Virtual Near-Radar Doppler Velocity Signatures of Low-Level Supercell Horizontal and Vertical Rotations: Simulation Study

A large number of studies within the severe storms community have demonstrated that the upward tilting of intense low-level horizontal baroclinically-generated vorticity is a primary source of low-level vertical vorticity in supercell mesocyclones. To assess the potential utility of Doppler radar to exploring this process, we first develop equations for the three-dimensional velocity and vorticity components of radar targets defined in a right-handed radar coordinate system that accounts for earth curvature and beam refraction. Then, we generate radar pseudo-observations of a supercell simulated using the Advanced Research Weather Research and Forecasting (WRF-ARW) model with 111-m horizontal grid spacing and typical cloud model settings. A tornado develops ~110 min into the simulation and becomes very intense with surface winds briefly exceeding 110 m s^-1 (EF5). Finally, we evaluate estimates of simulated low-level supercell azimuthal component of vorticity from the virtual Doppler radar data. Two salient findings are as follows: First, amplification of the Doppler azimuthal vorticity (horizontal rotation along the azimuthal direction) signatures at low levels were detected prior to the detection of Doppler normal vorticity (rotation perpendicular to the surface of constant elevation angle) signatures. Second, the Doppler azimuthal vorticity intensified as air advected spirally toward a region where upward tilting of low-level horizontal vorticity contributed to tornadogenesis. We hypothesize that these signatures may help forecasters issue earlier warnings of tornadogenesis.