Under What Conditions Can We Trust Retrieved Cloud Drop Concentrations in Broken Marine Stratocumulus?

Abstract

AbstractIn this study, we show how to obtain more reliable satellite‐retrieved cloud drop number concentrations (Nd) from Moderate Resolution Imaging Spectroradiometer (MODIS) standard cloud products. Nd is important as a fundamental cloud property that determines cloud albedo, precipitation‐forming processes, cloud longevity, and fractional cover, all of which determine cloud radiative effects. Nd is determined by cloud base aerosols and updrafts. Satellite retrievals of Nd for marine stratocumulus clouds are performed based on the assumption of a nearly adiabatic cloud column. The retrievals, however, are fraught with problems that cause large biases (e.g., the retrieved Nd for partially filled cloudy pixels are less than one third of the retrieved Nd for the convective cores) and limit the usefulness of Nd, especially in broken cloud fields. The Nd of the brightest 10% of cloudy pixels are selected because the brightest clouds are the convective cores that most closely follow adiabatic parcels, a key assumption of our retrieval. It is shown that this criterion is representative of pixels that are also fully surrounded by cloudy pixels (Nd agreement within 5%).