Diagnosis of the Windsor Tornado Event from Doppler Radar Data Assimilation and Ground-based Profiling

It is generally acknowledged that temporal changes in atmospheric moist static energy and vertical wind shear associated with the development of severe thunderstorms can display a variety of time scales prior to convective initiation. The present study demonstrates that extremely rapid changes in shear, potential instability, Convective Available Potential Energy (CAPE), and storm-relative helicity occurred in the local vicinity of a rare intense tornado in Windsor, Colorado just 2 h prior to the explosive development of the tornadic supercell. These rapid changes, which were not induced by convection, were superimposed on a general trend toward destabilization with a time scale of 6–12 hours.

These conclusions are drawn from the synthesis of 3–6-min resolution ground-based microwave radiometric and Doppler wind profiler data, in combination with a 1-km nested grid Numerical Weather Prediction (NWP) model initialized with WSR-88D radar rdata by way of a multi-scale variational assimilation method. Superior radiometric retrievals are achieved by coupling the radiometric data with background fields from the NWP model using a 1-D variational technique, as compared to the traditional pure statistical inversion method. The model successfully portrayed the rapid development of a supercell matching the general characteristics of the Windsor storm.

It is shown that the extremely rapid nature of the changes in the local storm environment were associated with development of a narrow low-level “jetlet” that advected moist, unstable air northwestward toward Windsor just ahead of a pronounced dryline. These details and the success in capturing the supercell were only evident in model analyses that utilized the WSR-88D reflectivity and radial velocity data in the initialization procedure.