. [ABSTRACT FROM AUTHOR]

Tropospheric reactive bromine is important for atmospheric chemistry, regional air pollution, and global climate. Previous studies have reported measurements of atmospheric reactive bromine species in different environments, and proposed their main sources, e.g. sea-salt aerosol (SSA), oceanic biogenic activity, polar snow/ice, and volcanoes. Typhoons and other strong cyclonic activities (e.g. hurricanes) induce abrupt changes in different earth system processes, causing widespread destructive effects. However, the role of typhoons in regulating reactive bromine abundance and sources remains unexplored. Here, we report field observations of bromine oxide (BrO), a critical indicator of reactive bromine, on the Huaniao Island (HNI) in the East China Sea in July 2018. We observed high levels of BrO below 500 m with a daytime average of 9.7 ± 4.2 pptv and a peak value of ∼26 pptv under the influence of a typhoon. Our field measurements, supported by model simulations, suggest that the typhoon-induced drastic increase in wind speed amplifies the emission of SSA, significantly enhancing the activation of reactive bromine from SSA debromination. We also detected enhanced BrO mixing ratios under high NOx conditions (ppbv level) suggesting a potential pollution-induced mechanism of bromine release from SSA. Such elevated levels of atmospheric bromine noticeably increase ozone destruction by as much as ∼40% across the East China Sea. Considering the high frequency of cyclonic activity in the northern hemisphere, reactive bromine chemistry is expected to play a more important role than previously thought in affecting coastal air quality and atmospheric oxidation capacity. We suggest that models need to consider the hitherto overlooked typhoon- and pollution-mediated increase in reactive bromine levels when assessing the synergic effects of cyclonic activities on the earth system. [ABSTRACT FROM AUTHOR]

Abstract Biogenic organic compounds in the surface ocean may significantly alter the cloud‐forming ability of sea spray aerosol and thereby affect the amount of solar radiation reaching the ocean surface. Estimates of the organic mass fraction of sea spray vary widely, and some results show a significant dependence on biological activity in the source seawater. We present airborne observations of the organic mass fraction of individual sea spray particles measured using the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument during the Atmospheric Tomography (ATom) mission, which sampled a wide range of latitudes and altitudes over the remote Atlantic and Pacific Oceans across four seasons, from the marine boundary layer to the upper troposphere. The measured sea spray particles of about 0.15–0.7 μm dry diameter showed higher average organic mass fractions at smaller sizes, but values were low overall, with regional integrated submicron means almost always

Surface waves grow through a mechanism in which atmospheric pressure is offset in phase from the wavy surface. A pattern of low atmospheric pressure over upward wave orbital motions (leeward side) and high pressure over downward wave orbital motions (windward side) travels with the water wave, leading to a pumping of kinetic energy from the atmospheric boundary layer into the waves. This pressure pattern persists above the air–water interface, modifying the turbulent kinetic energy in the atmospheric wave-affected boundary layer. Here, we present field measurements of wave-coherent atmospheric pressure and velocity to elucidate the transfer of energy from the atmospheric turbulence budget into waves through wave-coherent atmospheric pressure work. Measurements show that the phase between wave-coherent pressure and velocity is shifted slightly above 90° when wind speed exceeds the wave phase speed, allowing for a downward energy flux via pressure work. Although previous studies have reported wave-coherent pressure, to the authors' knowledge, these are the first reported field measurements of wave-coherent pressure work. Measured pressure work cospectra are consistent with an existing model for atmospheric pressure work. The implications for these measurements and their importance to the turbulent kinetic energy budget are discussed. Significance Statement: Surface waves grow through a pattern of atmospheric pressure that travels with the water wave, acting as a pump against the water surface. The pressure pumping, sometimes called pressure work, or the piston pressure, results in a transfer of kinetic energy from the air to the water that makes waves grow larger. To conserve energy, it is thought that the pressure work on the surface must extract energy from the mean wind profile or wind turbulence that sets the shape of the wind speed with height. In this paper, we present direct measurements of pressure work in the atmosphere above surface waves. We show that the energy extracted by atmospheric pressure work fits existing models for how waves grow and a simple model for how waves reduce energy in the turbulent kinetic energy budget. To our knowledge, these are the first reported field measurements of wave-coherent pressure work. [ABSTRACT FROM AUTHOR]

To assess dynamic loads, large offshore wind turbines need detailed and reliable statistical information on the inflow turbulence. We present a model that includes low frequencies down to ∼ 1 hr - 1 using the observed S (f) ∝ f - 5 / 3 in that range. The presented model contains a parameter representing the anisotropy of the two-dimensional, incompressible turbulence, and it assumes the low-frequency fluctuations to be homogeneous in the vertical direction. Combined with a three-dimensional model for the smaller scales, the model can predict correlations between different points. We have validated the model against two offshore wind data sets: a nacelle-mounted, forward-looking Doppler lidar with four beams at the Hywind Scotland offshore wind farm and sonic anemometer measurements at the FINO1 research platform in the North Sea. One-point auto spectra and two-point cross spectra were calculated after splitting the data into different atmospheric stability classes. The relative strength of the 2D low-frequency fluctuations to the 3D fluctuations was higher under stable conditions. The combined 2D+3D model was able to fit the measured spectra with good accuracy and could then predict the two-point cross spectra, co-coherences, and phase angles between wind fluctuations at different lateral and vertical separations. Good agreement was found between the measured and predicted values, albeit with exceptions. The model can generate stochastic wind fields for investigating wake meandering in wind farms or dynamic loads on floating wind turbines. [ABSTRACT FROM AUTHOR]

Low-level jets (LLJs), in which the wind speed attains a local maximum at low altitudes, have been found to occur in the U.S. mid-Atlantic offshore, a region of active wind energy deployment as of 2023. In contrast to widely studied regions such as the U.S. southern Great Plains and the California coastline, the mechanisms that underlie LLJs in the U.S. mid-Atlantic are poorly understood. This work analyzes floating lidar data from buoys deployed in the New York Bight to understand the characteristics and causes of coastal LLJs in the region. Application of the Hilbert–Huang transform, a frequency analysis technique, to LLJ case studies reveals that mid-Atlantic jets frequently occur during times of adjustment in synoptic-scale motions, such as large-scale temperature and pressure gradients or frontal passages, and that they do not coincide with motions at the native inertial oscillation frequency. Subsequent analysis with theoretical models of inertial oscillation and thermal winds further reveals that these jets can form in the stationary geostrophic wind profile from horizontal temperature gradients alone—in contrast to canonical LLJs, which arise from low-level inertial motions. Here, inertial oscillation can further modulate the intensity and altitude of the wind speed maximum. Statistical evidence indicates that these oscillations arise from stable stratification and the associated frictional decoupling due to warmer air flowing over a cold sea surface during the springtime land–sea breeze. These results improve our conceptual understanding of mid-Atlantic jets and may be used to better predict low-level wind speed maxima. Significance Statement: The purpose of this work is to identify and characterize the atmospheric mechanisms that result in an occasional low-level maximum in the wind speed off the U.S. mid-Atlantic coastline. Our findings show that these low-level jets form due to horizontal temperature gradients arising from fronts and synoptic systems, as well as from the land–sea breeze that forces warmer air over the cold ocean surface. This work aids predictability of such jets, improves our understanding of this coastal environment, and has implications for future deployment of offshore wind energy in this region. [ABSTRACT FROM AUTHOR]

Multiphase oxidation of methane sulfinic acid (MSIA) by ozone (O3) in deliquesced aerosol particles is an important but poorly constrained reaction in the marine boundary layer, hindering the accurate prediction of radiative forcing of natural aerosols. Large uncertainties arise from the unmeasured reaction rate constant for the aerosol multiphase reaction between O3 and methanesulfinate (MSI−), the dominant MSIA species in sea salt aerosols. Here, we directly measured kMSI−+O3 ${\mathrm{k}}_{{\text{MSI}}^{-}+{\mathrm{O}}_{3}}$ by performing flow cell experiments coupled with a Raman spectroscopy to investigate the kinetics of multiphase oxidation of MSI− by O3 in pH‐buffered deliquesced aerosol particles relevant to marine atmospheric conditions. kMSI−+O3 ${\mathrm{k}}_{{\text{MSI}}^{-}+{\mathrm{O}}_{3}}$ for aerosol particles was determined to be (2.2 ± 0.5) × 107 M−1 s−1, 10 times the value in dilute solutions. This higher rate constant would increase the yield of methane sulfonic acid from multiphase oxidation of MSIA and has important implications for accurate modeling of sulfur species. Plain Language Summary: Atmospheric oxidation of dimethyl sulfide (DMS) in the marine atmosphere ultimately forms methane sulfonic acid and sulfate aerosols, directly contributing to the global radiative forcing. They can also modify the properties of clouds and indirectly impact the climate. The poor understanding of the kinetics and mechanisms of atmospheric oxidation of DMS and its oxidation products in the marine atmosphere gives rise to large uncertainties in predicting the global radiative forcing. One of the key reactions is the multiphase oxidation of methane sulfinic acid (MSIA) and its deprotonated form methanesulfinate (MSI−) in deliquesced aerosol particles by ozone (O3). However, the reaction rate constant for MSI− and O3 in aerosol particles has never been experimentally determined. In this study, we conducted well‐controlled experiments and show that the reaction rate constant for MSI− and O3 in aerosol particles is 10 times that used in atmospheric models. The new kinetic data provides a better constraint on the product distributions from the multiphase oxidation of MSIA and should be incorporated into atmospheric models to improve the prediction of the radiative forcing of natural aerosols and clouds in the marine atmosphere. Key Points: The kinetics of multiphase oxidation of MSI− by O3 was investigated in pH‐buffered aerosol droplets coupling with Raman characterizationThe rate constant of MSI− and O3 in deliquesced aerosol particles was determined to be 10 times that used in chemical transport modelsThe reaction between MSI− and O3 would dominate the aqueous oxidation of MSIA species by O3 in the marine atmosphere [ABSTRACT FROM AUTHOR]

Marine cold-air outbreaks (CAOs) occur in the postfrontal sector of midlatitude storms, usually accompanied by dry intrusions (DIs) shaping the free-tropospheric (FT) air aloft. Substantial rain initiates overcast to broken regime transitions in marine boundary layer (MBL) cloud decks that form where cold air first meets relatively high sea surface temperatures. An exemplary CAO in the northwest Atlantic shows earlier transitions (corresponding to reduced extents of overcast clouds) closer to the low pressure center. We hypothesize that gradients in the meteorological pattern imposed by the prevailing DI induced a variability in substantial rain onset and thereby transition. We compile satellite observations, reanalysis fields, and Lagrangian large-eddy simulations (LES) translating along MBL trajectories to show that postfrontal trajectories closer to the low pressure center are more favorable to rain formation (and thereby cloud transitions) because of 1) weaker FT subsidence rates, 2) greater FT humidity, 3) stronger MBL winds, and 4) a colder MBL with reduced lower-tropospheric stability. LES confirms the observed variability in transitions, with substantial rain appearing earlier where there is swifter reduction of cloud condensation nucleus (CCN) concentration and increase of liquid water path (LWP). Prior to substantial rain, CCN budgets indicate dominant loss terms from FT entrainment and hydrometeor collisions. LWP-enhancing cloud thickness increases more rapidly for weaker large-scale subsidence that enables faster MBL deepening. Mere MBL warming and moistening cannot explain cloud thickness increases. The generality of such a DI-imposed cloud transition pattern merits further investigation with more cases that may additionally be convoluted by onshore aerosol gradients. Significance Statement: Cold-air outbreaks (CAOs) lead to marine boundary layer (MBL) clouds that commonly undergo rain-initiated overcast to broken cloud regime transitions that can drastically impact reflected solar radiation. We aim to better understand what mechanisms control these transitions. For a CAO event in the northwest Atlantic that shows earlier transitions closer to the low pressure center, we find the transition timing to be largely governed by the coinciding dry intrusion that imposes an inhomogeneous large-scale meteorological pattern onto the overlying free troposphere and thereby affects MBL rain formation. Our findings update conceptual understanding of extratropical cyclones and motivate analyzing observations and conducting simulations for more postfrontal cases through a Lagrangian perspective as done here for one case, to assess the generality of our findings. [ABSTRACT FROM AUTHOR]

While GCM horizontal resolution has received the majority of scale improvements in recent years, ample evidence suggests that a model's vertical resolution exerts a strong control on its ability to accurately simulate the physics of the marine boundary layer. Here we show that, regardless of parameter tuning, the ability of a single-column model (SCM) to simulate the subtropical marine boundary layer improves when its vertical resolution is improved. We introduce a novel objective tuning technique to optimize the parameters of an SCM against profiles of temperature and moisture and their turbulent fluxes, horizontal winds, cloud water, and rainwater from large-eddy simulations (LES). We use this method to identify optimal parameters for simulating marine stratocumulus and shallow cumulus. The novel tuning method utilizes an objective performance metric that accounts for the uncertainty in the LES output, including the covariability between model variables. Optimization is performed independently for different vertical grid spacings and value of time step, ranging from coarse scales often used in current global models (120 m, 180 s) to fine scales often used in parameterization development and large-eddy simulations (10 m, 15 s). Uncertainty-weighted disagreement between the SCM and LES decreases by a factor of ∼5 when vertical grid spacing is improved from 120 to 10 m, with time step reductions being of secondary importance. Model performance is shown to converge at a vertical grid spacing of 20 m, with further refinements to 10 m leading to little further improvement. Significance Statement: In successive generations of computer models that simulate Earth's atmosphere, improvements have been mainly accomplished by reducing the horizontal sizes of discretized grid boxes, while the vertical grid spacing has seen comparatively lesser refinements. Here we advocate for additional attention to be paid to the number of vertical layers in these models, especially in the model layers closest to Earth's surface where climatologically important marine stratocumulus and shallow cumulus clouds reside. Our experiments show that the ability of a one-dimensional model to represent physical processes important to these clouds is strongly dependent on the model's vertical grid spacing. [ABSTRACT FROM AUTHOR]

Recent studies indicate that the dominant mechanism for generating sprays in hurricane winds is a "bag breakup" fragmentation. This fragmentation process is typically characterized by inflation and consequent bursting of short-lived objects, referred to as "bags" (sail-like pieces of water film surrounded by a rim). Both the number of spray droplets and their size distribution substantially affect the air--sea heat and momentum exchange. Due to a lack of experimental data, the early spray generation function (SGF) for the bag breakup mechanism was based on the assumed similarity with resembling processes. Here we present experimental results for the case with a single isolated bag breakup fragmentation event. These experiments revealed several differences from similar fragmentation events that control the droplet sizes, such as secondary disintegration of droplets in gaseous flows and bursting of bubbles. In contrast to the bubble bursting, the film thickness of the bag canopy is not constant but is random with lognormal distribution. Additionally, its average value does not depend on the canopy radius but is determined by the wind speed. The lognormal size distribution of the canopy droplets is observed in conjunction with the established mechanism of liquid film fragmentation. The rim fragmentation results in two types of droplets, and their size distribution has been found to be lognormal distribution. The constructed SGF is verified by comparing it with experimental data from the literature. The perspectives of transferring the results from laboratory to field environment have also been discussed. [ABSTRACT FROM AUTHOR]

Abstract Multiphase oxidation of methane sulfinic acid (MSIA) by ozone (O3) in deliquesced aerosol particles is an important but poorly constrained reaction in the marine boundary layer, hindering the accurate prediction of radiative forcing of natural aerosols. Large uncertainties arise from the unmeasured reaction rate constant for the aerosol multiphase reaction between O3 and methanesulfinate (MSI−), the dominant MSIA species in sea salt aerosols. Here, we directly measured kMSI−+O3 by performing flow cell experiments coupled with a Raman spectroscopy to investigate the kinetics of multiphase oxidation of MSI− by O3 in pH‐buffered deliquesced aerosol particles relevant to marine atmospheric conditions. kMSI−+O3 for aerosol particles was determined to be (2.2 ± 0.5) × 107 M−1 s−1, 10 times the value in dilute solutions. This higher rate constant would increase the yield of methane sulfonic acid from multiphase oxidation of MSIA and has important implications for accurate modeling of sulfur species.

Abstract The maximum supersaturation (Sx) in clouds is a key parameter affecting the cloud's microphysical and radiative properties. We investigate the Sx of the marine boundary layer clouds by combining airborne and surface observations in the Eastern North Atlantic. The cloud droplet number concentration (Nc) in the least diluted cloud cores agrees well with the number concentration of particles larger than the Hoppel Minimum (HM) (N>HM) below clouds, indicating that the HM represents the average size threshold above which particles are activated to form cloud droplets. The Sx values derived from surface observations vary from 0.10% to 0.50% from June 2017 to June 2018, with a clear seasonal variation exhibiting higher values during winter. Most of the Sx variance (∼60%) can be explained by the cloud condensation nuclei (CCN) concentration and updraft velocity (w), with the CCN concentration playing a more important role than w in explaining the variation of Sx. The influence of CCN concentration on Sx leads to a buffered response of Nc to aerosol perturbations. The response of Nc to low aerosol concentration during winter is further buffered by the high w. The global Community Earth System Model (CESM) simulated Sx values in the Azores have a positive bias compared to measured Sx, likely due to overestimated w and underestimated CCN concentration. The CESM simulated Sx exhibits higher values further north over the North Atlantic Ocean, which is attributed to stronger w. The suppression of Sx by aerosol is also evident in regions with high CCN concentrations.

This study investigates the use of numerical weather prediction (NWP) ensembles to aid refractivity inversion problems during surface ducting conditions. Thirteen sets of measured thermodynamic atmospheric data from an instrumented helicopter during the Wallops Island field experiment are fit to a two-layer parametric surface duct model to characterize the duct. This modeled refractivity is considered "ground truth" for the environment and is used to generate the synthetic radar propagation loss field that then drives the inversion process. The inverse solution (refractivity derived from the synthetic radar data) is compared with this ground truth refractivity. For the inversion process, parameters of the two-layer model are iteratively estimated using genetic algorithms to determine which parameters likely produced the synthetic radar propagation field. Three numerical inversion experiments are conducted. The first experiment utilizes a randomized set of two-layer model parameters to initialize the inversion process, while the second experiment initializes the inversion using NWP ensembles, and the third experiment uses NWP ensembles to both initialize and restrict the parameter search intervals used in the inversion process. The results show that incorporation of NWP data benefits the accuracy and speed of the inversion result. However, in a few cases, an extended NWP ensemble forecast period was needed to encompass the ground truth parameters in the restricted search space. Furthermore, it is found that NWP ensemble populations with smaller spreads are more likely to hinder the inverse process than to aid it. [ABSTRACT FROM AUTHOR]

The wind drag on the sea surface is characterized by the aerodynamic roughness of the sea surface, z0, which is regulated by surface wind waves. Many studies have related the dimensionless form of z0 to the wave age parameter estimated from spectral peak information. These parametric relationships have been well developed for the wind-driven sea but not for mixed seas. Based on an analysis using observations from a fixed platform in the northern South China Sea, the deficiency of the spectral peak information in the parametric description z0 when swells dominate is indicated. Instead, a consistent parametric description of z0 can be obtained by using the wave age estimated from the mean wave period, and normalizing z0 by the mean wavelength. Normalizing z0 by the significant wave height introduces a spurious residual dependence of z0 on the wave steepness. A parametric relationship is developed between the dimensionless z0 (normalized by the mean wavelength) and the wave age from the mean wave period. A comparison of this new relationship to the wind-speed-only formulation in COARE 3.5 is provided. Significance Statement: In this paper, a consistent parametric description of the wave age dependence of the surface aerodynamic roughness is presented, with a wide range of sea states from dominant wind-driven seas to mixed seas in which the swells are dominant. [ABSTRACT FROM AUTHOR]

A mesoscale model is used to systematically investigate how the incoming flow incidence angle affects the development of atmospheric von Kármán vortex streets for non‐axisymmetric islands. The analysis is focused on an event observed on the leeward side of Guadalupe Island. By keeping the synoptic conditions the same, several simulations are performed for rotated orientations of the island topography, which correspond to a change in the angle of attack (AoA) relative to the upstream flow. The asymmetry of the vortex shedding and the role of the leading and trailing edge are in line with what was observed in laboratory von Kármán vortex streets past a flat plate. The eddies become larger with increasing AoA while the shedding frequency decreases, and the asymmetry between cyclonic and anticyclonic eddies weakens. Cyclonic vortices are more developed and stronger under typical conditions when they are shed from the trailing edge. We also demonstrate that the wake asymmetry is fundamentally between trailing‐edge and leading‐edge eddies rather than between cyclonic and anticyclonic eddies. Plain Language Summary: Low‐level marine clouds in satellite images often reveal vortex streets formed by two rows of counter‐rotating vortices on the leeward side of isolated mountainous islands. This phenomenon is called the "atmospheric von Kármán vortex street" (AVKVS) and has been investigated by satellite observations, laboratory experiments, and numerical modeling. Past studies highlighted an asymmetry between clockwise and counterclockwise rotating vortices, which was attributed to elongated island shapes and the effect of the Coriolis force. However, the origin of the AVKVS asymmetry is not yet fully understood. We use a mesoscale model to evaluate the effect of the incoming flow incidence angle (or angle of attack, i.e., AoA) on vortex street morphology for an elongated island. We first realistically simulate a vortex street observed downstream of Guadalupe Island and then, by keeping the same meteorological conditions, we simulate the phenomenon with artificially rotated island orientations to modify the AoA. The results show that the dimension of the vortices increases but their asymmetry weakens with increasing AoA. We also demonstrate that the wake asymmetry is fundamentally between trailing‐edge and leading‐edge eddies rather than between cyclonic and anticyclonic eddies. Key Points: The mesoscale Weather Research and Forecasting model (WRF) is used to study the characteristics of atmospheric von Kármán vortex streets (AVKVS) behind an elongated islandWe have convincingly demonstrated that the wake asymmetry of an AVKVS decreases with increasing angle of attackThe wake asymmetry is fundamentally between trailing‐edge and leading‐edge eddies rather than between cyclonic and anticyclonic eddies [ABSTRACT FROM AUTHOR]

Chemical processing of reactive nitrogen species, especially of NOx (= NO + NO2) and nitrous acid (HONO), determines the photochemical ozone production and oxidation capacity in the troposphere. However, sources of HONO and NOx in the remote marine atmosphere are still poorly understood. In this work, the multiphase chemistry mechanism CAPRAM in the model framework SPACCIM was used to study HONO formation at Cape Verde (CVAO) in October 2017, adopted with the input of current parameterizations for various HONO sources. Three simulations were performed that adequately reproduced ambient HONO levels and its diurnal pattern. The model performance for NOx and O3 improves significantly when considering dust‐surface‐photocatalytic conversions of reactive nitrogen compounds with high correlation coefficients up to 0.93, 0.56, and 0.89 for NO, NO2, and O3, respectively. Photocatalytic conversion of the adsorbed HNO3 on dust is modeled to be the predominant contributor for daytime HONO at CVAO, that is, accounting for about 62% of the chemical formation rate at noontime. In contrast, the ocean‐surface‐mediated conversion of NO2 to HONO and other discussed pathways are less important. The average OH levels at midday (9:00–16:00) modeled for cluster trajectory 1, 2, and 3 are 5.2, 5.1, and 5.2 × 106 molecules cm−3, respectively. Main OH formation is driven by O3 photolysis with a contribution of 74.6% to the total source rate, while HONO photolysis is negligible (∼1.8%). In summary, this study highlights the key role of dust aerosols for HONO formation and NOx cycling at CVAO and possibly in other dust‐affected regions, urgently calling for further investigations using field and model studies. Plain Language Summary: Chemical processing of NOx (= NO + NO2) and nitrous acid (HONO) is important for the tropospheric O3 budget and oxidation capacity. However, the sources of HONO and cycling of NOx in the remote marine atmosphere are still poorly explored. A detailed multiphase chemistry model simulation showed a better performance of HONO, NOx and O3 when considering dust‐surface‐photocatalytic conversions of reactive nitrogen compounds, especially the photocatalytic conversion of the adsorbed HNO3 on dust. The simulations demonstrated that OH formation is mainly driven by the O3 photolysis, while HONO photolysis is a negligible OH radical source due to its low concentration levels at Cape Verde. The study highlights the key role of dust aerosols for HONO and NOx chemistry in the remote marine boundary layer. Key Points: The sources of HONO and NOx at Cape Verde are well modeled with CAPRAMPhotocatalytic conversion of adsorbed HNO3 on dust is the predominant contributor for daytime HONOPhotolysis of O3 is the prevailing source of OH radical at Cape Verde, while HONO photolysis is a negligible OH radical source [ABSTRACT FROM AUTHOR]

Lagrangian stochastic models (LSM) are widely used to model the dispersion of sea spray droplets injected from the water surface into the marine atmospheric boundary layer (MABL) and for evaluation of the spray impact on the exchange fluxes between the atmosphere and the ocean. While moving through the MABL the droplets pass through the region of high gradients of air velocity, temperature and humidity occurring in the vicinity of the air–water interface. In this case, the applicability of LSMs constructed under the assumption of weakly inhomogeneous flows is questionable. In this work, we develop a Lagrangian stochastic model taking into account the strongly inhomogeneous structure of the airflow in MABL and, in particular, the anisotropy of turbulence dissipation rate. The model constants and the diffusion matrix coefficients are calibrated by comparison of the LSM prediction for the profiles of droplet concentration and the exchange fluxes of sensible and latent heat against the results of direct numerical simulation of turbulent, droplet-laden airflow over a waved water surface. [ABSTRACT FROM AUTHOR]

AbstractThe role of nonlocal transport on the development and maintenance of marine stratocumulus (Sc) clouds in coarse-resolution models is investigated, with a special emphasis on the downdraft contribution. A new parameterization of cloud-top-triggered downdrafts is proposed and validated against large-eddy simulation (LES) for two Sc cases. The applied nonlocal mass-flux scheme is part of the stochastic multiplume eddy-diffusivity/mass-flux (EDMF) framework decomposing the turbulent transport into local and nonlocal contributions. The complementary local turbulent transport is represented with the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme. This EDMF version has been implemented in the Weather Research and Forecasting (WRF) single-column model (SCM) and tested for three model versions: without mass flux, with updrafts only, and with both updrafts and downdrafts. In the LES, the downdraft and updraft contributions to the total heat and moisture transport are comparable and significant. The WRF SCM results show a good agreement between the parameterized downdraft turbulent transport and LES. While including updrafts greatly improves the modeling of Sc clouds over the simulation without mass flux, the addition of downdrafts is less significant, although it helps improve the moisture profile in the planetary boundary layer.

Comprehensive observations of hydroxyl (OH) and hydroperoxy (HO2) radicals were conducted in October 2019 at a coastal continental site in the Pearl River Delta (YMK site, 22.55°N, 114.60°E). The average daily maximum OH and HO2 concentrations were (4.7-9.5) × 106 cm-3 and (4.2-8.1) × 108 cm-3, respectively. The synchronized air mass transport from the northern cities and the South China Sea exerted a time-varying influence on atmospheric oxidation. Under a typical ocean30 atmosphere (OCM), reasonable measurement model agreement was achieved for both OH and HO2 using a 0-D chemical box model incorporating the regional atmospheric chemistry mechanism version 2-Leuven isoprene mechanism (RACM2-LIM1). Land mass (LAM) influence promoted more active photochemical processes, with daily averages of 7.1 × 106 cm-3 and 5.2 × 108 cm-3 for OH and HO2, respectively. Intensive photochemistry occurred after precursor accumulation, allowing local net ozone production comparable with surrounding suburban environments (5.52 ppb/h during the LAM period). The rapid oxidation process was accompanied by a higher diurnal nitrous acid (HONO) concentration (> 400 ppt). After a sensitivity test, HONO-related chemistry elevated the ozone production rate by 33% and 39% during the LAM and OCM periods, respectively, while the nitric acid and sulfuric acid formation rates were 52% and 35% higher, respectively. The simulated daytime HONO and ozone concentrations were reduced to a low level (~70 ppt and ~35 ppb) without the HONO constraint. This work challenges the conventional recognition of the MBL in a complex atmosphere. For coastal cities, the particularity of the HONO chemistry in the MBL tends to influence the ozone-sensitive system and eventually magnifies the background ozone. Therefore, the promotion of oxidation by elevated precursor concentrations is worth considering when formulating emission reduction policies. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) is characterized by abundant warm and cold structures that influence the overlying atmospheric boundary layer dynamics through two different mechanisms. First, turbulence and large eddies in the lower troposphere are affected by atmospheric stability, which can be modified by local SST, resulting in enhanced vertical mixing and larger surface winds over warmer waters. Second, the thermodynamic adjustment of air density to the underlying SST structures and the subsequent changes in atmospheric pressure drive secondary circulations. This paper aims to disentangle the effects of these processes and explore the environmental conditions that favor them. Two main environmental variables are considered: the large-scale air–sea temperature difference (proxy for stability) and wind speed. Using 5 years of daily reanalyses data, we investigate the 10-m wind response to SST structures. Based on linear regression between wind divergence and SST derivatives, we show that both mechanisms operate over a large spectrum of conditions. Ten-meter wind divergence is strongly impacted by the local SST via its effect on vertical mixing for midwind regimes in slightly unstable to near-neutral conditions, whereas the secondary circulation is important in two distinct regimes: low wind speed with a slightly unstable air column and high background wind speed with a very unstable air column. The first regime is explained by the prolonged Lagrangian time that the air parcel stays over an SST structure while the second one is related to strong heat fluxes at the air–sea interface, which greatly modify the marine atmospheric boundary layer properties. Location and frequency of the environmentally favorable conditions are discussed, as well as the response in low-cloud cover and rainfall. Significance Statement: The main objective of this study is to explore the wind response to thermal structures at the sea surface under different environmental conditions using the latest atmospheric reanalysis. Recent literature suggests that fine-scale air–sea interactions affect a large spectrum of atmospheric dynamics, from seasonal to weather-type regimes. It is thus important to characterize the atmospheric response to ocean surface variability. Our findings describe the environmental conditions for which the two main physical processes through which the atmosphere responds to sea surface temperature structures are active the most and can guide the development of high-resolution observing missions and campaigns in specific geographical locations and seasons to retrieve data that can be used to improve parameterization in models. [ABSTRACT FROM AUTHOR]

We explore the effect of aerosol perturbations on stratocumulus clouds in the context of marine cloud brightening (MCB) using high-resolution large-eddy simulations. We use a Lagrangian cloud microphysical model with a very detailed treatment of aerosol activation and droplet growth. The aerosol forcing is represented as a finite-width rectangular pulse in time (uniform in space). We analyze three stratocumulus cloud systems differing in their surface precipitation rate: namely, nonprecipitating, intermediate, and precipitating. We report on the diurnal evolution of these cloud systems subjected to a range of perturbations characterized by varying the amplitude and duration of the aerosol forcing pulse. Our simulations show that in the nonprecipitating system, the clouds are relatively insensitive to duration and amplitude, and are sensitive only to the total number concentration of the injected aerosol. In contrast, the precipitating cloud system is affected by the duration and the amplitude of the forcing, with the sensitivity conditional on the state of the cloud system before the injection of aerosol particles. We use these case studies to assess the efficacy of potential MCB spraying strategies. Our analysis shows that negative LWP adjustments offset a substantial fraction of the Twomey-induced brightening in all three cloud systems. This is countered by substantial cloud brightening obtained through precipitation-suppression-induced cloud-fraction adjustments. [ABSTRACT FROM AUTHOR]

The transition layer in the trades has long been observed and simulated, but the physical processes producing its structure remain little investigated. Using extensive observations from the Elucidating the Role of Clouds–Circulation Coupling in Climate (EUREC4A) field campaign, we propose a new conceptual picture of the trade wind transition layer, occurring between the mixed-layer top (around 550 m) and subcloud-layer top (around 700 m). The theory of cloud-free convective boundary layers suggests a transition-layer structure with strong jumps at the mixed-layer top, yet such strong jumps are only observed rarely. Despite cloud-base cloud fraction measured as only 5.3% ± 3.2%, the canonical cloud-free convective boundary layer structure is infrequent and confined to large [O(200) km] cloud-free areas. We show that the majority of cloud bases form within the transition layer, instead of above it, and that the cloud-top height distribution is bimodal, with a first population of very shallow clouds (tops below 1.3 km) and a second population of deeper clouds (extending to 2–3 km depth). We then show that the life cycle of this first cloud population maintains the transition-layer structure. That is, very shallow clouds smooth vertical thermodynamic gradients in the transition layer by a condensation–evaporation mechanism, which is fully coupled to the mixed layer. Inferences from mixed-layer theory and mixing diagrams, moreover, suggest that the observed transition-layer structure does not affect the rate of entrainment mixing, but rather the properties of the air incorporated into the mixed layer, primarily to enhance its rate of moistening. Significance Statement: The physical processes producing the structure of the trade wind transition layer, a thin atmospheric layer thought to be important for regulating convection, are not yet well understood. Using extensive observations from a recent field campaign, we find that the cloud-free convective boundary layer structure, with an abrupt discontinuity in thermodynamic variables, is infrequent, despite cloud-base cloud fraction being small. We show that very shallow clouds both forming and dissipating within the transition layer smooth vertical gradients compared to a jump, except in large [O(200) km] cloud-free areas. This condensation–evaporation mechanism, which is fully coupled to the mixed layer, does not appear to affect the rate of entrainment mixing, but rather the properties of air incorporated into the mixed layer. [ABSTRACT FROM AUTHOR]

The wintertime Kuroshio sea surface temperature (SST) front has the significant climate effects on Southern China. The study demonstrates a close relationship between heavy precipitation over Southern China and Kuroshio SST front in winter. More than half winter heavy rainfall events in Southern China are proved to be resulted from strong High‐frequency Variability events of the sea surface Wind Coupled with Precipitation (HV‐WCP events) over Kuroshio SST front. One day before strong HV‐WCP events, the initial precipitation appears over Middle‐lower Yangtze River due to the significantly enhanced frontal intensity. Then, precipitation generates low‐level cyclone and southeasterly wind anomalies, after it moving into Kuroshio front area because of the winter monsoon. The significant marine atmospheric boundary layer (MABL) height gradient over Kuroshio leads to plentiful moisture transporting from MABL into the free atmosphere and enhances the local precipitation again. This process further causes the large‐scale stratus rainband extending to Southern China and enhancing the heavy rainfall locally. Especially in 2008 winter, several processes of a strong HV‐WCP event followed by continuous weak ones are conducive to the low‐temperature‐precipitation disaster in Southern China. Plain Language Summary: The wintertime Kuroshio sea surface temperature front in the East China Sea is turned out to be relevant with the heavy precipitation in Southern China significantly. The strong HV‐WCP events over Kuroshio front area, defined as the High‐frequency Variability events of the sea surface Wind Coupled with Precipitation, determine more than half winter heavy precipitation events in Southern China. Due to the significantly enhanced frontal intensity on the day before strong HV‐WCP events, the initial precipitation appears in Middle‐lower Yangtze River. After it moving to the Kuroshio front, the positive feedback between circulation, vapor transport, and rainfall makes precipitation enhanced over Kuroshio front. Moreover, this positive feedback causes a large‐scale stratus precipitation band extending to the Southern China and strengthening the heavy rainfall there. Further results suggest that, on the interannual scale, the more C‐events defined as a strong HV‐WCP event followed by continuous weak ones over Kuroshio lead to more continuous low‐temperature‐precipitation winter days in Southern China, typically for the severe disaster period in 2008 early winter. Therefore, the HV‐WCP events over Kuroshio front have potential prediction significance on persistent winter disasters in Southern China, which can be used to prevent social damages caused by the extreme synoptic events. Key Points: The high‐frequency wind‐coupled‐precipitation events over East China Sea contribute more than half winter heavy‐rainy days in Southern ChinaThe vapor sources of most winter heavy precipitation in Southern China are observed cross boundary layer from warm flank of Kuroshio frontThe winter low‐temperature‐precipitation disasters in Southern China are revealed to be connected with the special aforementioned events [ABSTRACT FROM AUTHOR]

The refractive index structure constant C n 2 near the ocean surface is an important parameter for studying atmospheric optical turbulence over the ocean. The measured refractive index structure constant and meteorological parameters, such as temperature and three-dimensional wind speed, near the sea surface on the South China Sea during the period from January to November 2020 were analyzed. On the basis of Monin–Obukhov similarity theory, the dimensionless temperature structure parameter function f T near the sea surface was established, and a new parameterized model of the near-sea surface was proposed. The new model improved the error of the widely used model proposed by Wyngaard in 1973 (W73) and better reproduced the daily variation in the measured C n 2 . Further analysis of the seasonal applicability of the new model indicated that the correlation coefficients between the estimated and measured C n 2 in the spring, summer, autumn, and winter were 0.94, 0.94, 0.95, and 0.89, respectively, and the root mean square errors were 0.32, 0.41, 0.46, and 0.40 m−2/3, respectively. Compared with the C n 2 estimated by the W73 model, the correlation coefficient of C n 2 estimated by the new model and measured by the micro-thermometer increased by 0.05–0.27 and the root mean square error decreased by 0.04–0.56. The improved f T demonstrated higher accuracy than the existing models, which can lay a foundation for estimating turbulence parameters in different sea areas. [ABSTRACT FROM AUTHOR]

The climatic effects of aerosols derived from dimethyl sulfide (DMS) (e.g., methanesulfonic acid [MSA]) have long been of concern, particularly in the rapidly warming Arctic Ocean. Melting sea ice and increase in primary productivity can result in increased DMS emissions and MSA. However, the processes affecting MSA are complex. In addition to local sources, atmospheric chemistry processes, deposition, long‐distance transport, and regional heterogeneities influence MSA. This study used aerosol samples from the Chinese Polar Research Expedition during the summer of 2010–2016 and divided them into the Chukchi Sea (CS), high Arctic Ocean (HAO), and sea near Greenland (NG) to study the factors that influence the changes in MSA. For CS, MSA concentrations were primarily influenced by long‐distance transport from the Bering Sea air mass. Moreover, owing to the North Atlantic Oscillation, an increase in the Bering Sea temperature and a decrease in rainfall during air mass transport led to a significant increase in the concentration of MSA in the CS in 2016. When the sea ice concentration was between 0.2 and 0.6, high solar radiation likely promoted MSA formation in the HAO. For NG, the sources, rainfall and atmospheric chemical processes influenced the variations in MSA. This study highlights the importance of MSA transport in the CS and sea ice for MSA in the HAO under global warming. Plain Language Summary: The Arctic is the most sensitive region in the world to climate change. Melting sea ice and rising temperatures are affecting the ecosystems of the Arctic Ocean. Methanesulfonic acid (MSA) are generated by the oxidation of dimethyl sulfide (DMS) produced by marine phytoplankton after being released into the atmosphere. DMS‐derived aerosols in the Arctic Ocean substantially contribute to regional new particle formation and cloud condensation nuclei formation. This could have implications for climate in the Arctic Ocean. As MSA is a good indicator of DMS‐derived aerosols, comprehensively understanding how environmental changes influence MSA concentrations is crucial to clarify how emission sources and aerosol‐cloud interactions. This study demonstrates that the spatial and temporal distribution characteristics of MSA and their major influencing factors during summer in different sea areas of Arctic Ocean. The findings highlight that although the Chukchi Sea is one of the regions where the Arctic Ocean has warmed and marine primary productivity has increased significantly, the interannual variability of the MSA in this region is mainly influenced by the long‐distance transport from the Bering Sea. However, for the High Arctic Ocean regions, sea ice contributes significantly to local MSA concentrations. Key Points: The significant increase in methanesulfonic acid (MSA) in the Chukchi Sea is primarily influenced by the long‐distance transport of air masses from the Bering SeaWhen the sea ice concentration is between 0.2 and 0.6, high solar radiation may promote MSA formation in the high Arctic OceanSources, air temperature and rainfall mainly influenced the variations in MSA in the sea near Greenland [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO) plays a key role in atmospheric chemistry. Nevertheless, the HONO formation mechanism in the atmosphere, especially in the marine boundary layer, remains to be fully understood. Here, Born–Oppenheimer molecular dynamic and metadynamics simulations were performed to study the formation mechanism of HONO from the oxidation reactions of ClO radical and NO with the addition of (H2O)1–2, considering a monohydrated system ((ClO)(NO)(H2O)1) and dihydrated system ((ClO)(NO)(H2O)2), as well as at the air-water interface. This study shows that HONO formation follows a single-water mechanism in gas-phase and air-water interface systems. The free-energy barrier of the (ClO)(NO)(H2O)1 system was 9.66 kJ mol−1, whereas the (ClO)(NO)(H2O)2 system was a barrierless reaction. HONO formation at the air-water interface was faster than that in monohydrated and dihydrated systems. Although the concentration of ClO radical in the marine boundary layer is two orders higher than that of Cl radical, the production rates of HONO from the (ClO)(NO)(H2O)1 system are six orders lower than that from the (Cl)(NO)(H2O)1 system, which means that Cl radical dominates HONO formation rather than ClO radical in the marine boundary layer. These results can deepen our understanding of the HONO formation mechanism and be used to reduce HONO emissions and establish HONO-control strategies. [ABSTRACT FROM AUTHOR]

Monin–Obukhov similarity theory (MOST) is a well-tested approach for specifying the fluxes when the roughness surfaces are homogeneous. For flow over waves (inhomogeneous surfaces), phase-averaged roughness length scales are often prescribed through models based on the wave characteristics and the wind speed. However, such approaches lack generalizability over different wave ages and steepnesses due to the reliance on model coefficients tuned to specific datasets. In this paper, a sea surface–based hydrodynamic drag model applicable to moving surfaces is developed to model the pressure-based surface drag felt by the wind due to the waves. The model is based on the surface gradient approach of Anderson and Meneveau applicable to stationary obstacles and extended here to the wind–wave problem. The wave drag model proposed specifies the hydrodynamic force based on the incoming momentum flux, wave phase speed, and the surface frontal area. The drag coefficient associated with the wind–wave momentum exchange is determined based on the wave steepness. The wave drag model is used to simulate turbulent airflow above a monochromatic wave train with different wave ages and wave steepnesses. The mean velocity profiles and model form stresses are validated with available laboratory-scale experimental data and show good agreement across a wide range of wave steepnesses and wave ages. The drag force is correlated with the wave surface gradient and out-of-phase with the wave height distribution by a factor of π/2 for the sinusoidal wave train considered. These results demonstrate that the current approach is sufficiently general over a wide parameter space compared to wave phase-averaged models with a minimal increase in computational cost. Significance Statement: Understanding the physics of wind waves plays an important role in the context of numerous geophysical and engineering applications. A drag-based model is developed that characterizes the effect of the sea surface waves on the wind above. The model is validated with existing experimental datasets and is shown to be effective in predicting the average wind velocity and stress over waves with varied steepnesses and phase speeds. The ease of implementation and low computational cost of the model make it useful for studying turbulent atmospheric-scale flows over the sea surface important in offshore wind energy research as well as for modeling air–sea fluxes of momentum, heat, and mass. [ABSTRACT FROM AUTHOR]

We analyze approximately four years of heat-flux measurements at two levels, profiles of air temperature, and multiple measurements of the water temperature collected at a coastal zone site. Our analysis considers underestimation of the sea surface flux resulting from vertical divergence of the heat flux between the surface and the lowest flux level. We examine simple relationships of the heat flux to the wind speed and stratification and the potential influence of fetch and temperature advection. The fetch ranges from about 4 to near 400 km. For a given wind-direction sector, the transfer coefficient varies only slowly with increasing instability but decreases significantly with increasing stability. The intention here is not to recommend a new parameterization but rather to establish relationships that underlie the bulk formula that could lead to assessments of uncertainty and improvement of the bulk formula. Significance Statement: The behavior of surface heat fluxes in the coastal zone is normally more complex than over the open ocean but has a large impact on human activity. Our study examines extensive flux measurements on a tower in the Baltic Sea that allows partitioning of the fluxes according to wind direction without seriously depleting the data for a given wind-direction sector. Because some of the normal assumptions for the usual parameterization are not met, our study examines relationships behind the parameterization of the surface fluxes. [ABSTRACT FROM AUTHOR]

A winter storm triggered a significant sea smoke with the northwesterly wind to the Yellow Sea, China, on 7 January 2021. The ocean responses to this event lasted about 3 days. Satellite observations show that the sea surface temperature dropped from 5.7° to 4.7°C on the following day and then recovered to the previous level; the chlorophyll-a, a bio-growth indicator, increased from 3.6 to 3.9 mg m−3 due to cooling-induced coastal upwelling between 7 and 9 January. Two buoys measurements showed that the air temperature dropped to −13.3°C and high relative humidity with a maximum value of 89.0% above the sea surface, creating favorable conditions for sea smoke generation. A Regional Ocean Modeling System (ROMS) and Weather Research and Forecasting (WRF) Model coupled model with the Goddard Chemistry Aerosol Radiation and Transport (GOCART) chemical module was implemented to reproduce this sea smoke phenomenon and analyze the air–sea interaction. The 20°C temperature difference between extreme cold air (−13.3°C) and the relatively warm stable sea surface (4.7°–5.7°C) enhanced the seawater evaporation. In addition, we suppose the concentration of sea salt, a kind of condensation nucleus, with a particle diameter of 0.5–1.5 μm above the sea surface increased quickly on 7 January. The boiling-water-like sea surface was imaged on a synthetic aperture image. We developed an image analysis method to describe the cell-shaped texture characteristics imaged by Synthetic Aperture Radar (SAR). We also found that the sea surface imprints of sea smoke are governed by the thermal, not the dynamical instability. Significance Statement: On 7 January 2021, a significant sea smoke event happened in the Yellow Sea. The ocean response to the event lasted 3 days. First, on a synoptic scale, this study presents the comprehensive satellite observations of the sea surface temperature drop and chlorophyll-a increase associated with the sea smoke. Second, a coupled air–sea interaction model with the Goddard Chemistry Aerosol Radiation and Transport (GOCART) chemical module was implemented to reproduce this sea smoke phenomenon and identify which condensation nucleus induced such heavy sea smoke. Third, we developed an image analysis method to analyze high-resolution synthetic aperture radar images and found that the sea surface imprints of sea smoke are governed by the thermal, not the dynamical instability. [ABSTRACT FROM AUTHOR]

Observed stratocumulus to cumulus transitions (SCTs) and their sensitivity to aerosols are studied using a large‐eddy simulation (LES) model that simulates the aerosol lifecycle, including aerosol sources and sinks. To initialize, force, and evaluate the LES, we used a combination of reanalysis, satellite, and aircraft data from the 2015 Cloud System Evolution in the Trades field campaign over the Northeast Pacific. The simulations follow two Lagrangian trajectories from initially overcast stratocumulus (Sc) to the tropical shallow cumulus region near Hawaii. The first trajectory is characterized by an initially clean, well‐mixed Sc‐topped marine boundary layer (MBL), then continuous MBL deepening and precipitation onset followed by a clear SCT and a consistent reduction of aerosols that ultimately leads to an ultra‐clean layer in the upper MBL. The second trajectory is characterized by an initially polluted and decoupled MBL, weak precipitation, and a late SCT. Overall, the LES simulates the observed general MBL features. Sensitivity studies with different aerosol initial and boundary conditions reveal aerosol‐induced changes in the transition, and albedo changes are decomposed into the Twomey effect and adjustments of cloud liquid water path and cloud fraction. Impacts on precipitation play a key role in the sensitivity to aerosols: for the first case, runs with enhanced aerosols exhibit distinct changes in microphysics and macrophysics such as enhanced cloud droplet number concentration, reduced precipitation, and delayed SCT. Cloud adjustments are dominant in this case. For the second case, enhancing aerosols does not affect cloud macrophysical properties significantly, and the Twomey effect dominates. Key Points: A large‐eddy simulation (LES) is used to study the response of clouds to initial and boundary aerosol perturbations in two marine stratocumulus to cumulus transition casesAlthough the interactive aerosol scheme within the LES adds new degrees of freedom, the results agree well with observationsPrecipitation regulates the sensitivity to aerosols and the relative contributions of cloud adjustments to radiative forcing [ABSTRACT FROM AUTHOR]

Low‐level clouds over the tropical and sub‐tropical oceans play a crucial role in the planetary radiative energy budget. However, they are challenging to model in climate simulations because they are affected by local processes that are still partially unknown. The control that mesoscale sea surface temperature (SST) structures have on the dynamics of the lower atmosphere on daily scales is emerging to be non‐negligible and calls for more effort to be understood. During the EUREC4A field campaign, two of the research vessels (R/Vs) involved in the experiment sampled the edge of a cold mesoscale SST patch in the north‐western tropical Atlantic, crossing a gradient of roughly 0.75°C/100 km. The comprehensive set of instruments carried by the R/Vs allows an unprecedented characterization of the atmospheric response to the cold water forcing. The cold ocean patch weakens the vertical atmospheric mixing, reducing the boundary layer depth of roughly 200 m and the horizontal wind intensity of approximately 3 m s−1. At the same time, the humidity content in the sub‐cloud layer increases and these conditions decrease the latent heat flux (by roughly 80 W m−2) and reduce vertical velocity fluctuations, making it less likely that moisture exceeds the lifting condensation level. As a consequence, fewer and thinner low‐level clouds form over cold water. Independent satellite measurements are found to agree with the in situ observations. The observed link between sea temperature and low‐level clouds highlights its importance in the puzzle of modeling the sea‐air‐cloud interactions. Plain Language Summary: Puffy clouds typically visible at sea strongly challenge climate models that struggle to represent their interaction with the sea surface and solar radiation. And as a consequence, these climate models cannot precisely estimate how much the Earth's temperature will increase 100 years from now and its uncertainty. We went into the western Atlantic ocean for a month in January–February 2020 to measure sea surface temperature and cloud properties and observe how these changes occur. We saw clouds grow deeper over the warm water patches, holding more water and eventually raining. Weaker winds and more humid air occur on cold patches. Satellite observations seem to record the same behavior over a larger area in the same region. We detected a clear difference in cloud properties and amounts over warm and cold patches from all sensors, recording essential evidence of a feature that is hard to predict. We hope these observations will help to properly simulate the intensity and signs of low‐cloud feedback over warm and cold oceanic patches of water to improve climate models. Key Points: Ship‐based observations are used to characterize the lower atmospheric response to a cold patch in the north‐western subtropical AtlanticSignatures in dynamical and thermodynamical atmospheric properties agree with a reduced vertical mixing over the cold patchSuch a weaker vertical mixing is linked to a reduced shallow cloud cover because less moisture reaches the level of saturation [ABSTRACT FROM AUTHOR]

As a passive remote sensing technique, MAX-DOAS method was widely used to investigate the vertical profiles of aerosol and trace gases in the lower troposphere. However, the measurements for midlatitude marine boundary layer are rarely reported, especially during the storm weather system. In this study, the MAX-DOAS was used to retrieve the aerosol, HCHO and NO 2 vertical distribution at Huaniao Island of East China Sea in summer 2018, during which a strong tropical cyclone developed and passed through the measurement site. The observed aerosol optical depth (AOD), HCHO- and NO 2 - VCDs (Vertical Column Density) were in the range of 0.19-0.97, (2.57-12.27) × 1015 molec/cm2, (1.24-4.71) × 1015 molec/cm2, which is much higher than remote ocean area due to the short distance to continent. The vertically resolved aerosol extinction coefficient (AEC), HCHO and NO 2 presented the decline trend with the increase of height. After the typhoon passing through, the distribution of high levels of aerosol and HCHO stretched to about 1 km and the abundances of the bottom layer were found as double higher than before, reaching 0.51 km−1 and 2.44 ppbv, while NO 2 was still constrained within about 300 m with 2.59 ppbv in the bottom layer. The impacts of typhoon process forced air mass were also observed at the suburban site in Shanghai in view of both the aerosol extinction and chemical components. The different changes on air quality associated with typhoon and its mechanism in two different environments: coastal island and coastal city are worthy of further investigation as it frequent occurred in East Asia during summer and fall. [Display omitted] [ABSTRACT FROM AUTHOR]

The development of sea surface temperature (SST) anomalies over the northeast Pacific and their impacts on lower-tropospheric air temperatures over the Pacific Northwest are examined. Northeast Pacific SST anomalies are influenced by the synoptic-scale flow, with high pressure and weak surface winds associated with developing warm SST anomalies, while large pressure gradients and strong surface winds result in SST declines. SST over the northeast Pacific correlates significantly with surface air temperatures over the Pacific Northwest, with correlations increasing when high-frequency variability is filtered out. The correlations between unfiltered time series of SST and surface air temperature are largest for a zero-day lag and are strongest near the coast, contrasting with weaker correlations over the Columbia basin east of the Cascade Mountains. SST correlations with minimum surface air temperature are largest during the warm season, and maximum temperature correlations are highest in March; both have low correlations during autumn. Model simulations of periods with warm and cold northeast Pacific SST anomalies possess lower-tropospheric air temperature warming or cooling over the coastal zone, with SST influence weakening east of the Cascade crest. Eastern Pacific SST anomalies influence sea level pressure and lower-tropospheric heights, with warm SST anomalies resulting in simulated lowered pressure near the surface and increased heights aloft. The relationship between northeast Pacific SST and surface air temperature over land evince complex feedbacks: SST temperature anomalies can be advected inland from the Pacific, the SST anomalies can influence the synoptic-scale flow that affects the SST anomalies, and the synoptic-scale anomalies that produce the SST anomalies can directly influence temperatures over land. Significance Statement: Understanding the connection between northeast Pacific sea surface temperatures and low-level air temperatures over land is valuable for both subseasonal prediction and for examining the fidelity of model physics. [ABSTRACT FROM AUTHOR]

Significant discrepancies persist between field observations and model simulations regarding the strength of marine-derived HONO sources, underscoring the urgency to resolve unidentified HONO sources. In this study, sodium dodecyl sulfate (SDS) was chosen as a proxy for marine surfactants to investigate its impact on aqueous nitrate photolysis for the first time. Remarkable increases in HONO and NO 2 production rates by factors of 3.3 and 5.6, respectively, along with a 1.9-fold rise in NO 2 - concentration, were observed at a very low SDS concentration of 0.01 mM, strongly illustrating the promoting effect on nitrate photolysis. Furthermore, at an SDS concentration of 2 mM, intriguingly aligned with the critical micelle concentration, there was an additional 41.7% increase in HONO production rates. Vertically resolved Raman measurements indicated that SDS anions at the aqueous-air interface attracted NO 3 - closer to the aqueous surfaces, increasing the amount of incompletely solvated surface nitrate. Importantly, the anionic surfactant exhibited a greater promoting effect on HONO production compared to other typical nitrate photochemistry systems with the addition of a marine dissolved organic matter proxy, halogen, photosensitizer, or OH scavenger. These findings offer new insights into marine-derived HONO sources and should be considered in model simulations concerning the budgets of NO x , OH, and O 3 .

The trade wind subcloud layer is an important structural component of the atmosphere. Its thermodynamic variability has long been characterized using simple frameworks, of which mixed-layer theory is the simplest kind. Past studies qualitatively support such a description, yet the adequacy of mixed-layer theory as a quantitative description has not been tested. Here we use observations collected during the Elucidating the Role of Clouds–Circulation Coupling in Climate (EUREC4A) field campaign to test this framework and evaluate our understanding of the trade wind subcloud layer. We find evidence for a transition layer separating the mixed-layer and subcloud-layer tops. The presence of such a finitely thick transition layer with vertical gradients complicates the application of mixed-layer theory, which assumes an abrupt gradient, or "jump" at the subcloud-layer top. This ambiguity introduces effective parameters and motivates their estimation through a Bayesian methodology. Results from this Bayesian inversion further reflect a finite-depth entrainment zone. We find that subcloud-layer moisture and heat budgets close for synoptic variability and a monthly campaign mean, yielding a campaign-mean residual of 3.6 W m−2 for moisture and 2.9 W m−2 for heat. Surface wind speed variability influences the subcloud-layer depth and fluxes, yet thermodynamic variability above the subcloud-layer top emerges as the primary control on subcloud-layer moisture and heat variability. Given that this simple theoretical framework can explain observed variability, it offers an appealing framework for evaluating larger-scale models that must parameterize the processes regulating this fundamental part of the atmosphere. [ABSTRACT FROM AUTHOR]

Horizontal organization or mesoscale variability is an important mechanism in the interaction of the boundary layer with the large-scale conditions. The development of organization in a precipitating cumulus trade wind boundary layer is studied using large-eddy simulations with extensive horizontal domains, up to 160 × 160 km2 and fine grid resolution (40 m). The cloud fields vary between different computational domain sizes. Mean profiles and vertical velocity statistics do not vary significantly, both with respect to the domain size and when large-scale organization develops. Turbulent kinetic energy (TKE) rapidly increases when organization develops. The increase of TKE is attributed to the horizontal component, whereas the vertical velocity variance does not change significantly. The large computational domains blend the boundary between local convective circulations and mesoscale horizontal motions leading to the dependence of horizontal TKE on the LES domain size. Energy-containing horizontal length scales are defined based on the premultiplied spectra. When large-scale organization develops, the premultiplied spectra develop multiple peaks corresponding to the characteristic horizontal scales in the boundary layer. All flow variables have a small length scale of 1–2 km, which corresponds to local convective motions, e.g., updrafts and cumulus clouds. Organization development creates additional larger length scales. The growth rate of the large length scale is linear and it is about 3–4 km h−1, which agrees well with the growth rate of the cold pool radii. A single energy containing length scale is observed for vertical velocity for the entire run (even after organized convection develops) that is fairly constant with height. [ABSTRACT FROM AUTHOR]

Summertime remote sensor and in situ data from 2016 to 2019 collected at the ARM Eastern North Atlantic (ENA) Observatory are combined with aircraft measurements from the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign to quantify marine boundary layer (MBL) cloud, thermodynamic, and drizzle morphology in the region. A radar reflectivity–rainfall rate relationship (Z–R) is developed from aircraft data and 6-h cloud morphological regimes are identified from ENA data using a k-means algorithm driven by three independent inputs quantifying cloud thickness, drizzle intensity, and cloud field geometric complexity. Four separate MBL structural regimes representing non- or weakly drizzling single-layer stratocumulus, drizzling stratocumulus and cumulus-coupled stratocumulus, deep convection, and broken clouds embedded in northerly flow are identified. Single-layer stratocumulus is indicated when weak subtropical anticyclones are significantly west of the ENA site, and the MBL is cooler and drier than when drizzling and cumulus-coupled stratocumulus and broken clouds are observed. Drizzling and cumulus-coupled stratocumulus clouds are observed on the eastern flank of strong subtropical anticyclones in deep warm moist air masses with wind speeds exceeding 7 m s−1 and strong near-surface wind shear. Broken clouds exhibit strong wind shear near the inversion, while single-layer stratocumulus clouds have lower wind speeds and minimal shear. Net latent heat fluxes in the subcloud layer resulting from a combination of the ocean surface heat flux and evaporating drizzle average near zero over long periods in drizzling and cumulus-coupled stratocumulus. The ECMWF reanalysis version 5 (ERA5) is found to accurately represent single-layer stratocumulus properties, while producing significant discrepancies when drizzling stratocumulus and cumulus-coupled stratocumulus are observed. [ABSTRACT FROM AUTHOR]

We analyzed the variations in the mean wind field, turbulence, and turbulent flux of three typhoons using observational data obtained from an offshore monitoring platform. The turbulent fluctuation intensity and friction velocity are found to increase with wind speed both before and after landfall with two different relationships as two distinct branches on the turbulence‐wind diagram. Turbulent fluctuation intensity is found to decrease with increasing wind speed during landfall because of the transition of land‐based turbulence to sea‐based turbulence. The relationships between the friction velocity and drag coefficient and the wind speed were similarly affected by whether the typhoon makes landfall, and the relative position of the landfall site of the typhoon to the observation site. Plain Language Summary: Winds blowing across the surface of the sea cause waves, which, in turn, affect the wind flow. This interaction has been well defined at low wind speeds, but has not previously been studied for strong winds such as typhoons. We found that this interaction is related to the location of landfall site within the typhoon, and it is different from the interaction at low wind speeds. By analyzing the turbulent data obtained from the costal platform in the track of typhoons, we found that the relationships between the turbulence fluctuation intensity, the friction velocities, the drag coefficients and wind speed at 10 m height are primarily determined by the wind direction such as onshore and offshore winds that are related to the landfall position of the typhoons. Key Points: We analyze four different landfall processes for typhoonsThe turbulent variables change differently with wind speeds during different typhoons periodThese changes are affected by the landfall site [ABSTRACT FROM AUTHOR]

This article presents a simulated and experimental analysis of the effects of the evaporation duct on microwave propagation in the Irish Sea. The evaporation duct is a phenomenon that occurs almost permanently over all the world’s oceans and allows electromagnetic waves to travel beyond the horizon. Weather data logged over several years from four buoys off the east coast of Ireland has been analyzed to find the probability and strength of the evaporation duct. Signal propagation in the evaporation duct has been simulated using the parabolic equation model and compared to results obtained from an experimental setup in the Irish Sea. The best antenna heights and frequencies to maximize signal propagation at this location are also found. Results show that the evaporation duct can be used to provide high bandwidth communications beyond the horizon with an uptime of approximately 40%, and that weather data from buoys can be used to predict the performance of the communications link. While this method of communication is not one that can be relied on all the time, it could be useful to reduce dependency on expensive satellite links or provide nontime critical communications. [ABSTRACT FROM AUTHOR]

Satellite observations have revealed that mesoscale sea surface temperature (SST) perturbations can exert distinct influence on sea surface wind by modifying the overlying atmospheric boundary layer. Recently, spectral transfer functions have been shown to be useful to elucidate the wind response features. Spectral transfer functions can represent spatially lagged responses, their horizontal scale dependence, and background wind speed dependence. By adopting the transfer function analysis, the present study explores seasonality and regional differences in the wind response over the major western boundary current regions. Transfer functions estimated from satellite observations are found to be largely consistent among seasons and regions, suggesting that the underlying dominant dynamics are ubiquitous. Nevertheless, the wind response exhibits statistically significant seasonal and regional differences depending on background wind speed. When background wind is stronger (weaker) than 8.5 m s−1, the wind response is stronger (weaker) in winter than in summer. The Agulhas Retroflection region exhibits stronger wind response typically by 30% than the Gulf Stream and Kuroshio Extension regions. Although observed wind distributions are reasonably reconstructed from the transfer functions and observed SST, surface wind convergence zones along the Gulf Stream and Kuroshio Extension are underrepresented. The state-of-the-art atmospheric reanalysis and regional model represent well the structure of the transfer functions in the wavenumber space. The amplitude is, however, underestimated by typically 30%. The transfer function analysis can be adapted to many other atmospheric responses besides sea surface wind, and thus provide new insights into the climatic role of the mesoscale air–sea coupling. [ABSTRACT FROM AUTHOR]

From 2014 to 2017, two Department of Energy buoys equipped with Doppler lidar were deployed off the U.S. East Coast to provide long-term measurements of hub-height wind speed in the marine environment. We performed simulations of selected cases from the deployment using a 5-km configuration of the Weather Research and Forecasting (WRF) Model, to see if simulated hub-height speeds could produce closer agreement with the observations than existing reanalysis products. For each case we performed two additional simulations: one in which marine surface roughness height was one-way coupled to forecast wave parameters from a stand-alone WaveWatch III (WW3) simulation, and another in which WRF and WW3 were two-way coupled using the Coupled Ocean–Atmosphere–Wave–Sediment–Transport (COAWST) framework. It was found that all the 5-km WRF simulations improved 90-m wind speed statistics for the tropical cyclone case of 8 May 2015 and the cold frontal case of 25 March 2016, but not the nor'easter of 18 January 2016. The impact of wave coupling on buoy-level (4 m) wind speed was modest and case dependent, but when present, the impact was typically seen at 90 m as well, being as large as 10% in stable conditions. One-way wave coupling consistently reduced wind speeds, improving biases for 25 March 2016 but worsening them for 8 May 2015. Two-way wave coupling mitigated these negative biases, improved wave field representation and statistics, and mostly improved 4-m wind field correlation coefficients, at least at the Virginia buoy, largely due to greater self-consistency between wind and wave fields. Significance Statement: Using atmospheric models to forecast winds in the environments of offshore wind turbines will be critical in the new energy economy. The models used are imperfect, however, being sometimes too coarse, and may not properly represent the wind field at typical turbine hub heights of 90 m, for which we have limited observations in the marine environment. To help address this gap, two buoys equipped with lidars that measured hub-height winds continuously were deployed off the U.S. East Coast from 2014 to 2017. We used the lidar buoy data to show the benefits of a relatively high-resolution atmospheric model over existing reanalysis products, as well as including both the impacts of waves on winds and vice versa. [ABSTRACT FROM AUTHOR]