2009 Volume 87 Issue 4 Pages 593-600
We derived rain rate, cloud optical thickness and the effective radius in water clouds by a combined use of the Precipitation Radar and the Visible and Infrared Scanner onboard the Tropical Rainfall Measuring Mission. The derived data were used to study how cloud optical thickness relates to precipitation. In particular, we focused on the changes in cloud optical thickness resulted from changes in the size distributions of cloud droplets associated with precipitation. There were considerable scatter between cloud optical thickness and rain rate on a global scale. However, cloud optical thickness was found to increase with rain rate on average. The tendency to increase was mostly due to increases in liquid water path and depended on rain rate. For strong rain, relatively small increases in the optical thickness with rain rate were observed. Whereas, for weak rain, larger increases with rain rate were found, which is related to considerable changes in liquid water path and in the effective radius of cloud droplets. To study the effects of drop size variation, the relationships between cloud optical thickness and rain rate for same values of liquid water path were analyzed. Results show that there were no significant dependences of cloud optical thickness on rain rate for strong rain. For weak rain, cloud optical thickness was found to decrease with rain rate. In particular, significant differences of optical thickness were found between non-precipitating clouds and precipitating clouds: smaller cloud optical thickness was observed for precipitating clouds. Dispersion of cloud drop size was found in the rain formation process, which may relate to changes in the shape of drop size spectra and leads to the decreases in the cloud optical thickness for precipitating clouds.