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Key Points A preexisting stratocumulus deck is more persistent when experiencing warm‐air advection than cold‐air advection This persistence is due to reduced entrainment drying as a result of decoupling, which outweighs decreased cloud‐base moisture transport The mechanism is more notable when free‐tropospheric humidity is higher 

The Amazon Basin, which plays a critical role in the carbon and water cycle, is under stress due to changes in climate, agricultural practices, and deforestation. The effects of thermodynamic and microphysical forcing on the strength of thunderstorms in the Basin (75–45°W, 0–15°S) were examined during the pre‐monsoon season (mid‐August through mid‐December), a period with large variations in aerosols, intense convective storms, and plentiful flashes. The analysis used measurements of radar reflectivity, ice water content (IWC), and aerosol type from instruments aboard the CloudSat and CALIPSO satellites, flash rates from the ground‐based Sferics Timing and Ranging Network, and total aerosol optical depth (AOD) from a surface network and a meteorological re‐analysis. After controlling for convective available potential energy (CAPE), it was found that thunderstorms that developed under dirty (high‐AOD) conditions were 1.5 km deeper, had 50% more IWC, and more than two times as many flashes as storms that developed under clean conditions. The sensitivity of flashes to AOD was largest for low values of CAPE where increases of more than a factor of three were observed. The additional ice water indicated that these deeper systems had higher vertical velocities and more condensation nuclei capable of sustaining higher concentrations of water and large hydrometeors in the upper troposphere. Flash rates were also found to be larger during periods when smoke rather than dust was common in the lower troposphere, likely because smoky periods were less stable due to higher values of CAPE and AOD and lower values of mid‐tropospheric relative humidity.

 

Aerosol-boundary layer interactions play an important role in affecting atmospheric thermodynamics and air pollution. As a key factor in dictating the development of the boundary layer, the entrainment process in the context of aerosol-boundary layer interactions is still poorly understood. Using comprehensive field observations made at a superstation in Beijing, we gain insight into the response of the entrainment process to aerosols. We found that high aerosol loading can significantly suppress the entrainment rate, breaking the conventional linear relationship between sensible heat fluxes and entrainment fluxes. Related to aerosol vertical distributions, aerosol heating effects can alter vertical heat fluxes, leading to a strong interaction between aerosols and the entrainment process in the upper boundary layer. Such aerosol-entrainment coupling can inhibit boundary layer development and explains the great sensitivity of observed entrainment rates to aerosols than can traditional calculations. The notable impact of aerosols on the entrainment process raises holistic thinking about the dynamic framework of the boundary layer in a polluted atmosphere, which may have a significant bearing on the dispersion of air pollutants and the land-atmosphere coupling.

 

Abstract. The states of coupling between clouds andsurface or boundary layer have been investigated much more extensively formarine stratocumulus clouds than for continental low clouds, partly due tomore complex thermodynamic structures over land. A manifestation is a lackof robust remote sensing methods to identify coupled and decoupled cloudsover land. Following the idea for determining cloud coupling over the ocean,we have generalized the concept of coupling and decoupling to low cloudsover land, based on potential temperature profiles. Furthermore, by usingample measurements from lidar and a suite of surface meteorologicalinstruments at the U.S. Department of Energy's Atmospheric RadiationMeasurement Program's Southern Great Plains site from 1998 to 2019, we havedeveloped a method to simultaneously retrieve the planetary boundary layer(PBL) height (PBLH) and coupled states under cloudy conditions during thedaytime. The new lidar-based method relies on the PBLH, the liftedcondensation level, and the cloud base to diagnose the cloud coupling. Thecoupled states derived from this method are highly consistent with thosederived from radiosondes. Retrieving the PBLH under cloudy conditions, whichhas been a persistent problem in lidar remote sensing, is resolved in thisstudy. Our method can lead to high-quality retrievals of the PBLH undercloudy conditions and the determination of cloud coupling states. With thenew method, we find that coupled clouds are sensitive to changes in the PBLwith a strong diurnal cycle, whereas decoupled clouds and the PBL are weaklyrelated. Since coupled and decoupled clouds have distinct features, our newmethod offers an advanced tool to separately investigate them in climatesystems. 

Due to surface heating, the morning boundary layer transits from stable to neutral or convective conditions, exerting critical influences on low tropospheric thermodynamics. Low clouds closely interact with the boundary layer development, yet their interactions bear considerable uncertainties. Our study reveals that cloud‐surface coupling alters the morning transition from stable to unstable boundary layer and thus notably affects the diurnal variation of the boundary layer. Specifically, due to the reduction in surface fluxes, decoupled clouds can delay the process of eroding nocturnal inversion by 0.8‐hr and even prevent the transition of the boundary layer from happening for 12% of decoupled cases, keeping the boundary layer in a stable state during the noontime. On the other hand, when clouds are coupled with the surface, cloud‐top radiative cooling can directly cool the upper boundary layer to facilitate sub‐cloud convection, leading to an unstable boundary layer in the earlier morning.

 

Abstract. Aerosol–cloud interactions remain largely uncertain with respect to predicting theirimpacts on weather and climate. Cloud microphysics parameterization is oneof the factors leading to large uncertainty. Here, we investigate the impactsof anthropogenic aerosols on the convective intensity and precipitation of athunderstorm occurring on 19 June 2013 over Houston with the Chemistryversion of Weather Research and Forecast model (WRF-Chem) using the Morrisontwo-moment bulk scheme and spectral bin microphysics (SBM) scheme. We findthat the SBM predicts a deep convective cloud that shows better agreement withobservations in terms of reflectivity and precipitation compared with theMorrison bulk scheme that has been used in many weather and climate models.With the SBM scheme, we see a significant invigoration effect on convectiveintensity and precipitation by anthropogenic aerosols, mainly throughenhanced condensation latent heating. Such an effect is absent withthe Morrison two-moment bulk microphysics, mainly because the saturationadjustment approach for droplet condensation and evaporation calculationlimits the enhancement by aerosols in (1) condensation latent heat byremoving the dependence of condensation on droplets and aerosols and (2) ice-related processes because the approach leads to stronger warm rain andweaker ice processes than the explicit supersaturation approach. 

Abstract. Changes in land cover and aerosols resulting from urbanization may impactconvective clouds and precipitation. Here we investigate how Houstonurbanization can modify sea-breeze-induced convective cloud and precipitation through the urban land effect and anthropogenic aerosol effect. The simulations are carried out with the Chemistry version of the WeatherResearch and Forecasting model (WRF-Chem), which is coupled with spectral-bin microphysics (SBM) and the multilayer urban model with abuilding energy model (BEM-BEP). We find that Houston urbanization (thejoint effect of both urban land and anthropogenic aerosols) notably enhancesstorm intensity (by ∼ 75 % in maximum vertical velocity) andprecipitation intensity (up to 45 %), with the anthropogenic aerosoleffect more significant than the urban land effect. Urban land effectmodifies convective evolution: speed up the transition from the warm cloudto mixed-phase cloud, thus initiating surface rain earlier but slowing down the convective cell dissipation, all of which result from urban heating-induced stronger sea-breeze circulation. The anthropogenic aerosol effectbecomes evident after the cloud evolves into the mixed-phase cloud,accelerating the development of storm from the mixed-phase cloud to deepcloud by ∼ 40 min. Through aerosol–cloud interaction (ACI), aerosols boost convective intensity and precipitation mainly by activatingnumerous ultrafine particles at the mixed-phase and deep cloud stages. Thiswork shows the importance of considering both the urban land and anthropogenic aerosol effects for understanding urbanization effects on convective cloudsand precipitation.