November 6, 2010 Shallow Convection / Graupel / TBSS’s

Shallow Convection TBSS’

11/06 RDU AMDAR Soundings in Vicinity of 12Z

We had some interesting weather yesterday (11/06) as the deep upper level trough over the eastern US slowly progressed eastward through central NC. While there weren’t any significant operational implications, from a science perspective I thought this was worth looking at. An H5 low (or H5 cold pool)  embedded within the mean trough was located in vicinity of northwest NC at 12Z yesterday, with H5 temps of -33C, colder than anywhere else in the CONUS. Mid-level (H7-H5) lapse rates were ~7.5 C/km over central NC, surface temps were in the lower 40s, and surface dewpoints were in the upper 30s. Critical thicknesses off the 12Z 11/06 GSO sounding were 1310/1506 meters. The freezing level was between 2500-3000 ft AGL, and the -15C isotherm was at 9-10 KFT AGL. The primary forcing for precipitation yesterday was probably DPVA associated with the deep upper level trough (and perhaps smaller amplitude waves within the trough).

Shallow convection was ongoing across central NC at 12Z 11/06, with echo tops of 12-15 KFT. Although no instability (MLCAPE, MUCAPE or otherwise) was analyzed on the SPC mesoanalysis website, the presence of shallow convection was evidence enough that sufficient instability was present. The aformentioned instability was most likely the result of steep mid-level lapse rates and strong synoptic forcing (layer-lifting, convective instability). While instability was sufficient for convection to be present, it was insufficient for updrafts strong enough to produce lightning – although 30-35 dBZ echoes were frequently present at 10-11 KFT AGL, suggesting that at least several updrafts made valiant attempts.

At approximately 815 am EDT (1215Z), Three Body Scatter Spikes (TBSS’s) were noted with convective cells in Lee and Chatham counties on the 0.5 degree elevation angle, with the 40-45 dBZ convective cores responsible for the TBSS signatures located at roughly 2500-3000 ft AGL (at/near the 0C isotherm).

So why did we get TBSS’s with such low reflectivities (relatively speaking) and minimal instability? I’m thinking that while convective updrafts were too weak to produce hail/lightning, they must have been sufficient enough to loft supercooled water 1-3 KFT above the freezing level, riming snowflakes and producing graupel. Although the 0.5 degree slice was at 2500-3000 ft AGL (in vicinity of the freezing level where you would expect higher reflectivities associated with melting), there were 40 dBZ echoes (albeit much fewer) on the 0.9 and 1.4 degree slices as well, indicating the possible presence of graupel at 4-5 KFT AGL (reflectivites with dry snow above the freezing level shouldn’t be that high). I’ve never seen a TBSS outside of convection purely in association with a melting layer, so I’m assuming that melting layers alone are generally not sufficient to produce a TBSS, and that more reflective hydrometeors (graupel or hail) are required.

Dual Pol data (if it had been available) could have provided us with more insight. Correlation Coefficient (CC) on the 0.9 and 1.4 degree slices would have been able to tell us if we were looking at predominately one hydrometeor above the freezing level (i.e. dry snow) or more than one (i.e. dry snow and graupel). Differential reflectivity could have also helped, given that dry snow, graupel/hail and liquid hydrometeors have different ZDR characteristics. Looking at Doppler velocities from an MRR would have also been helpful.

We made numerous calls to Lee/Chatham counties and only rain was reported with the aformentioned convection. The fact that only rain was reported when the freezing level was ~3 KFT AGL suggests that large amounts of small (but highly reflective) ice hyrdrometeors were responsible for the TBSS’s. Of course, it’s entirely possible something other than rain made it to the ground and nobody was there to witness or report it. Does anyone else have any thoughts?


About bvincentnws

Meteorologist National Weather Service 2003 - Current
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