Search

The development of a unified similarity scaling has so far failed over complex surfaces, as scaling studies show large deviations from the empirical formulations developed over flat and horizontally homogeneous terrain as well as large deviations between the different complex terrain data sets. However, a recent study of turbulence anisotropy for flat and horizontally homogeneous terrain has shown that separating the data according to the limiting states of anisotropy (isotropic, two‐component axisymmetric and one‐component turbulence) improves near‐surface scaling. In this paper we explore whether this finding can be extended to turbulence over inclined and horizontally heterogeneous surfaces by examining near‐surface scaling for 12 different data sets obtained over terrain ranging from flat to mountainous. Although these data sets show large deviations in scaling when all anisotropy types are examined together, the separation according to the limiting states of anisotropy significantly improves the collapse of data onto common scaling relations, indicating the possibility of a unified framework for turbulence scaling. A measure of turbulence complexity is developed, and the causes for the breakdown of scaling and the physical mechanisms behind the turbulence complexity encountered over complex terrain are identified and shown to be related to the distance to the isotropic state, prevalence of directional shear with height in mountainous terrain, and the deviations from isotropy in the inertial subrange., Key Points A pathway toward a unified theory of near‐surface atmospheric turbulence is providedThe separation according to the limiting states of anisotropy significantly improves the collapse of data onto common scaling relationsPhysical processes responsible for turbulence complexity are identified

Motivated by an observed relationship between marine low cloud cover and surface wind speed, this study investigates how vertical wind shear affects trade‐wind cumulus convection, including shallow cumulus and congestus with tops below the freezing level. We ran large‐eddy simulations for an idealized case of trade‐wind convection using different vertical shears in the zonal wind. Backward shear, whereby surface easterlies become upper westerlies, is effective at limiting vertical cloud development, which leads to a moister, shallower, and cloudier trade‐wind layer. Without shear or with forward shear, shallow convection tends to deepen more, but clouds tops are still limited under forward shear. A number of mechanisms explain the observed behavior: First, shear leads to different surface wind speeds and, in turn, surface heat and moisture fluxes due to momentum transport, whereby the weakest surface wind speeds develop under backward shear. Second, a forward shear profile in the subcloud layer enhances moisture aggregation and leads to larger cloud clusters, but only on large domains that generally support cloud organization. Third, any absolute amount of shear across the cloud layer limits updraft speeds by enhancing the downward oriented pressure perturbation force. Backward shear—the most typical shear found in the winter trades—can thus be argued a key ingredient at setting the typical structure of the trade‐wind layer., Key Points Shear in the zonal wind influences cloud‐top heights via the effect of momentum transport on the surface wind and surface fluxesBackward shear (surface easterlies turn westerlies) lowers cloud tops and shallows and moistens the trade‐wind layerAny absolute amount of wind shear limits in‐cloud updraft speeds and enhances low‐level cloud fraction

It is well known that subtropical shallow convection transports heat and water vapor upwards from the surface. It is less clear if it also transports horizontal momentum upwards to significantly affect the trade winds in which it is embedded. We utilize unique multiday large‐eddy simulations run over the tropical Atlantic with ICON‐LEM to investigate the character of shallow convective momentum transport (CMT). For a typical trade‐wind profile during boreal winter, CMT acts as an apparent friction to decelerate the north‐easterly flow. This effect maximizes below the cloud base while in the cloud layer, friction is very small, although present over a relatively deep layer. In the cloud layer, the zonal component of the momentum flux is counter‐gradient and penetrates deeper than reported in traditional shallow cumulus LES cases. The transport through conditionally sampled convective updrafts and downdrafts explains a weak friction effect, but not the counter‐gradient flux near the cloud tops. The analysis of the momentum flux budget reveals that, in the cloud layer, the counter‐gradient flux is driven by convectively triggered nonhydrostatic pressure‐gradients and horizontal circulations surrounding the clouds. A model set‐up with large domain size and realistic boundary conditions is necessary to resolve these effects., Key Points Shallow convective momentum transport decelerates northeasterly trade winds below cloud base and favors nonlocal, counter‐gradient momentum flux near cloud‐topsThe counter‐gradient momentum transport is arbitrated by horizontal circulations surrounding the clouds driven by cross‐cloud pressure gradientsAnalysis of conditional sampling through clouds confirm their small contribution to counter‐gradient fluxes and a so‐called “cumulus friction”

The goal of this study is to understand the mechanisms controlling the isotopic composition of the water vapor near the surface of tropical oceans, at the scale of about a hundred kilometers and a month. In the tropics, it has long been observed that the isotopic compositions of rain and vapor near the surface are more depleted when the precipitation rate is high. This is called the “amount effect.” Previous studies, based on observations or models with parameterized convection, have highlighted the roles of deep convective and mesoscale downdrafts and rain evaporation. But the relative importance of these processes has never been quantified. We hypothesize that it can be quantified using an analytical model constrained by large‐eddy simulations. Results from large‐eddy simulations confirm that the classical amount effect can be simulated only if precipitation rate changes result from changes in the large‐scale circulation. We find that the main process depleting the water vapor compared to the equilibrium with the ocean is the fact that updrafts stem from areas where the water vapor is more enriched. The main process responsible for the amount effect is the fact that when the large‐scale ascent increases, isotopic vertical gradients are steeper, so that updrafts and downdrafts deplete the subcloud layer more efficiently., Key Points The precipitation is more depleted when the precipitation rate is higher only if it is associated with stronger large‐scale ascentUpdrafts are responsible for most of the depletion of the near‐surface water vapor relative to equilibrium with the oceanWith large‐scale ascent, the near‐surface water vapor is more depleted because updrafts export enriched water more efficiently

The strength and structure of the Earth's magnetic field is gradually changing. During the next 50 years the dipole moment is predicted to decrease by ∼3.5%, with the South Atlantic Anomaly expanding, deepening, and continuing to move westward, while the magnetic dip poles move northwestward. We used simulations with the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model to study how predicted changes in the magnetic field will affect the climate of the thermosphere‐ionosphere system from 2015 to 2065. The global mean neutral density in the thermosphere is expected to increase slightly, by up to 1% on average or up to 2% during geomagnetically disturbed conditions ( Kp≥4). This is due to an increase in Joule heating power, mainly in the Southern Hemisphere. Global mean changes in total electron content (TEC) range from −3% to +4%, depending on season and UT. However, regional changes can be much larger, up to about ±35% in the region of ∼45°S to 45°N and 110°W to 0°W during daytime. Changes in the vertical E→×B→ drift are the most important driver of changes in TEC, although other plasma transport processes also play a role. A reduction in the low‐latitude upward E→×B→ drift weakens the equatorial ionization anomaly in the longitude sector of ∼105–60°W, manifesting itself as a local increase in electron density over Jicamarca (12.0°S, 76.9°W). The predicted increase in neutral density associated with main magnetic field changes is very small compared to observed trends and other trend drivers, but the predicted changes in TEC could make a significant contribution to observationally detectable trends., Key Points Predicted magnetic field changes from 2015 to 2065 cause a very minor (up to 1–2%) increase in global mean thermosphere neutral densityEffects on total electron content are larger, up to ±35% in the American sector ( ∼45°S to 45°N, 110°W to 0°W)Reduced vertical E→×B→ drifts at Jicamarca and surrounding longitudes ( ∼105–60°W) cause a weakening of the equatorial ionization anomaly

Cold pools influence convective initiation and organization, dust lofting, and boundary layer properties, but little is known about their interactions with the land surface, particularly in dry continental environments. In this study, two‐way cold pool‐land surface interactions are investigated using high‐resolution idealized simulations of an isolated, transient cold pool evolving in a dry convective boundary layer. Results using a fully interactive land surface demonstrate that sensible heat fluxes are suppressed at the center of the cold pool but enhanced at the edge due to the spatial patterns of land surface cooling and the air temperature and wind speed perturbations. This leads to cold pool dissipation from the edge inward. Latent heat fluxes are primarily suppressed within the cold pool, and the magnitude of this suppression is controlled by competition between atmospheric and land surface effects. By comparing the fully interactive land surface simulation to a simulation with imposed surface fluxes, the land surface‐cold pool feedbacks are shown to reduce the cold pool lifetime, extent, and intensity by up to 50% and influence the pattern of boundary layer turbulent kinetic energy recovery, which have significant implications for cold pool‐induced convective initiation., Key Points An isolated cold pool evolving in a dry convective boundary layer is simulated at high resolutionSensible heat fluxes are enhanced at the cold pool's edge and suppressed in the center, leading to dissipation from the edge inwardLand surface interactions reduce the cold pool lifetime, area, and intensity by up to 50% and affect the boundary layer evolution

Stratocumulus clouds are the most common type of boundary layer cloud; their radiative effects strongly modulate climate. Large eddy simulations (LES) of stratocumulus clouds often struggle to maintain fidelity to observations because of the sharp gradients occurring at the entrainment interfacial layer at the cloud top. The challenge posed to LES by stratocumulus clouds is evident in the wide range of solutions found in the LES intercomparison based on the DYCOMS‐II field campaign, where simulated liquid water paths for identical initial and boundary conditions varied by a factor of nearly 12. Here we revisit the DYCOMS‐II RF01 case and show that the wide range of previous LES results can be realized in a single LES code by varying only the numerical treatment of the equations of motion and the nature of subgrid‐scale (SGS) closures. The simulations that maintain the greatest fidelity to DYCOMS‐II observations are identified. The results show that using weighted essentially non‐oscillatory (WENO) numerics for all resolved advective terms and no explicit SGS closure consistently produces the highest‐fidelity simulations. This suggests that the numerical dissipation inherent in WENO schemes functions as a high‐quality, implicit SGS closure for this stratocumulus case. Conversely, using oscillatory centered difference numerical schemes for momentum advection, WENO numerics for scalars, and explicitly modeled SGS fluxes consistently produces the lowest‐fidelity simulations. We attribute this to the production of anomalously large SGS fluxes near the cloud tops through the interaction of numerical error in the momentum field with the scalar SGS model., Key Points Fidelity of LES simulations of stratocumulus clouds depends on advection schemes and SGS closuresWENO schemes without explicit SGS closures simulate stratocumulus wellA simple model shows how advection schemes and SGS closures interact and affect cloud simulations

Unique in situ observations of atmospheric conditions over the Red Sea and the coastal Arabian Peninsula are examined to study the dynamics and regional impacts of the local land‐sea breeze cycle (LSBC). During a 26‐month data record spanning 2008–2011, observed LSBC events occurred year‐round, frequently exhibiting cross‐shore wind velocities in excess of 8 m/s. Observed onshore and offshore features of both the land‐ and sea‐breeze phases of the cycle are presented, and their seasonal modulation is considered. Weather Research and Forecasting climate downscaling simulations and satellite measurements are used to extend the analysis. In the model, the amplitude of the LSBC is significantly larger in the vicinity of the steeper terrain elements encircling the basin, suggesting an enhancement by the associated slope winds. Observed and simulated conditions also reflected distinct gravity‐current characteristics of the intrinsic moist marine air mass during both phases of the LSBC. Specifically, the advance and retreat of marine air mass was directly tied to the development of internal boundary layers onshore and offshore throughout the period of study. Convergence in the lateral moisture flux resulting from this air mass ascending the coastal topography (sea‐breeze phase) as well as colliding with air masses from the opposing coastline (land‐breeze phase) further resulted in cumulous cloud formation and precipitation., Key Points sea‐breeze observationsland‐sea‐air interactionsmesoscale moisture transport dynamics

Atmospheric observations of greenhouse gases provide essential information on sources and sinks of these key atmospheric constituents. To quantify fluxes from atmospheric observations, representation of transport—especially vertical mixing—is a necessity and often a source of error. We report on remotely sensed profiles of vertical aerosol distribution taken over a 2 year period in Pasadena, California. Using an automated analysis system, we estimate daytime mixing layer depth, achieving high confidence in the afternoon maximum on 51% of days with profiles from a Sigma Space Mini Micropulse LiDAR (MiniMPL) and on 36% of days with a Vaisala CL51 ceilometer. We note that considering ceilometer data on a logarithmic scale, a standard method, introduces, an offset in mixing height retrievals. The mean afternoon maximum mixing height is 770 m Above Ground Level in summer and 670 m in winter, with significant day‐to‐day variance (within season σ = 220m≈30%). Taking advantage of the MiniMPL's portability, we demonstrate the feasibility of measuring the detailed horizontal structure of the mixing layer by automobile. We compare our observations to planetary boundary layer (PBL) heights from sonde launches, North American regional reanalysis (NARR), and a custom Weather Research and Forecasting (WRF) model developed for greenhouse gas (GHG) monitoring in Los Angeles. NARR and WRF PBL heights at Pasadena are both systematically higher than measured, NARR by 2.5 times; these biases will cause proportional errors in GHG flux estimates using modeled transport. We discuss how sustained lidar observations can be used to reduce flux inversion error by selecting suitable analysis periods, calibrating models, or characterizing bias for correction in post processing., Key Points Aerosol lidar maps LA mixing depth in space (pilot mobile study) and time (2 years data)Automatic mixing depth retrieval system finds daily variability far exceeds seasonal differencePBL heights in models used for GHG monitoring show biases that will carry over to flux estimates

Climate and reanalysis models contain large water and energy budget errors over tropical land related to the misrepresentation of diurnally forced moist convection. Motivated by recent work suggesting that the water and energy budget is influenced by the sensitivity of the convective diurnal cycle to atmospheric state, this study investigates the relationship between convective intensity, the convective diurnal cycle, and atmospheric state in a region of frequent convection—the Amazon. Daily, 3‐hourly satellite observations of top of atmosphere (TOA) fluxes from Clouds and the Earth's Radiant Energy System Ed3a SYN1DEG and precipitation from Tropical Rainfall Measuring Mission 3B42 data sets are collocated with twice daily Integrated Global Radiosonde Archive observations from 2002 to 2012 and hourly flux tower observations. Percentiles of daily minimum outgoing longwave radiation are used to define convective intensity regimes. The results indicate a significant increase in the convective diurnal cycle amplitude with increased convective intensity. The TOA flux diurnal phase exhibits 1–3 h shifts with convective intensity, and precipitation phase is less sensitive. However, the timing of precipitation onset occurs 2–3 h earlier and the duration lasts 3–5 h longer on very convective compared to stable days. While statistically significant changes are found between morning atmospheric state and convective intensity, variations in upper and lower tropospheric humidity exhibit the strongest relationships with convective intensity and diurnal cycle characteristics. Lastly, convective available potential energy (CAPE) is found to vary with convective intensity but does not explain the variations in Amazonian convection, suggesting that a CAPE‐based convective parameterization will not capture the observed behavior without incorporating the sensitivity of convection to column humidity., Key Points Amazon convective diurnal cycle characteristics vary with convective intensityColumn humidity is the most important factor explaining convective behaviorCAPE is a poor predictor of the Amazonian convective diurnal cycle

Large‐scale weather forecasting and climate models are beginning to reach horizontal resolutions of kilometers, at which common assumptions made in existing parameterization schemes of subgrid‐scale turbulence and convection—such as that they adjust instantaneously to changes in resolved‐scale dynamics—cease to be justifiable. Additionally, the common practice of representing boundary‐layer turbulence, shallow convection, and deep convection by discontinuously different parameterizations schemes, each with its own set of parameters, has contributed to the proliferation of adjustable parameters in large‐scale models. Here we lay the theoretical foundations for an extended eddy‐diffusivity mass‐flux (EDMF) scheme that has explicit time‐dependence and memory of subgrid‐scale variables and is designed to represent all subgrid‐scale turbulence and convection, from boundary layer dynamics to deep convection, in a unified manner. Coherent up and downdrafts in the scheme are represented as prognostic plumes that interact with their environment and potentially with each other through entrainment and detrainment. The more isotropic turbulence in their environment is represented through diffusive fluxes, with diffusivities obtained from a turbulence kinetic energy budget that consistently partitions turbulence kinetic energy between plumes and environment. The cross‐sectional area of up and downdrafts satisfies a prognostic continuity equation, which allows the plumes to cover variable and arbitrarily large fractions of a large‐scale grid box and to have life cycles governed by their own internal dynamics. Relatively simple preliminary proposals for closure parameters are presented and are shown to lead to a successful simulation of shallow convection, including a time‐dependent life cycle., Key Points An extended eddy‐diffusivity mass‐flux (EDMF) scheme is presented that is prognostic and has variable plume area fractionsThe new EDMF scheme consistently partitions flow variables (including turbulence kinetic energy) between plumes and the environmentIn first tests, it successfully reproduces the average condition and transient life cycles of shallow convection

Land surface albedo is a significant parameter for maintaining a balance in surface energy. It is also an important parameter of bare soil surface albedo for developing land surface process models that accurately reflect diurnal variation characteristics and the mechanism behind the solar spectral radiation albedo on bare soil surfaces and for understanding the relationships between climate factors and spectral radiation albedo. Using a data set of field observations, we conducted experiments to analyze the variation characteristics of land surface solar spectral radiation and the corresponding albedo over a typical Gobi bare soil underlying surface and to investigate the relationships between the land surface solar spectral radiation albedo, solar elevation angle, and soil moisture. Based on both solar elevation angle and soil moisture measurements simultaneously, we propose a new two‐factor parameterization scheme for spectral radiation albedo over bare soil underlying surfaces. The results of numerical simulation experiments show that the new parameterization scheme can more accurately depict the diurnal variation characteristics of bare soil surface albedo than the previous schemes. Solar elevation angle is one of the most important factors for parameterizing bare soil surface albedo and must be considered in the parameterization scheme, especially in arid and semiarid areas with low soil moisture content. This study reveals the characteristics and mechanism of the diurnal variation of bare soil surface solar spectral radiation albedo and is helpful in developing land surface process models, weather models, and climate models., Key Points Field observation experiment of surface solar spectral radiationThe impact of solar elevation angle and soil moisture on the diurnal variation characteristics of bare soil surface albedoSolar elevation angle is a significant factor of controlling the diurnal variation characteristics of bare soil surface albedo

A recent intercomparison exercise proposed by the Working Group for Numerical Experimentation (WGNE) revealed that the parameterized, or unresolved, surface stress in weather forecast models is highly model‐dependent, especially over orography. Models of comparable resolution differ over land by as much as 20% in zonal mean total subgrid surface stress (τtot). The way τtot is partitioned between the different parameterizations is also model‐dependent. In this study, we simulated in a particular model an increase in τtot comparable with the spread found in the WGNE intercomparison. This increase was simulated in two ways, namely by increasing independently the contributions to τtot of the turbulent orographic form drag scheme (TOFD) and of the orographic low‐level blocking scheme (BLOCK). Increasing the parameterized orographic drag leads to significant changes in surface pressure, zonal wind and temperature in the Northern Hemisphere during winter both in 10 day weather forecasts and in seasonal integrations. However, the magnitude of these changes in circulation strongly depends on which scheme is modified. In 10 day forecasts, stronger changes are found when the TOFD stress is increased, while on seasonal time scales the effects are of comparable magnitude, although different in detail. At these time scales, the BLOCK scheme affects the lower stratosphere winds through changes in the resolved planetary waves which are associated with surface impacts, while the TOFD effects are mostly limited to the lower troposphere. The partitioning of τtot between the two schemes appears to play an important role at all time scales., Key Points: Parameterized orographic drag affects the Northern Hemisphere winter circulation at all time scalesThe partition of the surface stress between various parameterizations affects circulation aspects

A description of the daily cycle of oceanic shallow cumulus for undisturbed boreal winter conditions in the North Atlantic trades is presented. Modern investigation tools are used, including storm‐resolving and large‐eddy simulations, runover large domains in realistic configurations, and observations from in situ measurements and satellite‐based retrievals. Models and observations clearly show pronounced diurnal variations in cloudiness, both near cloud base and below the trade inversion. The daily cycle reflects the evolution of two cloud populations: (i) a population of nonprecipitating small cumuli with weak vertical extent, which grows during the day and maximizes around sunset, and (ii) a population o deeper precipitating clouds with a stratiform cloud layer below the trade inversion, which grows during the night and maximizes just before sunrise. Previous studies have reported that cloudiness near cloud base undergoes weak variations on time scales longer than a day. However, here we find that it can vary strongly at the diurnal time scale. This daily cycle could serve as a critical test of the models' representation of the physical processes controlling cloudiness near cloud base, which is thought to be key for the determination of the Earth's climate response to warming., Key Points Trade wind cumuli exhibit a pronounced daily cycle, with an evening maximum near cloud base and a nighttime/dawn maximum near the inversionCloud‐base cloudiness during undisturbed periods can be much more variable at the diurnal than at longer time scalesThe representation of the daily cycle of inversion clouds is particularly sensitive to resolution