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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. 

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. 

Abstract. The aerosol–planetary boundary layer (PBL) interaction wasproposed as an important mechanism to stabilize the atmosphere andexacerbate surface air pollution. Despite the tremendous progress made inunderstanding this process, its magnitude and significance still have largeuncertainties and vary largely with aerosol distribution and meteorologicalconditions. In this study, we focus on the role of aerosol verticaldistribution in thermodynamic stability and PBL development by jointly usingmicropulse lidar, sun photometer, and radiosonde measurements taken inBeijing. Despite the complexity of aerosol vertical distributions,cloud-free aerosol structures can be largely classified into three types:well-mixed, decreasing with height, and inverse structures. The aerosol–PBLrelationship and diurnal cycles of the PBL height and PM2.5 associated with these different aerosol vertical structures showdistinct characteristics. The vertical distribution of aerosol radiativeforcing differs drastically among the three types, with strong heating in thelower, middle, and upper PBL, respectively. Such a discrepancy in the heatingrate affects the atmospheric buoyancy and stability differently in the threedistinct aerosol structures. Absorbing aerosols have a weaker effect ofstabilizing the lower atmosphere under the decreasing structure than underthe inverse structure. As a result, the aerosol–PBL interaction can bestrengthened by the inverse aerosol structure and can be potentiallyneutralized by the decreasing structure. Moreover, aerosols can both enhanceand suppress PBL stability, leading to both positive and negativefeedback loops. This study attempts to improve our understanding of theaerosol–PBL interaction, showing the importance of the observationalconstraint of aerosol vertical distribution for simulating this interactionand consequent feedbacks. 

Abstract. Twelve months of measurements collected during the Two-ColumnAerosol Project field campaign at Cape Cod, Massachusetts, which started inthe summer of 2012, were used to investigate aerosol physical, optical, andchemical properties and their influences on the dependence of clouddevelopment on thermodynamic (i.e., lower tropospheric stability, LTS)conditions. Relationships between aerosol loading and cloud properties underdifferent dominant air-mass conditions and the magnitude of the firstindirect effect (FIE), as well as the sensitivity of the FIE to differentaerosol compositions, are examined. The seasonal variation in aerosol numberconcentration (Na) was not consistent with variations in aerosoloptical properties (i.e., scattering coefficient, σs, andcolumnar aerosol optical depth). Organics were found to have a largecontribution to small particle sizes. This contribution decreased during theparticle growth period. Under low-aerosol-loading conditions, the liquidwater path (LWP) and droplet effective radius (DER) significantly increasedwith increasing LTS, but, under high-aerosol-loading conditions, LWP and DERchanged little, indicating that aerosols significantly weakened thedependence of cloud development on LTS. The reduction in LWP and DER fromlow- to high-aerosol-loading conditions was greater in stable environments,suggesting that clouds under stable conditions are more susceptible toaerosol perturbations than those under more unstable conditions. Highaerosol loading weakened the increase in DER as LWP increased andstrengthened the increase in cloud optical depth (COD) with increasing LWP,resulting in changes in the interdependence of cloud properties. Under bothcontinental and marine air-mass conditions, high aerosol loading cansignificantly increase COD and decrease LWP and DER, narrowing theirdistributions. Magnitudes of the FIE estimated under continental air-massconditions ranged from 0.07±0.03 to 0.26±0.09 with a meanvalue of 0.16±0.03 and showed an increasing trend as LWP increased.The calculated FIE values for aerosols with a low fraction of organics aregreater than those for aerosols with a high fraction of organics. Thisimplies that clouds over regions dominated by aerosol particles containingmostly inorganics are more susceptible to aerosol perturbations, resultingin larger climate forcing, than clouds over regions dominated by organicaerosol particles.