Using Mobile Doppler Radar Observations of Rear Flank Gust Fronts to Infer Outflow Buoyancy Deficits

Identifying the difference between tornadic and non-tornadic supercells remains an enigmatic challenge to operational forecasters and research meteorologists alike. Although an infallible answer to this question has yet to be discovered, many studies reveal a strong link between the thermodynamic characteristics of the rear flank downdraft (RFD) and tornado formation.  It has been shown that non-tornadic and weakly tornadic supercells are associated with RFDs containing large deficits in equivalent and density potential temperature, while strongly tornadic supercells produce RFDs with much weaker deficits of these quantities (and even surpluses).

This study proposes that the thermodynamics of an RFD can be directly inferred from the vertical structure of the rear flank gust front (RFGF, e.g., as observed with a mobile Doppler radar). Modeling studies have shown that the gust front leading thunderstorm outflow varies in speed and structure due to both the strength of the cold pool as well as the ambient environmental shear. However, there are no studies to the authors’ knowledge that corroborate these results using direct observations of the vertical structure of RFGFs.

In several ad-hoc field campaigns focused on the Southern Great Plains, the Texas Tech University Atmospheric Science Group used Ka-band mobile radars to document the vertical structure of a number of severe thunderstorm outflow events, both from supercells and upscale modes. In each of the cases presented, the outflow was sampled by in situ (e.g., StickNet, Oklahoma/West Texas Mesonet) instrumentation that recorded the thermodynamic state of both the inflow and outflow air.  Environmental wind shear during each event is identified using velocity azimuth displays from the NEXRAD WSR-88D in closest proximity to the mobile radar deployment site. Both the thermodynamic characteristics and the shear values from each case are then used to initialize two-dimensional CM1 cold pool simulations to quantify the similarities between observational and theoretical outflow structure and speed.

Initial results reveal an indirect relationship between the slope of a cold pool and its potential temperature deficit in the presence of ambient shear. For fixed environmental shear, the edge of a strong cold pool is less inclined than that of a weaker cold pool. However, for fixed cold-pool temperature, outflow in weak ambient shear has a steeper slope than the same outflow in stronger ambient shear. Ongoing work continues to explore potential temperature deficits in the presence of non-linear shear profiles, as well as investigate how the amount of turbulence produced along the upper interface of a cold pool varies with the temperature deficit. The results from this work will then be extended to investigate the association of RFGF properties with the occurrence and strength of tornadoes in the parent storm.