Considerations for Pulse-Doppler Radar Observations of Severe Thunderstorms

Abstract

Data show that Doppler shift of storm echoes and the range extent of severe storm systems encompass a span exceeding the unambiguous resolving capability of centimetric radars. Echo coherency which is related to Doppler spectrum width and radar pulse repetition frequency (PRF) places limits on pulse-Doppler radar's unambiguous range and velocity measurements. Statistics on severe storm Doppler velocities and spectrum widths are given to evaluate limits for radars operating at different wavelengths. The maximum range to which coherent Doppler measurements can be obtained is related to shear, turbulence and radar parameters. Data suggest that either eddy dissipation is high (? > 1 m2 s?3) in large (20%) portions of severe storms or that many of the measured Doppler spectrum widths arise from vortices of scales small compared to the pulse volume but outside the equilibrium range of turbulence scales. Rangescrambled echoes are most annoying because they obscure observation, whereas velocity ambiguities can usually be resolved. The extent to which overlaid echoes obscure observation of mesocyclones is estimated for scattered and squall line thunderstorms. Staggered PRF techniques are shown to increase the unambiguous range and velocity limits of pulse-Doppler radars. This study concludes that, aside from attenuation considerations, the biggest obstacle to shorter wavelength Doppler radar observation of severe storms is the larger occurrence of overlaid echoes resulting from the smaller unambiguous range necessary to meet echo coherency requirements.