Extended Abstract (200K)16.1
Investigating derecho and supercell proximity soundings
Jeffry S. Evans, NOAA/NWS/NCEP/SPC, Norman, OK; and C. A. Doswell
Proximity soundings are obtained within 2 hours and 167 km (100 mi) of 67 derechos and 98 discrete supercells, yielding around 100 soundings for each. Statistical analyses are computed for temperature and dew point at 25 mb intervals, and for wind component data at 50 m intervals. Following the procedure of Evans and Doswell (2001), the derecho data set is divided into events associated with "strong" forcing (SF), "weak" forcing (WF), or "hybrid" forcing. The supercell dataset is stratified into non-tornadic, weak tornadic (F0-F1), and significant tornadic (F2-F5) events.
Our results indicate WF derechos exist in a warmer environment and a moister boundary layer on average, as compared to the SF and Hybrid derecho cases. The WF events also occur within a fairly narrow range of vertical temperature and moisture profiles, while SF events exhibit a large range. It is readily apparent that SF events occur in stronger flow and vertical shear regimes than the hybrid and WF cases, with a much longer hodograph plot. However, all three events are associated with strong low level storm relative winds (SRW) (over 23 m s-1 from 0-2 km AGL) and weak mid and upper tropospheric SRW, with average inflow occurring through the lowest 5 km for each derecho.
Supercell thermodynamic and kinematic environments are surprisingly similar to those supporting derechos, especially the SF events, though our results indicate some discrepancies between the three supercell categories. Tornadic and non-tornadic supercells are best distinguished by EHI (mean value of 4 versus 1.5) and 0-3 km helicity (average of 255 m2 s-2 versus 133 m2 s-2), however neither parameter is statistically significant in discerning tornado intensity (weak versus strong and violent). Shear in the lowest 1 km AGL and BRN-shear appear most useful in assessing tornado intensity, however neither distinguish between non-tornadic and weakly tornadic supercells. Most importantly, SF derechos occur within identical 0-1 km shear (middle 50 percent values ranging between 9.5 m s-1 and 16.5 m s-1) and within similar 0-3 km helicity (median value near 250 m2 s-2) as discrete supercells producing significant tornadoes. Only EHI and upper level (6-10 km AGL) SRW appear to be statistically significant in distinguishing between the two convective modes. The strong similarities in the proximity soundings and shear parameters between SF derechos and discrete tornadic supercells highlight the difficulties in correctly anticipating convective mode on a day when severe thunderstorms are expected. Forecasters must be aware of other influences, such as the initiating mechanism, in forecasting whether thunderstorms will remain discrete or evolve into linear convection.
Session 16, Tornado and Severe Storms Environments
Friday, 16 August 2002, 8:00 AM-9:59 AM
Next paper