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Ann. Geophys., 24, 23-35, 2006
www.ann-geophys.net/24/23/2006/
© European Geosciences Union 2006
VHF signal power suppression in stratiform and convective precipitation
A. J. McDonald1, K. P. Monahan1, D. A. Hooper2, and C. Gaffard3
1Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand
2Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK
3Met Office, University of Reading, Meteorology Building, PO Box 243, Earley Gate, Reading, RG6 6BB, UK
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.
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