Using Mobile Doppler Radar Observations to Infer Buoyancy Deficits within Thunderstorm Outflow

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 radar observations of 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 radar observations of the slope and propagation speeds 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 Doppler 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 with both free-slip and semi-slip boundary conditions to quantify the similarities between observational and theoretical outflow structure and speed.

Two-dimensional free-slip model results reveal an indirect relationship between the slope of a cold pool and its potential temperature deficit in the presence in ambient positive shear. Semi-slip model results reveal a similar relationship, although slope dependence on internal potential temperature deficit lessens in the strongest shear. Horizontal wind output from the semi-slip model is used to create simulated radial velocity plots similar to those seen using a mobile Doppler radar. We will show how features in simulated cold pool structure are visible in observed structure using mobile Ka-band Doppler radars. If applicable, radar RHI observations from the 2017 RiVorS project will be presented to supplement the conclusions.