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Anthropogenic emissions were greatly constrained during COVID‐19 lockdown in China. Nevertheless, observations still showed high loadings of fine particles (PM2.5) over northern China with secondary aerosols increasing by 15 μg/m3 yet a ∼10% drop in light‐absorbing black carbon (BC). Such a chemical transition in aerosol composition tended to make the atmosphere more scattering, indicated by satellite‐retrieved aerosol absorption optical depth falling by 60%. Comparison between weather forecast and radiosonde observations illustrated that, without upper‐level heating induced by BC, the stabilized stratification diminished, which was conducive for planetary boundary layer (PBL) mixing and thus near‐surface pollution dispersion. Furthermore, coupled dynamic‐chemistry simulations estimated that emission reduction during the lockdown weakened aerosol‐PBL interaction and thus a reduction of 25 μg/m3 (∼50%) in PM2.5 enhancement. Based on the unique natural experiment, this work observationally confirmed and numerically quantified the importance of BC‐induced meteorological feedback, further highlighting the priority of BC control in haze mitigation., Key Points Heavy haze still engulfed northern China despite great emission reductions during COVID‐19 lockdownEnhanced secondary aerosols and decreased black carbon (BC) led to a more scattering atmosphere and weakened aerosol‐planetary boundary layer interactionObservational evidences and quantitative modeling confirmed the importance of BC during the unique natural experiment

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 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

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

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

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

International audience; This paper focuses on the structure of the marine boundary layer in an convergence region between easterly (trade winds) and westerly flows, over the Subtropical South Atlantic Ocean on 15 September 1994 using an unprecedented combination of spaceborne and airborne lidar observations, airborne Radar Ocean Wave Spectrometer and dropsondes system, and three-dimensional modeling. Methodologies previously developed to determine the marine atmospheric boundary layer (MABL) depth and cloud top heights from airborne lidar measurements are successfully applied to spaceborne measurements. Nested numerical simulations have been performed in this case study. It is shown that the MABL structure in this region (observed with lidar and simulated) collapsed dramatically in connection with a marked decrease of wind speed and near-zero turbulent kinetic energy due to specific synoptic conditions.

The article of record as published may be found at http://dx.doi.org/10.1029/2000JC000466 Basin-scale sea ice models are often run uncoupled to either an atmosphere or ocean model to evaluate the sea ice model, to compare different models, and to test changes in physical parameterizations. Such simulations require that the boundary forcing be specified. The specification of atmospheric forcing associated with the surface heat and freshwater fluxes has been done in various sea ice simulation using climatology, numerical weather prediction analyses, or and satellite data. However, the errors in the boundary forcing may be so large that it is difficult to determine whether discrepancies between simulated and observed properties of sea ice should be attributed to deficiencies in the sea ice model or to the boundary forcing. To assess the errors in boundary forcing, we use data from the Surface Heat Budget of the Arctic Ocean (SHEBA) to evaluate various data sets that have been used to provide boundary forcing for sea ice models that as associated with the surface heat and freshwater fluxes. The impact of errors in these data sets on a sea ice model is assessed by using a single-column ice thickness distribution mode, which is alternately forced with in situ measurements from SHEBA and output from large-scale analyses. Substantial discrepancies are found among the data sets. The response of the sea ice model to the different forcing data sets was considerable. This research was funded by several grants from NSF-OPP and by DOE ARM. Approved for public release; distribution is unlimited.