Tuesday, September 09, 2014

Warm Cloud Microphysics And Yesterday's Rain Event

It appears that warm cloud microphysics (droplet growth through collision and coalescence) likely played a significant role in yesterday's heavy rains. While there was CG to lightning (plot of detected flashes through midnight last night shown above), the number of flashes was relatively limited in comparison to significant thunderstorm events here. Here at the house we heard only a rumble or two of thunder during the entire event. If convective showers are generated entirely by warm cloud microphysics there is no electrification within the cloud and lightning does not occur.

The morning sounding at TWC, taken a couple of hours before the rains set in, is shown below. I have marked the -10 C level of one possible lifted parcel (CAPE for that parcel is shaded red and CIN blue). The -10 C level within that possible updraft is way up at 400 mb, indicating a very deep cloud layer within which warm cloud microphysics were at play.

Jack Diebolt sent a cross-section he generated for TUS radar data taken at about 1130 am yesterday morning. The cross-section is twisted in spatial orientation and runs from south-southwest to north-northeast through the stronger echoes at that time. The important thing to note is that the strongest echo cores are relatively shallow - a signature that indicates warm microphysics is generating the large cloud droplets and higher radar reflectivity. The green echo spike (28 to 30 dBZ) near the right side of the section extends only to about 27000 ft above the radar elevation (about 32,000 ft msl or 300 mb). Thus, most of the convective towers are not reaching above -10 C and are not dominated by cold-cloud microphysics. Warm convective towers can be very efficient rain producers, which is certainly what we witnessed yesterday.

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