Separating aerosol microphysical effects and satellite measurement artifacts of the relationships between warm rain onset height and aerosol optical depth

Abstract

AbstractThe high resolution (375 m) of the Visible Infrared Imaging Radiometer Suite on board the Suomi National Polar‐Orbiting Partnership satellite allows retrieving relatively accurately the vertical evolution of convective cloud drop effective radius (re) with height or temperature. A tight relationship is found over SE Asia and the adjacent seas during summer between the cloud‐free aerosol optical depth (AOD) and the cloud thickness required for the initiation of warm rain, as represented by the satellite‐retrieved cloud droplet re of 14 µm, for a subset of conditions that minimize measurement artifacts. This cloud depth (ΔT14) is parameterized as the difference between the cloud base temperature and the temperature at the height where re exceeds 14 µm (T14). For a unit increase of AOD, the height of rain initiation is increased by about 5.5 km. The concern of data artifacts due to the increase in AOD near clouds was mitigated by selecting only scenes with cloud fraction (CF) < 0.1. For CF > 0.1 and ΔT14 > ~20°C, the increase of ΔT14 gradually levels off with further increase of AOD, possibly because the AOD is enhanced by aerosol upward transport and detrainment through the clouds below the T14 isotherm. The bias in the retrieved re due to the different geometries of solar illumination was also quantified. It was shown that the retrievals are valid only for backscatter views or when avoiding scenes with significant amount of cloud self‐shadowing. These artifacts might have contributed to past reported relationships between cloud properties and AOD.