Assessment of Low Cloud Cover Changes in the Presence of Shortwave-Absorbing Smoke

This study will investigate how boundary layer cloud cover in the southeast Atlantic is altered when sunlight-absorbing smoke is present. At Ascension Island, biomass-burning aerosols are often present both in the boundary layer and in the free troposphere during the boreal fall. The low cloud is typically most present at two levels within the decoupled boundary layer: one at the lifting condensation level of near-surface air, and the other just below the trade-wind inversion. The lower cloud cover could increase if precipitation is suppressed due to more cloud-nucleating aerosol, or it could decrease if the absorbed sunlight increases the boundary layer temperature, thus reducing the relative humidity. The upper cloud cover could be similarly affected, and could also increase if sunlight absorbed by the free-tropospheric smoke acts to warm the lower troposphere and strengthen the trade-wind inversion. Thus, the goal of this study will be to assess how the cloud cover actually changes. The LASIC deployment on Ascension Island includes two 35-GHz (K-band) cloud radars, one upward-pointing only and the other a scanning radar. The data from the scanning radar is valuable because orography affects the local measurements, as oncoming winds at the LASIC site enhance the cloud cover at the lifting condensation level. This project will evaluate the cloud structure as ascertained by both radars.