Observing the diurnal characteristics of marine stratocumulus drizzle using the TRMM microwave imager

With the launch of the TRMM satellite in 1997, there is, for the first time, an opportunity to observe how drizzle formation varies diurnally. Modeling and limited observational studies have suggested that drizzle formation peaks in the evening, which is attributed to a decoupling of the cloud layer dynamics from the subcloud layer earlier in the day. However, long-term observations of drizzle frequency over extensive areas, which are needed to confirm this idea, are lacking.

This study uses the new passive microwave imager on TRMM (the TMI) to investigate the daily cycle of drizzle frequency. The TMI has the highest spatial resolution of any previous microwave radiometer (5 km at 85.5 GHz) making it ideal for detecting small drizzle cells in marine stratocumulus. The TMI is used rather than the precipitation radar (PR) because of its wider swath and, hence, greater areal coverage. Moderate to heavy drizzle is detected using a precipitation index consisting, in part, of the polarization difference at 85.5 GHz. Lower frequency channels of the TMI are also used to account for water vapor effects. An appropriate threshold for the index to separate drizzling clouds from thick nonprecipitating clouds is chosen based on comparisons with the PR data. This study focuses on a large region off the coast of California during the summer of 1998.

Topic to discuss: Assimilation of cloudy radiances from GOES-I/M imager into a mesoscale model