Satellite measurements of precipitation and critical size of cloud droplet

We have examined the cloud-precipitation interaction by a synergetic use of space-borne active and passive sensors. Increases in aerosol concentrations result in decreased cloud drop size and may also lead to suppression of precipitation. This process is known as ‘second indirect effect of aerosols' and is one of the major uncertainties in climate studies. Many factors like cloud drop concentration, the liquid water content and size distribution affect on the conversion process of cloud droplets to raindrops. Theoretical studies suggest that cloud droplet size is the most important factor in the process among these parameters. In particular, a key factor for better understanding the second indirect effects is a critical size of cloud drops below which precipitation hardly forms. We measured cloud drop size and precipitation by a combined use of precipitating radar and solar/infrared radiometer onboard the Tropical Rainfall Measuring Mission (TRMM). We found sudden changes in the cloud droplet effective radius in marine water clouds, and significant differences in the radius between non-precipitating clouds and precipitating clouds in which clouds and precipitation coexist. The largest radius was limited to between 15 and 20 ìm for non-precipitating clouds, whereas it increased discontinuously to 30 ìm for precipitating clouds. Detailed analysis suggests that the differences are likely due to the rapid growth of cloud drops into raindrops. This study revealed that the critical sizes are clearly observed and are consistent on a global scale, which improves our understanding of the second indirect effects of aerosols.