Objective Forecasting of Some Individual Cloud Characteristics in the 1989 Illinois Cloud Seeding Experiment

Robert W Scott, Robert R Czys


A simple objective procedure used exploratively to forecast the occurrence, height, and coalescence activity of summertime convective clouds in Illinois during the cloud seeding trials of the 1989 Precipitation Augmentation for Crops Experiment is described. The method used the temperature of the convective condensation level (TccL) and potential buoyancy (PB) at 500 mb, easily determined from morning National Weather Service sounding data, to forecast afternoon convection. Categories of maximum echo top heights were found to arrange according to TccL and PB. The physical basis of TccL and PB to implicitly represent a period of time for coalescence to produce supercooled drizzle and rain drops is discussed. The technique performed well at forecasting the occurrence and height of afternoon convective clouds, and the accuracy of the occurrence forecast improved if precipitable water was used as an additional criteria. Aircraft measurements of supercooled rain drop concentrations showed that a discriminator function, dependent only on TccL and PB, gave a good indication of the presence or absence of supercooled drizzle and rain drops in the updrafts of clouds at the -10°C seeding level. Median concentrations of supercooled drizzle and rain drops (ND>300) in updraft regions at the -10°C level were found to be best approximated by a third order polynomial dependent on TccL and PB, presenting a possible physical link between cloud scale environment and in-cloud conditions.

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