Detecting drizzle in marine warm clouds using combined visible, infrared, and microwave satellite data

Abstract

We present a method to detect drizzle in marine warm clouds by combining visible, near-infrared, and microwave measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite. A visible/near-infrared algorithm is used to simultaneously retrieve cloud optical depth (τ) and effective radius (r'e). A new microwave algorithm is developed to retrieve liquid water path (LWPm). First, the relationship among LWPm, re and τ are investigated by using radiative transfer model simulations and by analyzing satellite observations. Results from both the modeling and the analysis indicate that LWPm, re and τ satisfy a relation in the form of LWPm = αrβeτ(3β-1)/2, and that α and β stay relatively constant within a satellite scene for nondrizzle clouds, while they are different from those for drizzle clouds. On the basis of this result, a drizzle detection index (DI) is defined by the ratio of LWPm to the liquid water path calculated from τ and re using a relation derived for nondrizzle clouds. Model simulation results show that for nondrizzle clouds DI is close to 1, and for drizzle/rain clouds DI is greater than 1. Case studies using satellite data indicate that values of DI are greater in the regions with large re but relatively small τ, and/or near the center of convective cells. The relation among LWPm, re and τ derived from satellite retrievals is consistent with that of radiative transfer simulation results. Finally, coincident airborne cloud radar measurements were used to verify the capability of D1 for drizzle detection. The result indicates that D1 is useful in differentiating between drizzle and nondrizzle clouds, and suggesting that DI of .1 is the drizzle threshold, which is also consistent with our radiative transfer model simulations. Copyright 2004 by the American Geophysical Union.