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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.

Sea-to-air heat fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulas, these fluxes are a function of surface wind speed U10 and air–sea temperature and moisture disequilibrium (ΔT and Δq, respectively). Although many studies have explained TC intensification through the mutual dependence between increasing U10 and increasing sea-to-air heat fluxes, recent studies have found that TC intensification can occur through deep convective vortex structures that obtain their local buoyancy from sea-to-air moisture fluxes, even under conditions of relatively low wind. Herein, a new perspective on the bulk aerodynamic formulas is introduced to evaluate the relative contribution of wind-driven (U10) and thermodynamically driven (ΔT and Δq) ocean heat uptake. Previously unnoticed salient properties of these formulas, reported here, are as follows: 1) these functions are hyperbolic and 2) increasing Δq is an efficient mechanism for enhancing the fluxes. This new perspective was used to investigate surface heat fluxes in six TCs during phases of steady-state intensity (SS), slow intensification (SI), and rapid intensification (RI). A capping of wind-driven heat uptake was found during periods of SS, SI, and RI. Compensation by larger values of Δq > 5 g kg−1 at moderate values of U10 led to intense inner-core moisture fluxes of greater than 600 W m−2 during RI. Peak values in Δq preferentially occurred over oceanic regimes with higher sea surface temperature (SST) and upper-ocean heat content. Thus, increasing SST and Δq is a very effective way to increase surface heat fluxes—this can easily be achieved as a TC moves over deeper warm oceanic regimes.

The effect of a single water molecule on the reaction of dimethyl sulfide (DMS) with BrO reaction has been investigated using quantum chemical calculations at the CCSD(T)/6-311++G**//BH&HLYP/aug-cc-pVTZ level of theory. Two reaction mechanisms have been considered both in the absence and the presence of water, namely, oxygen atom transfer and hydrogen abstraction, among which the oxygen atom transfer was predominant. Five reaction channels were found in the absence of water, in which the channels starting from the cis-configuration of the pre-reaction complexes were more favorable because of the low energy barrier. The inclusion of water slightly decreased the energy barrier height of most oxygen atom transfer channels, while making the hydrogen abstraction channels more complex. While the effective rate coefficients for the oxygen atom transfer paths are found to have decreased by 3–7 orders of magnitude in the presence of water relative to the water-free reactions, the negligible fraction of reactants that are effectively clustered with water does not significantly change the overall rate of the formation of dimethyl sulfoxide and Br. The present results show that the overall mechanism and rate of the DMS + BrO reaction may not be affected by humidity under atmospheric conditions.

We analyze a multi-model ensemble at a convection-resolving resolution based on the DYAMOND models and a resolution ensemble based on the limited-area model COSMO over 40 days to study how tropical and subtropical marine low clouds are represented at a kilometer-scale resolution. The analyzed simulations produce low cloud fields that look in general realistic in comparison with satellite images. The evaluation of the radiative balance, however, reveals substantial inter-model differences and an under estimated low cloud cover in most models. Models that simulate increased low cloud cover are found to have a deeper marine boundary layer (MBL), stronger entrainment, and an enhanced latent heat flux. These findings demonstrate that some of the fundamental relations of the MBL are systematically represented by the model ensemble, which implies that the relevant dynamical processes start to become resolved on the model grid at a kilometer-scale resolution. A sensitivity experiment with the COSMO model suggests that differences in the strength of turbulent vertical mixing may contribute to the inter-model spread in cloud cover. ISSN:0026-1165

In this study, more than 4 years of ground-based observations and retrievals were collected and analyzed to investigate the seasonal and diurnal variations of single-layered MBL (with three subsets: nondrizzling, virga, and rain) cloud and drizzle properties, as well as their vertical and horizontal variations. The annual mean drizzle frequency was ~55%, with ~70% in winter and ~45% in summer. The cloud-top (cloud-base) height for rain clouds was the highest (lowest), resulting in the deepest cloud layer, i.e., 0.8 km, which is 4 (2) times that of nondrizzling (virga) clouds. The retrieved cloud-droplet effective radii rc were the largest (smallest) for rain (nondrizzling) clouds, and the nighttime values were greater than the daytime values. Drizzle number concentration Nd and liquid water content LWCd were three orders and one order lower, respectively, than their cloud counterparts. The rc and LWCc increased from the cloud base to zi ≈ 0.75 by condensational growth, while drizzle median radii rd increased from the cloud top downward the cloud base by collision–coalescence. The adiabaticity values monotonically increased from the cloud top to the cloud base with maxima of ~0.7 (0.3) for nondrizzling (rain) clouds. The drizzling process decreases the adiabaticity by 0.25 to 0.4, and the cloud-top entrainment mixing impacts as deep as upper 40% of the cloud layers. Cloud and drizzle homogeneities decreased with increased horizontal sampling lengths. Cloud homogeneity increases with increasing cloud fraction. These results can serve as baselines for studying MBL cloud-to-rain conversion and growth processes over the Azores.

Low-level cloud plays a key role in modulating air–sea interaction processes and sea surface temperature (SST) variability. The present study investigated the evolution process of oceanic low-level cloud cover (LCC) and related air–sea interaction processes on an intraseasonal time scale in the summertime (June–October) North Pacific (30°–40°N, 165°–175°E) based on satellite observational and reanalysis datasets from 2003 to 2016. The intraseasonal time scale (20–100 days) is dominant not only for the LCC, but also for LCC controlling factors, that is, SST, estimated inversion strength (EIS), and horizontal temperature advection (Tadv). To reveal the lead–lag relationship among these variables, we conducted phase composite analysis with a bandpass filter based on the intraseasonal variability (ISV) of LCC. It suggests that ISV of LCC leads to that of SST and that horizontal dry–cold advection from the poleward region leads to increasing LCC and decreasing SST. The increasing LCC corresponds to a positive relative humidity (RH) anomaly in the lower troposphere, which is due to adiabatic cooling with shallow convection, vertical moisture advection, and meridional RH advection associated with the anomalous cold Tadv. Heat budget analysis of the ocean mixed layer suggests the importance of anomalous dry–cold advection for cooling SST, not only via enhanced latent heat release but also via decreased downward shortwave radiation at the sea surface according to cloud radiative effect with a positive LCC anomaly. Determining the detailed lead–lag relationship between LCC and its controlling factor is a good approach to understand mechanisms of the local processes of both low-level cloud evolution and air–sea interaction.

Convection initiation (CI) and the subsequent upscale convective growth (UCG) at the coast of South China in a warm-sector heavy rainfall event are shown to be closely linked to a varying marine boundary layer jet (MBLJ) over the northern South China Sea (NSCS). To elucidate the dynamic and thermodynamic roles of the MBLJ in CI and UCG, we conducted and analyzed convection-permitting numerical simulations and observations. Compared to radar observations, the simulations captured CI locations and the following southwest–northeast-oriented convection development. The nocturnal MBLJ peaks at 950 hPa and significantly intensifies with turning from southwesterly to nearly southerly by inertial oscillation. The strengthened MBLJ promotes mesoscale ascent on its northwestern edge and terminus where enhanced convergence zones occur. Located directly downstream of the MBLJ, the coastal CI and UCG are dynamically supported by mesoscale ascent. From a thermodynamic perspective, a warm moist tongue over the NSCS is strengthened by the MBLJ-driven mesoscale ascent as well as by a high sea surface temperature. The warm moist tongue farther extends northeastward by horizontal transport and arrives at the coast where CI and UCG occur. Near the CI location, rapid development of a low-level saturated layer is mainly attributed to the mesoscale ascent and low-level moistening associated with the MBLJ. In addition, subsequent CI happens on either side of the original CI along the coast due to the delay of low-level moistening, which partly contributes to linear convective growth. Furthermore, ensemble simulations confirmed that a stronger MBLJ is more favorable to CI and UCG near the coast.

The marine boundary layer (MBL) is the largest transport place and reaction vessel of atmospheric mercury (Hg). The transformations of atmospheric Hg in the MBL are crucial for the global transport and deposition of Hg. Herein, Hg isotopic compositions of total gaseous mercury (TGM) and particle-bound Hg (PBM) collected during three cruises to Chinese seas in summer and winter were measured to reveal the transformation processes of atmospheric Hg in the MBL. Unlike the observation results at inland sites, isotopic compositions of TGM in the MBL were affected not only by mixing continental emissions but also largely by the oxidation of Hg0 primarily derived by Br atoms. Δ199Hg values of TGM were significantly positively correlated with air temperature in summer, indicating that processes inducing positive mass-independent fractionation of odd isotopes in TGM could be more active at low temperatures, while the relative processes might be weak in winter. In contrast, the positive Δ199Hg and high ratios of Δ199Hg∕Δ201Hg in PBM indicated that alternative oxidants other than Br or Cl atoms played a major role in the formation of Hg(II) in PBM, likely following the nuclear volume effect. Our results suggest the importance of local Hg environmental behaviors caused by an abundance of highly reactive species and provide new evidence for understanding the complicated transformations of atmospheric Hg in the MBL.

Marine boundary layer (MBL) cloud morphology associated with two summertime cold fronts over the eastern North Atlantic (ENA) is investigated using high-resolution simulations from the Weather Research and Forecasting (WRF) Model and observations from the Atmospheric Radiation Measurement (ARM) ENA Climate Research Facility. Lagrangian trajectories are used to study the evolution of post-cold-frontal MBL clouds from solid stratocumulus to broken cumulus. Clouds within specified domains in the vicinity of transitions are classified according to their degree of decoupling, and cloud-base and cloud-top breakup processes are evaluated. The Lagrangian derivative of the surface latent heat flux is found to be strongly correlated with that of the cloud fraction at cloud base in the simulations. Cloud-top entrainment instability (CTEI) is shown to operate only in the decoupled MBL. A new indicator of inversion strength at cloud top that employs the vertical gradients of equivalent potential temperature and saturation equivalent potential temperature, which can be computed directly from soundings, is proposed as an alternative to CTEI. Overall, results suggest that the deepening–warming hypothesis suggested by Bretherton and Wyant explains many of the characteristics of the summertime postfrontal MBL evolution of cloud structure over the ENA, thereby widening the phase space over which the hypothesis may be applied. A subset of the deepening–warming hypothesis involving warming initially dominating over moistening is proposed. It is postulated that changes in climate change–induced modifications in cold-frontal structure over the ENA may be accompanied by coincident changes in the location and timing of MBL cloud transitions in the post-cold-frontal environment.

This paper develops a method to estimate the shallow-convective mass flux M at the top of the subcloud layer as a residual of the subcloud-layer mass budget. The ability of the mass-budget estimate to reproduce the mass flux diagnosed directly from the cloud-core area fraction and vertical velocity is tested using real-case large-eddy simulations over the tropical Atlantic. We find that M reproduces well the magnitude, diurnal cycle, and day-to-day variability of the core-sampled mass flux, with an average root-mean-square error of less than 30% of the mean. The average M across the four winter days analyzed is 12 mm s−1, where the entrainment rate E contributes on average 14 mm s−1 and the large-scale vertical velocity W contributes −2 mm s−1. We find that day-to-day variations in M are mostly explained by variations in W, whereas E is very similar among the different days analyzed. Instead E exhibits a pronounced diurnal cycle, with a minimum of about 10 mm s−1 around sunset and a maximum of about 18 mm s−1 around sunrise. Application of the method to dropsonde data from an airborne field campaign in August 2016 yields the first measurements of the mass flux derived from the mass budget, and supports the result that the variability in M is mostly due to the variability in W. Our analyses thus suggest a strong coupling between the day-to-day variability in shallow convective mixing (as measured by M) and the large-scale circulation (as measured by W). Application of the method to the EUREC4A field campaign will help evaluate this coupling, and assess its implications for cloud-base cloudiness.

The 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.

Carbonyl sulfide (OCS) and carbon disulfide (CS2) are volatile sulfur gases that are naturally formed in seawater and exchanged with the atmosphere. OCS is the most abundant sulfur gas in the atmosphere, and CS2 is its most important precursor. They have attracted increased interest due to their direct (OCS) or indirect (CS2 via oxidation to OCS) contribution to the stratospheric sulfate aerosol layer. Furthermore, OCS serves as a proxy to constrain terrestrial CO2 uptake by vegetation. Oceanic emissions of both gases contribute a major part to their atmospheric concentration. Here we present a database of previously published and unpublished (mainly shipborne) measurements in seawater and the marine boundary layer for both gases, available at https://doi.org/10.1594/PANGAEA.905430 (Lennartz et al., 2019). The database contains original measurements as well as data digitalized from figures in publications from 42 measurement campaigns, i.e., cruises or time series stations, ranging from 1982 to 2019. OCS data cover all ocean basins except for the Arctic Ocean, as well as all months of the year, while the CS2 dataset shows large gaps in spatial and temporal coverage. Concentrations are consistent across different sampling and analysis techniques for OCS. The database is intended to support the identification of global spatial and temporal patterns and to facilitate the evaluation of model simulations.

The initial oxidation product of dimethyl sulfide in the marine boundary layer, the methyl thiomethyl radical, has remained elusive. A structurally analogous biradical with one radical center in the α-position to a sulfur atom could now be obtained by UV irradiation of

10 pags. 4 figs., Heterogeneous uptake of hypoiodous acid (HOI), the dominant inorganic iodine species in the marine boundary layer (MBL), on sea-salt aerosol (SSA) to form iodine monobromide and iodine monochloride has been adopted in models with assumed efficiency. Recently, field measurements have reported a much faster rate of this recycling process than previously assumed in models. Here, we conduct global model simulations to quantify the range of effects of iodine recycling within the MBL, using Conventional, Updated, and Upper-limit coefficients. When considering the Updated coefficient, iodine recycling significantly enhances gaseous inorganic iodine abundance (∼40%), increases halogen atom production rates (∼40% in I, >100% in Br, and ∼60% in Cl), and reduces oxidant levels (−7% in O, −2% in OH, and −4% in HO) compared to the simulation without the process. We appeal for further direct measurements of iodine species, laboratory experiments on the controlling factors, and multiscale simulations of iodine heterogeneous recycling., This study received funding from the European Research Council Executive Agency under the European Union's Horizon 2020 Research and Innovation Programme (Project ERC-2016- COG 726349 CLIMAHAL). Y.J.T. acknowledges the financial support by the National Natural Science Foundation of China (Grant number: 42175118). R.P.F. would like to thank financial support from ANPCyT (PICT 2019-2187). Computing resources were provided by NCAR's Computational and Information Systems Laboratory (CISL).

Adaptive sampling and situational awareness for autonomous underwater vehicles (AUVs) is a major improvement in ocean research. By only sampling the feature of interest in a feature-relevant domain instead of a covering a whole area expensive ship time can be saved and at the same time a more comprehensive data set can be obtained. A classical marine example where adaptive sampling is useful is sampling of boundary layers such as the thermocline because the boundary layer thickness is very small compared to the depth of the water column. These boundary layers play an important role in many ocean related disciplines such as marine biology, physical oceanography and underwater acoustics. In this paper an unscented Kalman filter (UKF) based extremum seeking control (ESC) approach is presented to detect and track such boundary layers. Simulation results for different use cases are presented to show its effectiveness.

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.

Marine stratocumulus clouds are intimately coupled to the turbulence in the boundary layer and drizzle is known to be ubiquitous within them. Six years of data collected at the Atmospheric Radiation Measurement (ARM)’s Eastern North Atlantic site are utilized to characterize turbulence in the marine boundary layer and air motions below stratocumulus clouds. Profiles of variance of vertical velocity binned by wind direction (wdir) yielded that the boundary layer measurements are affected by the island when the wdir is between 90° and 310° (measured clockwise from North where air is coming from). Data collected during the marine conditions (wdir310) showed that the variance of vertical velocity was higher during the winter months than during the summer months due to higher cloudiness, wind speeds, and surface fluxes. During marine conditions the variance of vertical velocity and cloud fraction exhibited a distinct diurnal cycle with higher values during the nighttime than during the daytime. Detailed analysis of 32 cases of drizzling marine stratocumulus clouds showed that for a similar amount of radiative cooling at the cloud top, within the sub-cloud layer 1) drizzle increasingly falls within downdrafts with increasing rain rates, 2) the strength of the downdrafts increases with increasing rain rates, and 3) the correlation between vertical air motion and rain rate is highest in the middle of the sub-cloud layer. The results presented herein have implications for climatological and model evaluation studies conducted at the ENA site, along with efforts of accurately representing drizzle-turbulence interactions in a range of atmospheric models.

Marine boundary layer clouds tend to organize into closed or open mesoscale cellular convection (MCC). Here, two-dimensional wavelet analysis is applied for the first time to passive microwave retr...

Flight data from the Cloud System Evolution over the Trades (CSET) campaign over the Pacific stratocumulus-to-cumulus transition are organized into 18 Lagrangian cases suitable for study and future modeling, made possible by the use of a track-and-resample flight strategy. Analysis of these cases shows that 2-day Lagrangian coherence of long-lived species (CO and O3) is high (r = 0.93 and 0.73, respectively), but that of subcloud aerosol, MBL depth, and cloud properties is limited. Although they span a wide range in meteorological conditions, most sampled air masses show a clear transition when considering 2-day changes in cloudiness (−31% averaged over all cases), MBL depth (+560 m), estimated inversion strength (EIS; −2.2 K), and decoupling, agreeing with previous satellite studies and theory. Changes in precipitation and droplet number were less consistent. The aircraft-based analysis is augmented by geostationary satellite retrievals and reanalysis data along Lagrangian trajectories between aircraft sampling times, documenting the evolution of cloud fraction, cloud droplet number concentration, EIS, and MBL depth. An expanded trajectory set spanning the summer of 2015 is used to show that the CSET-sampled air masses were representative of the season, with respect to EIS and cloud fraction. Two Lagrangian case studies attractive for future modeling are presented with aircraft and satellite data. The first features a clear Sc–Cu transition involving MBL deepening and decoupling with decreasing cloud fraction, and the second undergoes a much slower cloud evolution despite a greater initial depth and decoupling state. Potential causes for the differences in evolution are explored, including free-tropospheric humidity, subsidence, surface fluxes, and microphysics.

Modeling marine low clouds and fog in coastal environments remains an outstanding challenge due to the inherently complex ocean–land–atmosphere system. This is especially important in the context of global circulation models due to the profound radiative impact of these clouds. This study utilizes aircraft and satellite measurements, in addition to numerical simulations using the Weather Research and Forecasting (WRF) Model, to examine three well-observed coastally trapped disturbance (CTD) events from June 2006, July 2011, and July 2015. Cloud water-soluble ionic and elemental composition analyses conducted for two of the CTD cases indicate that anthropogenic aerosol sources may impact CTD cloud decks due to synoptic-scale patterns associated with CTD initiation. In general, the dynamics and thermodynamics of the CTD systems are well represented and are relatively insensitive to the choice of physics parameterizations; however, a set of WRF simulations suggests that the treatment of model physics strongly influences CTD cloud field evolution. Specifically, cloud liquid water path (LWP) is highly sensitive to the choice of the planetary boundary layer (PBL) scheme; in many instances, the PBL scheme affects cloud extent and LWP values as much as or more than the microphysics scheme. Results suggest that differences in the treatment of entrainment and vertical mixing in the Yonsei University (nonlocal) and Mellor–Yamada–Janjić (local) PBL schemes may play a significant role. The impact of using different driving models—namely, the North American Mesoscale Forecast System (NAM) 12-km analysis and the NCEP North American Regional Reanalysis (NARR) 32-km products—is also investigated.

East Asia is the largest emitter of mercury (Hg) in the world, but only a few studies of speciated Hg have been carried out in South Korea. In this study, total gaseous mercury (TGM), gaseous oxidized mercury (GOM), and particulate bound mercury (PBM) were measured at one of the most northwestern islands of South Korea. Concentrations of TGM and GOM were similar to those measured in other regions of Korea; however, PBM was found in much higher concentrations during cold months, most likely from coal and biomass burning in North Korea. Sources outside Korea significantly affected TGM and PBM, and GOM increased as air masses transported from domestic coal-burning power plants. During the warm months, GOM increased as gaseous elemental mercury (GEM) decreased when air masses were confined within the ocean area with minimal influence from anthropogenic emissions. This implies that there is an efficient process for the oxidation of GEM and GOM production in the marine boundary layer. The concentrations of ozone also decreased as an increase in GOM was observed, suggesting that halogens are primarily responsible for GEM oxidation.

A single-column model (SCM) is used to simulate a variety of environmental conditions between Los Angeles, California, and Hawaii in order to identify physical elements of parameterizations that are required to reproduce the observed behavior of marine boundary layer (MBL) cloudiness. The SCM is composed of the JPL eddy-diffusivity/mass-flux (EDMF) mixing formulation and the RRTMG radiation model. Model forcings are provided by the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA2). Simulated low cloud cover (LCC), rain rate, albedo, and liquid water path are compared to collocated pixel-level observations from A-Train satellites. This framework ensures that the JPL EDMF is able to simulate a continuum of real-world conditions. First, the JPL EDMF is shown to reproduce the observed mean LCC as a function of lower-tropospheric stability. Joint probability distributions of lower-tropospheric cloud fraction, height, and lower-tropospheric stability (LTS) show that the JPL EDMF improves upon its MERRA2 input but struggles to match the frequency of observed intermediate-range LCC. We then illustrate the physical roles of plume lateral entrainment and eddy-diffusivity mixing length in producing a realistic behavior of LCC as a function of LTS. In low-LTS conditions, LCC is mostly sensitive to the ability of convection to mix moist air out of the MBL. In high-LTS conditions, LCC is also sensitive to the turbulent mixing of free-tropospheric air into the MBL. In the intermediate LTS regime typical of stratocumulus–cumulus transition there is proportional sensitivity to both mixing mechanisms, emphasizing the utility of a combined eddy-diffusivity/mass-flux approach for representing mixing processes.

An artificial neural network is trained to reproduce thermodynamic tendencies and boundary layer properties from ERA5 HIRES reanalysis data over the summertime Northeast Pacific stratocumulus to trade cumulus transition region. The network is trained prognostically using 7-day forecasts rather than using diagnosed instantaneous tendencies alone. The resulting model, Machine Assisted Reanalysis Boundary Layer Emulation (MARBLE), skillfully reproduces the boundary layer structure and cloud properties of the reanalysis data in 7-day single-column prognostic simulations over withheld testing periods. Radiative heating profiles are well-simulated, and the mean climatology and variability of the stratocumulus to cumulus transition are accurately reproduced. MARBLE more closely tracks the reanalysis than does a comparable configuration of the underlying forecast model.

The marine boundary layer (MBL) is a key component of Earth’s climate system, and its main characteristics (height, entrainment efficiency, energy and mass fluxes, cloud formation processes, etc.) are closely linked to the properties of the inversion layer, which generally determines its height. Furthermore, cloud response to a warmer climate, one of the main sources of uncertainty in future climate projections, is highly dependent on changes in the MBL and in the inversion-layer properties. Long-term trends of the time series of MBL parameters at 32 stations in the Atlantic Ocean have been analyzed using conveniently homogenized radiosonde profiles from 1981 to 2010. In general, decreasing trends are found in the strength and thickness of the inversion layer and in the difference between the precipitable water vapor (PWV) in the free troposphere and the MBL. In contrast, positive trends are found in the height of the bottom of the inversion layer, the lapse rates of virtual and equivalent potential temperatures, the PWV within the boundary layer, and the sea surface temperature (SST). The weakening trend of the inversion layer and the increasing desiccation of the free troposphere relative to the MBL could have important consequences for both the evolution of low cloud cover in a greenhouse-warming climate and the fragile local ecosystems, such as “cloud forests.”

Organosulfur compounds are important components of secondary organic aerosols (SOA). While the Aerodyne high-resolution time-of-flight aerosol mass spectrometer (AMS) has been extensively used in aerosol studies, the response of the AMS to organosulfur compounds is not well-understood. Here, we investigated the fragmentation patterns of organosulfurs and inorganic sulfates in the AMS, developed a method to deconvolve total sulfate into components of inorganic and organic origins, and applied this method in both laboratory and field measurements. Apportionment results from laboratory isoprene photooxidation experiment showed that with inorganic sulfate seed, sulfate functionality of organic origins can contribute ∼7% of SOA mass at peak growth. Results from measurements in the Southeastern U.S. showed that 4% of measured sulfate is from organosulfur compounds. Methanesulfonic acid was estimated for measurements in the coastal and remote marine boundary layer. We explored the application of this method to unit mass-resolution data, where it performed less well due to interferences. Our apportionment results demonstrate that organosulfur compounds could be a non-negligible source of sulfate fragments in AMS laboratory and field data sets. A reevaluation of previous AMS measurements over the full range of atmospheric conditions using this method could provide a global estimate/constraint on the contribution of organosulfur compounds.

The kinetics and mechanism of H–abstraction reaction between CF3CH2OCH2CF3 and Cl atoms are studied in the temperature range of 250–1000 K using CCSD(T)//BHandHLY/6-311++G(d, p) method and canonical variational transition state theory (CVT). Two potential pathways are observed for H–abstraction reaction and one of them is the predominant channel over the whole temperature range. IRC calculation reveals an indirect reaction process through the formation of pre- and post-reaction complexes. The calculated rate coefficient kCl = 6.95 × 10−13 cm3 molecule−1 s−1 is in good agreement with the experimental results. Reaction with Cl atoms may constitute a significant sink in the marine boundary layer.

The Lagrangian particle dispersion model FLEXPART was used to simulate atmospheric dispersion of radionuclides from the Fukushima nuclear power plant (FNPP) towards the Tropical Western Pacific (TWP) and Southeast Asia (SEA). The simulation model distinguished between hemispherical transport via the jet stream and regional transport within the marine boundary layer by the East Asian northeast monsoon. This regional transport was driven by anticyclonic circulation over southern Japan and the western Pacific resulting from a recurrent eastward extension of the Siberian High to the Pacific Ocean. Activity concentrations of 131I and 137Cs measured at ten monitoring stations in TWP and SEA were used to validate the particle dispersion model. Good agreement between the FLEXPART model and observations yields confidence regarding its application to assess radiation impacts and support emergency planning in response to a possible future nuclear accident in the region.

The marine boundary layer jets (MBLJs) over the northern South China Sea during the early summer rainy season over Taiwan are analyzed using 5-yr (2008–12) National Centers for Environmental Prediction Climate Forecast System Reanalysis data with a 6-h interval. The MBLJ is distinctly different from the low-level jets associated with the subsynoptic frontal systems. During this period, the MBLJ events over the northern South China Sea mainly occur during the second half of the monsoon rainy season over Taiwan (after 1 June) and have a wind speed maximum around the 925-hPa level. The MBLJs are mainly related to the subsynoptic-scale pressure gradients related to a relatively deep mei-yu trough over southeastern China and a stronger-than-normal west Pacific subtropical high. Within the MBL, there is a three-way balance among pressure gradients, Coriolis force, and surface friction, with cross-isobar ageostrophic winds pointing toward the mei-yu trough throughout the diurnal cycle. At the jet core, the vertical wind profile resembles an Ekman spiral with supergeostrophic winds >12 m s−1 near the top of the MBL. The MBLJs are strongest at night and close to geostrophic flow in the late afternoon/early evening. This is because the friction velocity and ageostrophic wind decrease during daytime in response to mixing in the lowest levels. The MBLJs play an important role in horizontal moisture transport from the northern South China Sea to the Taiwan area. In the frontal zone, the moisture tongue extends vertically upward. The rainfall production is related to vertical motions in the frontal zone or localized lifting due to orographic effects.

The low cloud bias in global climate models (GCMs) remains an unsolved problem. Coarse vertical resolution in GCMs has been suggested to be a significant cause of low cloud bias because planetary boundary layer parameterizations cannot resolve sharp temperature and moisture gradients often found at the top of subtropical stratocumulus layers. This work aims to ameliorate the low cloud problem by implementing a new computational method, the Framework for Improvement by Vertical Enhancement (FIVE), into the Energy Exascale Earth System Model (E3SM). Three physics schemes representing microphysics, radiation, and turbulence as well as vertical advection are interfaced to vertically enhanced physics (VEP), which allows for these processes to be computed on a higher vertical resolution grid compared to the rest of the E3SM model. We demonstrate the better representation of subtropical boundary layer clouds with FIVE while limiting additional computational cost from the increased number of levels. When the vertical resolution approaches the large eddy simulation‐like vertical resolution in VEP, the climatological low cloud amount shows a significant increase of more than 30% in the southeastern Pacific Ocean. Using FIVE to improve the representation of low‐level clouds does not come with any negative side effects associated with the simulation of mid‐ and high‐level cloud and precipitation, that can occur when running the full model at higher vertical resolution.

Aerosol-cloud interactions remain the largest uncertainty in assessments of anthropogenic climate forcing, while the complexity of these interactions require methods that enable abstractions and simplifications that allow their improved treatment in climate models. Marine boundary layer clouds are an important component of the climate system as their large albedo and spatial coverage strongly affect the planetary radiative balance. High resolution simulations of clouds provide an unprecedented understanding of the structure and behavior of these clouds in the marine atmosphere, but the amount of data is often too large and complex to be useful in climate simulations. Data reduction and inference methods provide a way that to reduce the complexity and dimensionality of datasets generated from high-resolution Large Eddy Simulations.In this study we use network analysis, (the δ-Maps method) to study the complex interaction between liquid water, droplet number and vertical velocity in Large Eddy Simulations of Marine Boundary Layer clouds. δ-Maps identifies domains that are spatially contiguous and possibly overlapping and characterizes their connections and temporal interactions. The objective is to better understand microphysical properties of marine boundary layer clouds, and how they are impacted by the variability in aerosols. Here we will capture the dynamical structure of the cloud fields predicted by the MIMICA Large Eddy Simulation (LES) model. The networks inferred from the different simulation fields are compared between them (intra-comparisons) using perturbations in initial conditions and aerosol, using a set of four metrics. The networks are then evaluated for their differences, quantifying how much variability is inherent in the LES simulations versus the robust changes induced by the aerosol fields.

Satellite observations are used in regional machine learning models to quantify sensitivities of marine boundary-layer clouds (MBLC) to aerosol changes.MBLCs make up a large part of the global cloud coverage as they are persistently present over more than 20% of the Earth’s oceans in the annual mean.They play an important role in Earth’s energy budget by reflecting solar radiation and interacting with thermal radiation from the surface, leading to a net cooling effect. Cloud properties and their radiative characteristics such as cloud albedo, horizontal and vertical extent, lifetime and precipitation susceptibility are dependent on environmental conditions. Aerosols in their role as condensation nuclei affect these cloud radiative properties through changes in the cloud droplet number concentration and subsequent cloud adjustments to this perturbation. However, the magnitude and sign of these effects remain among the largest uncertainties in future climate predictions.In an effort to help improve these predictions a machine learning approach in combination with observational data is pursued:Satellite observations from the collocated A-Train dataset (C3M) for 2006-2011 are used in combination with ECMWF atmospheric reanalysis data (ERA5) to train regional Gradient Boosting Regression Tree (GBRT) models to predict changes in key physical and radiative properties of MBLCs. The cloud droplet number concentration (Nd) and the liquid water path (LWP) are simulated for the eastern subtropical oceans, which are characterised by a high annual coverage of MBLC due to the occurrence of semi-permanent stratocumulus sheets. Relative humidity above cloud, cloud top height and temperature below the cloud base and at the surface are identified as important predictors for both Nd and LWP. The impact of each predictor variable on the GBRT model's output is analysed using Shapley values as a method of explainable machine learning, providing novel sensitivity estimates that will improve process understanding and help constrain the parameterization of MBLC processes in Global Climate Models.

Oceanic low level clouds strongly affect the atmospheric radiation budget. Uncertainties in their microphysical properties and cover currently limit the accuracy of climate predictions. Further, studies quantifying the relative importance of aerosol and dynamics on cloud properties in specific meteorological regimes are poorly constrained by observations in the Western North Atlantic boundary layer.Low level clouds were measured during the Aerosol Cloud meTereology Interactions oVer the western ATlantic Experiment (ACTIVATE) campaign in winter and summer 2020. The two NASA LaRC research aircraft HU-25 Falcon and UC-12 B-200 King Air conducted 35 simultaneous flights to investigate aerosol-cloud interactions of maritime clouds and their impact on radiation. Number concentration, liquid water content, ice water content, and particle size distribution in the size range of 3 µm to 1460 µm in diameter were measured with the fast forward scattering cloud probe (FCDP) and 2-dimensional optical array imaging probe (2D-S) onboard the Falcon. Here, we present an overview of late winter (February-March) and late summer (August-September) oceanic cloud properties in the region 65°W to 80°W and 30°N to 40°N. We compare cloud properties in these two seasons and investigate their dependence on meteorological parameters and aerosol abundance. In a case study, we present cloud observations in a cold air outbreak event on 1 March 2020 with a specific focus on mixed-phase clouds.

Ambient radioactivity reflects a wide range of physical processes, including atmospheric and geological processes, as well as space weather and solar conditions. Gamma radiation near the Earth’s surface comes from diverse sources, including space (cosmic radiation), the earth’s atmosphere, and solid earth. In addition to the terrestrial gamma radiation originating from the radioactive decay of primordial radionuclides present in every soil and rock, gamma radiation is also continuously produced in the atmosphere from the interaction of secondary cosmic rays and upper-atmosphere gases, as well as from the decay of airborne radon (Rn-222) progeny. Therefore the temporal variability of gamma radiation contains information on a wide range of physical processes and space-earth interactions, but disentangling the different contributions remains a challenging endeavor. Continuous monitoring of gamma radiation at sea enables to remove both the terrestrial and radon exhalation contributions, allowing to examine in detail the space and atmospheric sources of ambient gamma radiation.Gamma radiation over the Atlantic Ocean was measured on board the ship-rigged sailing ship NRP Sagres in the framework of the SAIL (Space-Atmosphere-Ocean Interactions in the marine boundary Layer) project. The measurements were performed continuously (every 1-second) with a NaI(Tl) scintillator counting all the gamma rays from 475 keV to 3 MeV. The casing of the instrument was adapted in order to endure the harsh oceanic conditions and installed in the mizzen mast of the ship. The counts were linked to a rigorous temporal reference frame and precise positioning through GNSS.Here preliminary results based on the gamma radiation measurements performed from January 5th to May 9th 2020 are presented, corresponding to the journey of the ship from Lisboa to Cabo Verde, Rio de Janeiro, Montevideu, Cape Town, and back to Lisboa. The data exhibit a clear transition from the coastal to the marine environment, enabling to study in detail the temporal variation of gamma radiation in the marine boundary layer, as well as the interface between land and marine conditions in terms of environmental radioactivity.

The marine boundary layer offers a unique opportunity to investigate the electrical properties of the atmosphere, as the effect of natural radioactivity in driving near surface ionization is significantly reduced over the ocean, and the concentration of aerosols is also typically lower than over land. This work addresses the temporal variability of the atmospheric electric field in the South Atlantic marine boundary layer based on measurements from the SAIL (Space-Atmosphere-Ocean Interactions in the marine boundary Layer) project. The SAIL monitoring campaign took place on board the Portuguese navy tall ship NRP Sagres during its circumnavigation expedition in 2020. Two identical field mills (CS110, Campbell Scientific) were installed on the same mast but at different heights (about 5 and 22 meters), recording the atmospheric electric field every 1-second. Hourly averages of the atmospheric electric field are analyzed for the ship’s leg from 3rd to 25th March, between Buenos Aires (South America) and Cape Town (South Africa). The median daily curve of the electric field has a shape compatible with the Carnegie curve, but significant variability is found in the daily pattern of individual days, with only about 30% of the days exhibiting a diurnal pattern consistent with the Carnegie curve.

This study relies on ground-based observations to quantify the limitations of current spaceborne radars when it comes to capturing the coverage and vertical structure of Warm Marine Boundary Layer (WMBL) clouds. Its finding that neither the CloudSat-Cloud Profiling Radar (CPR) nor the EarthCARE-CPR will be able to accurately characterize cloud cover, cloud fraction and the location of a large fraction of WMBL clouds further encourages the development of concepts for new spaceborne radar systems targeting WMBL clouds. Given the properties of WMBL clouds extracted from ground-based radar measurements collected in the eastern northern Atlantic, we recommend that the next generation of space-borne radars targeting WMBL science shall operate interlaced pulse modes including both a highly sensitive long-pulse and a less sensitive but clutter limiting short-pulse mode. The findings of this study are particularly relevant for shaping the Aerosol and Cloud, Convection and Precipitation (ACCP) mission currently under investigation at the National Aeronautics and Space Administration (NASA).