High resolution 95 GHz radar reflectivity and Doppler images of tornadoes

During the spring of 1999, engineers from the University of Massachusetts (UMass) Microwave Remote Sensing Laboratory (MIRSL) and meteorologists from the University of Oklahoma (OU) School of Meteorology collected the highest resolution radar reflectivity and Doppler velocity images of tornadoes to date. Image sequences were captured from three tornadoes (including the May 3rd, 1999 Oklahoma City tornado) using the UMass truck mounted 95 GHz (3-mm wavelength) polarimetric Doppler radar system. The radar resolved spatial variations as fine as 12 by 12 by 15 m from a range of 4 km with a 4 ft diameter Cassegrain antenna and by transmitting 100 ns long pulses. Using a Polarization Diversity Pulse Pair (PDPP) sequence, the radar unambiguously measured the Doppler velocity field in tornado eye-walls. For the first time, the PDPP technique measured radial wind speeds in high wind and high shear conditions where conventional pulse pair techniques broke down. This paper documents their measurement and signal processing techniques and presents images revealing previously unseen high-resolution structures and dynamics inside tornadoes.