Towards Distinguishing the Synoptic-Scale Sensitivity of Low-Level Clouds to Aerosol and Meteorology over the Southeast Atlantic

One of the largest low-level cloud decks in the world occurs over the southeast Atlantic, next to the largest global source of biomass-fire emissions from continental Africa. Between July and October, the shortwave-absorbing biomass burning aerosols are transported westward over the marine low-level clouds, a regime characterized by strong lower tropospheric stability (LTS) and enhanced climatological subsidence. The clouds are hypothesized to respond to both the presence of the aerosol, and to meteorological variability, on synoptic timescale. However, due to the coupled interaction of the aerosols and the meteorology, it remains a difficult challenge to completely isolate the sensitivity of the low-level cloud to either the aerosol alone, or the individual meteorological predictor over the southeast Atlantic. Though cloud sensitivity represents the change in cloud properties per unit change in individual environmental predictor, differing units of cloud sensitivity estimates often make comparison very difficult. Here we use satellite observations and ERA-Interim reanalysis dataset in standardize multiple regression analysis, to estimate the independent effect of meteorology and aerosol on the southeast Atlantic low-level clouds. One advantage of this method over traditional regression analysis is that, it normalizes the variability of the interacting components to one standard deviation, allowing for comparison between different environmental variables. Preliminary results identify LTS with the strongest meteorological effect, increasing cloudiness everywhere over the low cloud deck. In addition, enhanced 800 hPa subsidence increases the cloudiness between 5^o-20^oS, but decreases cloudiness south of 20^oS. Cloud sensitivity to shortwave-absorbing aerosols increases to the west, where the aerosols are more likely to be closer to and/or in contact with the low-level cloud. In the presence of aerosol, the sensitivity of low-level cloudiness to LTS increases by about 32% where the aerosols are likely in contact with the cloud, and by about 13% where the aerosol are likely separated from the underlying low-level cloud.