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.
Highlights
Wind profiler radars operating at very high and ultra high frequencies (VHF and UHF, respectively) are sensitive to both clear air returns, from radio refractive index irregularities, and Rayleigh scattering, from hydrometeors
Separation of precipitation periods into stratiform and convective regions is performed to examine the affect of these precipitation types on the VHF signal power return
Statistical analysis suggests that both precipitation types produce similar magnitude reductions in VHF signal power on average
Summary
Wind profiler radars operating at very high and ultra high frequencies (VHF and UHF, respectively) are sensitive to both clear air returns, from radio refractive index irregularities, and Rayleigh scattering, from hydrometeors. Whereas light rain or drizzle is all that is necessary for the return from hydrometeors to be larger than that from the clearair for UHF wind-profilers While this is true in general, several studies (Chu and Lin, 1994; Cohn et al, 1995; Vaughan and Worthington, 2000; McDonald et al, 2004) have suggested that processes associated with precipitation can directly impact the magnitude of the clear-air signal. The relationship between temperature and humidity fluctuations associated with this process would act to reduce the magnitude of refractive index irregularities, to which the radar reflectivity is directly related This possibility is supported by Rao et al (1999) which indicated that the weakening of the clear air echo observed at the bright band in stratiform precipitation might be due to the turbulent mixing between warm and humid in-cloud air and colder and drier surrounding air. Work by McDonald et al (2004)
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