Using mobile doppler radar observations of gust fronts to infer buoyancy deficits
Hutson, Abby L.
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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 propagation speed and 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 and multicell outflow. 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 with both free-slip and semi-slip lower boundary contitions to quantify the similarities between observational and theoretical outflow structure and speed. Simulated cold pools achieve the propagation speeds predicted by cold pool theory, after scaled by a factor of two. Cold pool theory also accurately predicted the observed propagation speed for 4 of the 5 cases. Given the environmental vertical shear is known, the theoretical cold pool speed equation could be used to infer the buoyancy deficit of observed outflow. Two-dimensional free-slip model results also reveal an indirect relationship between the slope of a cold pool and its potential temperature deficit in the presence of ambient positive shear. In the same environment, the edge of a strong cold pool is less inclined than that of a weaker cold pool. However, outflow in weak ambient shear has a steeper slope than the same outflow in stronger ambient shear. The semi-slip model results reveal a similar relationship, although slope dependence on internal potential temperature deficit lessens in the strongest shear. An ordinary least squares regression is performed on the free-slip model data to quantify cold pool slope given ambient shear, shear depth, and potential temperature deficit. The regression equation is applied each of the observed cases and it is revealed that shear over the lowest half of the cold pool better predicts the slope of cold pool than shear over the full depth of the cold pool. More cases are needed to best understand how cold pools behave in our atmosphere given varying buoyancy deficits.