VHF signal power suppression in stratiform and convective precipitation

Abstract

Abstract. Previous studies have indicated that VHF clear-air radar return strengths are reduced during periods of precipitation. This study aims to examine whether the type of precipitation, stratiform and convective precipitation types are identified, has any impact on the relationships previously observed and to examine the possible mechanisms which produce this phenomenon. This study uses a combination of UHF and VHF wind-profiler data to define periods associated with stratiform and convective precipitation. This identification is achieved using an algorithm which examines the range squared corrected signal to noise ratio of the UHF returns for a bright band signature for stratiform precipitation. Regions associated with convective rainfall have been defined by identifying regions of enhanced range corrected signal to noise ratio that do not display a bright band structure and that are relatively uniform until a region above the melting layer.This study uses a total of 68 days, which incorporated significant periods of surface rainfall, between 31 August 2000 and 28 February 2002 inclusive from Aberystwyth (52.4 • N, 4.1 • W). Examination suggests that both precipitation types produce similar magnitude reductions in VHF signal power on average. However, the frequency of occurrence of statistically significant reductions in VHF signal power are very different. In the altitude range 2-4 km stratiform precipitation is related to VHF signal suppression approximately 50% of the time while in convective precipitation suppression is observed only 27% of the time. This statistical result suggests that evaporation, which occurs more often in stratiform precipitation, is important in reducing the small-scale irregularities in humidity and thereby the radio refractive index. A detailed case study presented also suggests that evaporation reducing small-scale irregularities in humidity may contribute to the observed VHF signal suppression.

Multiple bands near fronts in VHF wind‐profiling radar and radiosonde data

“…In the lower troposphere, however, specific humidity gradients dominate the power (May et al ., 1991;Vaughan and Worthington, 2000). A departure from the general dependence of power on log(M 2 /z 2 ) occurs in precipitation, especially stratiform precipitation which causes a greater suppression of the signal than convective precipitation (Vaughan and Worthington, 2000;McDonald et al ., 2006).…”

Evaluating the Effects of Height-Variable Reflectivity and Antenna Sidelobes on the Radar Equation

“…Some of these studies claim for a re-duction of the radio signal in rainy conditions (e.g., McDonald et al 2004McDonald et al , 2006 while others claim for an enhancement (e.g., Cohn et al 1995). Unfortunately, those analyses had not considered possible effects from height-variable reflectivity and antenna sidelobes.…”