A Preliminary Appraisal of the Natural Structure and Seadability of Updrafts in Midwestern Cumulus at the -10º C Level

Robert R Czys

Abstract


The properties of 40 updrafts in 11 warm-based Midwestern cumulus congestus are characterized on the basis of aircraft data collected at the -10°C level during the 1986 Precipitation Augmentation for Crops Experiment field program. Typically, clouds in this sample were found to be composed of multiple updrafts, with one updraft encountered on average for every 1.5 km of cloud penetrated. Mean updraft velocities ranged from 1 to 12 m s^-1 with a sample average of 4.2 m s^-1. All updrafts contained at least some supercooled liquid water content in the size range of cloud droplets (D < 50 um). Cloud droplet liquid water content was low, typically 0.3 g m^3, and bimodal cloud droplet size distributions were occasionally observed. Most updrafts contained supercooled drizzle and raindrops. The mass of supercooled drizzle and raindrops was often as large or larger then the mass of supercooled cloud droplets, indicating an efficient coalescence process. Sub-millimeter-size graupel was the apparent dominant first ice form, often occurring in concentrations that exceeded those conventionally expected from ice nuclei by factors from 10 to 100. Images of vapor-grown ice crystals were not often identified in the records from 2D optical array probes. Thus it is likely that the very first ice initiated from the freezing of supercooled drizzle and raindrops. Aircraft instrumentation available to the 1986 field program was not adequate to detect ice smaller than approximately 150 um diameter. Secondary ice production (SIP) by Hallett-Mossop rime-splintering could not be verified on the basis of available information. Natural updraft buoyancies were often close to neutral, and the amount of loading by the condensate was found to have deciding influence on net buoyancy. The results of calculations suggest that buoyancy enhancements are feasible by seeding, particularly for clouds with substantial water leads and moderate updrafts. However, discerning seeding effects will be complicated by large variations in initial conditions for seeding.

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