Space-borne Measurements of Changes in Cloud Optical Thickness and Cloud Drop Size Associated with Precipitation

The cloud feedback problem is one of the largest uncertainties in climate studies because cloud formation and dissipation are complicated processes. Clouds have mutual relations to aerosols and precipitation. Increases in aerosol concentrations result in a decreased drop size and modify the cloud radiative forcing. Decreased drop size may also suppress precipitation. Precipitation is generated from cloud drops by condensation and coalescence, which in turn generates cloud drops by breakup, and removes cloud drops and controls cloud amount. A study on clouds-precipitation interaction is, therefore, critical to improve our understanding of the cloud feedback problem. Despite extensive studies on cloud-precipitation interactions, our understanding is very limited because of their complex nature. We examined how cloud optical thickness changes in rain formation process by a combined use of the Precipitation Radar and the Visible and Infrared Scanner (VIRS) onboard the Tropical Rainfall Measuring Mission. The Tropical Rainfall Measuring Mission satellite primarily intends to measure temporal and spatial variations of precipitation with Precipitating Radar (PR) but also measures cloud properties with Visible and Infrared Scanner. Consequently, combined use of the PR and VIRS is well suited for studies of cloud-precipitation interactions. In the analysis, pixels for which the brightness temperature at Ch4 (10.8μm) of VIRS ranged from 273 to 290 K were selected. Thus we examine water cloud properties. Cloud optical thickness and the effective radius were derived from the reflected radiances at Ch1 (0.63μm) and Ch3 (3.75μm) measured with the VIRS. The derived data were used to study how cloud optical thickness relates to precipitation. In particular, we focused on the 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 increases with rain rate on average. The tendency to increase was mostly due to increases in liquid water path and depended on rain rate. For heavier 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. Cloud optical thickness is determined primarily by liquid water path but is also influenced by size distributions of cloud droplets. To remove the effects of the changes in liquid water path to cloud optical thickness and to study the …