Observations in supercells with a mobile, 3-mm-wavelength Doppler radar

A group from the University of Oklahoma has been using a truck-mounted, mobile, 3-mm wavelength Doppler radar designed and constructed by a group at the Microwave Remote Sensing Laboratory at the University of Massachusetts at Amherst. This radar, which has a beamwidth of 0.18 deg and a range of about 10-15 km (2.5-3 km in clear air), has been used to probe tornadoes, dust devils, and other substorm-scale phenomena in supercells with spatial resolution as high as 5-10 m or less in the azimuthal direction. In the polarization diversity pulse-pair (PDPP) mode, maximum unambiguous velocities of 79 m s-1 are achieved; in the normal pulse-pair mode, maximum unambiguous velocities of only 6 m s-1 are available, but are good enough for low-velocity phenomena. In previous talks we have highlighted some of our observations of tornadoes. The focus of this presentation, however, is a discussion of several datasets collected in 1999 and 2000 (datasets collected in 2001 will be discussed if they are significant and if we can process them in time for the conference): We will correlate visual features with the radar data. We will also correlate features seen on WSR-88D radars with what we found with the mobile Doppler radar. The cases we have selected to show are (1) a supercell in the northern Texas Panhandle on 24 May 2000 and (2) a developing supercell/updraft base in northwest Texas on 25 May 1999. The 24 May 2000 supercell had a spectacular bell-shaped appearance with a flared-out base. Although it produced large hail, no observers saw any tornado. However, a well defined hook echo and coincident cyclone vortex signature of 40 m s-1 azimuthal shear over about 100 m were noted just above the ground. Maximum ground-relative Doppler velocities of only 25 m s-1 were detected. Apparently the cyclonic vortex was just under tornadic intensity. The developing supercell/updraft base on 25 May 1999, on the other hand, did not exhibit any strong rotation at low levels. However, clockwise and counterclockwise hook echoes and parallel bands of weak reflectivity were found. There was evidence of a weak cyclone (in both the reflectivity and Doppler velocity data) near the ground at the rear of the storm. In a sequence of photographs of the updraft base it appears as if dry air was intruding from the rear, as the back edge of the base was eroding. A slight enhancement in rear-front flow is evident in the Doppler-velocity field near the center of the base. It is hypothesized that a weak cyclonic-anticyclonic couplet of vorticity and rear-inflow jet was produced through tilting along the edges of the updraft base.