A review of ground-based, mobile, W-band Doppler-radar observations of tornadoes and dust devils

Abstract

A ground-based, mobile, W-band Doppler-radar has been used in the U.S. during the last decade to obtain high-spatial resolution maps of the radar reflectivity and wind fields in tornadoes and dust devils. This radar is one of the best tools available for studying the substructure of intense, small-scale vortices in the boundary layer. The most significant findings to date are summarized. In one case, it was found that just prior to tornadogenesis in a supercell, a 100–200 m scale cyclonic vortex formed at the leading edge of a bulge in the rear–flank gust front. This vortex appeared to interact with a larger-scale (500 m to 1 km wide) cyclonic vortex, just as the tornado formed. Other small-scale cyclonic vortices were present along the rear–flank gust, but they did not develop into tornadoes. The mature tornado-vortex was dominated by quasi-stationary wavenumber-two disturbances, while the mean vortex resembled a two-celled, Rankine combined vortex. The diameter of the mean vortex narrowed as it intensified and widened as it weakened, even though the tornado condensation funnel narrowed as the tornado was dissipating. Evidence was also found of short-term, inertial-like oscillations in vortex diameter and intensity. Spiral bands and eyes were ubiquitous. The eye in one well-documented case was broader in the lowest few hundred meters than it was aloft. Multiple vortices and “umbilical” cords of very narrow bands of reflectivity have also been found. Both cyclonic and anticyclonic dust devils have been documented. Some dust devils resemble a relatively narrow, Rankine combined vortex, while others are wider and have a broad, calm eye and a narrow annulus of intense vorticity just within the radius of maximum wind (RMW), and rising motion just inside the RMW and sinking motion well inside the RMW. Multiple-vortex structure, Rossby-like wave motion, and the Fujiwhara effect have also been documented.