11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Wednesday, 5 June 2002: 9:00 AM
Droplet collision rates in turbulent clouds: theory and observations
Raymond A. Shaw, Michigan Technological University, Houghton, MI
Often, droplets in a turbulent cloud are not distributed in a perfectly random manner. Correlations in droplet position can be caused by turbulent mixing of cloudy and clear air, by the inertial response of cloud droplets to fluid accelerations (preferential concentration), or even by simple sedimentation in still air. When droplets are not distributed with perfect randomness the coagulation equation (stochastic collection equation) used in cloud physics will give erroneous results. The coagulation equation can be modified, however, if the droplet pair correlation function is known. An analytical form of the modified coagulation equation appropriate for a turbulent mixing zone can be formulated and will be discussed. Also, the pair correlation function can be measured directly using a Fast FSSP, allowing the modified droplet collision rate to be calculated. In order to obtain the 3-dimensional pair correlation function necessary for modifying the coagulation equation, the 1-dimensional FSSP data must be inverted and this procedure, including its assumptions and limitations, will be discussed using data from cumulus clouds.

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