Aerosols are usually released and activated as CCN in the boundary layer below clouds. All else equal, an increase of CCN increases cloud droplet number concentrations (Nd) and suppresses surface precipitation. In the SEA, these changes occur after BB aerosols have entered the MBL, either through entrainment from the cloud top or advection off the African continent and entrainment from below clouds. A few recent studies tried to use satellite observations to assess the aerosol microphysical effects exerted by BB aerosols in the SEA (23, 24). However, because of the covariation of aerosols and meteorology, it is hard to determine the aerosol effects on MBL clouds solely from observations, and even more challenging to quantify the relative importance of the microphysical effects, which are inherently entangled with semidirect effects. In addition, the aerosol microphysical effects on MBL clouds may vary strongly within a day because the MBL clouds exhibit strong diurnal cycles through coupling (during the night) and decoupling (during the day) processes (6). Therefore, to unravel the dominating mechanisms, it is also critical to examine the diurnal variation of aerosol microphysical effects.
Cloud-top entrainment describes the mixing between cloud and dry air that occurs at cloud top that influences cloud-top height and cloud-top microphysics. If cloud droplet number concentrations increase, without a corresponding increase in LWP, entrainment can increase on a time scale comparable with the eddy turnover time scale (1 h or less) because faster evaporation of the smaller cloud drops at cloud top can decrease temperatures enhancing turnover of eddies. In turn, the enhanced entrainment can cause the stratocumulus-topped boundary layer (STBL) to warm and dry, thereby reducing CF and LWP on a time scale that is markedly longer (6). Due to such complex negative feedbacks, previous studies have found that the aerosol microphysical effects on clouds may be diminished or even cancelled under some scenarios [e.g., in cases with small precipitation but strong entrainment (28) or with relatively high cloud base (29)]. Therefore, it is critical to assess the total net microphysical effect of BB aerosols over the SEA during the fire season using a model that can account for all of the relevant cloud processes in these scenarios. Due to the complex interactions between microphysics, radiation, turbulence, and entrainment processes associated with MBL clouds (30), simulating MBL clouds has proven to be a challenging task, especially in GCMs (6). Over the SEA, 12 models employed in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) underestimate the magnitude of annual mean shortwave cloud radiative forcing by 10–20 W m−2 (31). This large discrepancy motivates us to use high-resolution models instead of GCMs with coarser resolutions to study the effect of BB aerosols on MBL clouds.
In this work, we use advanced modeling techniques (large eddy simulations [LES] nested within Weather Research and Forecasting with Chemistry [WRF-Chem]) to study the role of BB aerosols in regulating the properties of MBL clouds and the resulting radiative energy budget in the SEA, with a specific focus on the relative importance between the semidirect and microphysical effects. In WRF-Chem, 2-mo simulations from August 1 to September 30, 2014, are conducted at a convection-permitting scale of 3 km in three contrasting aerosol scenarios, namely a case with only sea salt and DMS emissions (clean or “C” case), a case with BB aerosol, sea salt, and DMS emissions (polluted or “P” case), and a case similar to P case, but with the radiative effect of biomass aerosols turned off and with only the microphysical effect of aerosols on clouds included (microphysics only or “M” case). The difference between the P and C cases is a measure of the total effect of BB aerosols, whereas the difference between the P and M cases represents the sum of direct and semidirect effects of BB aerosols. In addition, we also conduct WRF-LES simulations to corroborate the performance of WRF-Chem. All of the WRF-LES domains have 97 (vertical levels) × 500 × 500 model grids with horizontal resolutions of 66.7 m and 52 layers from 0 to 1 km and 25 layers from 1 to 2 km. In both WRF-Chem and WRF-LES, the Morrison two-moment cloud microphysical scheme is adopted to treat sophisticated aerosol–cloud interactions.