A Simple Objective Method Used to Forecast Convective Activity during the 1989 PACE Cloud-seeding Experiment

Abstract

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. Maximum echo top heights were found to group 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 raindrops is discussed. The technique performed well at forecasting the occurrence and height of afternoon convective clouds. Aircraft measurements of supercooled raindrop 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 raindrops in the updrafts of clouds at the ? 10°C seeding level. Median concentrations of supercooled drizzle and raindrops (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 PR, presenting a possible physical link between cloud-scale environment and in-cloud conditions.