Estimation of cloud liquid water path and detection of rain using the vertical profile of cloud droplet size

Liquid water path (LWP) is an important cloud variable. Remote sensing of LWP has relied on the retrieval of cloud optical depth (COD) and cloud droplet effective radius (DER) using visible (~600nm) and mid-infrared (3700nm) channels, respectively. It is widely known that the cloud particle size retrieval can only represent cloud droplet size for a very thin layer near the cloud tops. Plentiful of in-situ observations showed that cloud droplet size varies with cloud height. As such, systematic bias in the estimates of LWP was often incurred. To evaluate the problem, we applied a method that can be used to retrieve the vertical profile of cloud droplet size (Chang and Li 2002, 2003, JGR) to global MODIS data for liquid water clouds. By integrating the DER profile, LWP is computed with a knowledge of COD. The LWP retrievals are then compared with the microwave retrieval from collocated AMSU-E. Their differences exhibit a systematic dependence on the profile of DER. Correcting the dependence leads to a better retrieval of LWP. In addition, the profile of DER is also found to be useful in detecting if a water cloud is drizzling or not. The majority of the drizzling clouds show larger DER near cloud bottom than near cloud tops, and vice versa.