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

In this study, we show how to obtain more reliable satellite-retrieved cloud drop number concentrations (N_d) from Moderate Resolution Imaging Spectroradiometer (MODIS) standard cloud products. N_d 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. N_d is determined by cloud base aerosols and updrafts. Satellite retrievals of N_d 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 N_d for partially filled cloudy pixels are less than one third of the retrieved N_d for the convective cores) and limit the usefulness of N_d, especially in broken cloud fields. The N_d 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 (N_d agreement within 5%).