Guiding Cloud Seeding by Aircraft and Satellite Measurements of the Height for Onset of Precipitation Above Cloud Base in Growing Convective Clouds

The action of cloud seeding for rain enhancement and possibly for severe storm mitigation is aimed at accelerating the formation of precipitation in the clouds. Rain forms naturally when cloud drops grow with height above cloud base to the size of 14 µm in effective radius (r_e), above which they can readily coalesce into raindrops and freeze into ice hydrometeors. The vertical growth rate of re, dr_e/dH, is determined to a large extent by the CCN concentrations below cloud base. When larger concentrations of CCN occur, the same amount of cloud water is distributed to smaller drops, which need to ascend higher in the cloud for reaching the critical size of 14 µm for initiating rain. The importance of CCN was demonstrated and quantified by aircraft measurements during the CAIPEEX project, which took place during the summers of 2009 and 2010 in India. It made vertical profiles of r_e in convective clouds and related them to the CCN concentrations measured below the cloud base at a wide range of conditions. A systematic linear dependence was found between the CCN concentrations and the vertical distance above cloud base that was required for r_e to reach a given size, including the precipitation threshold of 14 µm. The growth of cloud drop r_e with height can be observed with satellite, using the method developed by Rosenfeld and Lensky (1998). The height at which the satellite retrieved r_e exceeds 15 µm is considered as the temperature for initiating rain. Here we use slightly larger r_e with the satellite (15 instead of 14 µm), because the satellite tends to overestimate r_e slightly. The height can be obtained from the temperature by the use of a sounding. The cloud depth for rain initiation is then calculated by subtracting the retrieved cloud base (as obtained from the warmest cloudy data points) from the height for r_e=15 µm. Larger rain-free cloud depth means slower rain initiation and smaller precipitation efficiency of the clouds. Satellites can be used over large areas for assessing this cloud property, and apply it to various objectives, such as aerosol impacts on precipitation, and assessment of the potential seedability of clouds. Examples from various parts of the world will be given.