The effect of precipitation on wind-profiler clear air returns

Abstract

A small number of studies have indicated that reductions in the signal strength of clear air returns can be observed at low altitudes in regions of precipitation. This study uses data from the NERC MST radar facility in Aberystwyth (52.4° N, 4.1° W) and co-located tipping bucket rain gauge data to determine whether this effect can be observed for all periods where high rainfall rates were observed at the ground. The period selected for examination includes all of the days where a peak rainfall rate of 6mm h-1 was exceeded in 2001. A statistical examination of VHF radar signal power during periods with and without surface rainfall suggests that the returned power is reduced by the presence of precipitating clouds. The corrected spectral width of the Doppler spectra is also significantly wider during periods of precipitation. The process which causes the decrease in the VHF signal power seems to be associated with a reduction in Fresnel reflection within precipitating clouds. This, in turn, may be due to a reduction of humidity gradients in clouds. UHF wind profiler data is also used to show that there is a relationship between enhanced UHF returns (signifying precipitation) and reduced VHF returns. To clarify the processes and effects observed we examine three case studies which show typical relationships between the VHF signal power and surface rainfall or enhanced UHF signal-to-noise ratios. The effect of precipitation on the signal processing scheme's derivation of signal power and spectral width is explored using individual Doppler spectra.