P2.2 Vertical Profiles of Ice Cloud Microphysical Properties Observed behind Convective Lines during the Bow Echo and Mesoscale Convective Vortices Experiment (BAMEX)

Wednesday, 12 July 2006
Grand Terrace (Monona Terrace Community and Convention Center)
Greg McFarquhar, University of Illinois, Urbana, IL; and M. S. Timlin, R. M. Rauber, B. F. Jewett, J. Grim, and D. P. Jorgensen

During the Bow Echo and Mesoscale Convective Vortices Experiment (BAMEX), the NOAA P-3 executed 17 Lagrangian spiral descents behind convective lines to document the vertical variability above within and below the stratiform melting layer. For 16 spirals, the ambient relative humidity with respect to ice (RHi) was at or near 100% for temperatures above the melting layer but exhibited steady decreases below to an average RH with respect to water (RHw) of 77% at 9oC. For one spiral conducted June 29, 2003 in a notch behind a developing bow, RHi averaged 85% above 0oC and decreased below to RHw of 48% at 9oC.

Vertical profiles of particle shapes, size distributions, total number concentrations, mass contents and fit parameters of gamma distributions are computed using data acquired with optical array probes on the P-3 in conjunction with radar reflectivity measured by the NOAA tail radar. For spirals at or near saturation above the melting layer, the concentration of hydrometeors smaller than 2 mm and the slope parameter of the gamma distribution decreased whereas the concentration of hydrometeors larger than 3 mm and the maximum particle size increased with temperature, suggesting aggregation dominates the evolution of size distributions in saturated regions. For the spiral conducted in sub-saturated air above the melting layer on June 29, there was a slight decrease in the concentration of particles smaller than 2 mm with increasing temperature, but no increase in concentration of particles larger than 3 mm or maximum diameter or decrease in slope parameter. As sublimation preferentially removes small ice, this suggests competing influences of aggregation and sublimation keep size distributions relatively constant.

Sublimative cooling would only be significant for the June 29 spiral executed in sub-saturated air above the melting layer in the notch behind the developing bow. For the spirals at other locations and times in the lifetime of the convective line, evaporation below the melting layer in sub-saturated regions would dominate contributions to the diabatic processes that ultimately help in generating downdrafts.

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