Proceedings with Extended Abstracts (single PDF file) - Radio ...

THREE-METRE-SCALE TURBULENCE ANISOTROPY AS APRECURSOR TO RAINAnna Hocking 1 and W.K. Hocking 21. Mardoc Inc., London, Ontario, Canada.2. Dept. of Physics and Astronomy, University of Western Ontario, London, Ontario. Canada.Abstract.It is shown that the anisotropy parameter, θ s , which is often measured with VHF radars, canserve as a forecast diagnostic for the occurrence of local liquid precipitation. We demonstratethis effect using VHF and S-band radars at McGill University, near Montreal, QC, Canada,for the months of October and November, 2002. The study is preliminary, and is to beextended to include other months of the year. Current observations suggest that thestatement is true for non-winter months, but is not valid once the precipitation becomespredominantly snow and ice. It is not clear whether this phenomenon is specific to theMcGill site, or can be applied to other locations as well.Introduction.It has long been recognized that atmospheric scatter detected by ~50 MHz VHF windprofilerradars is anisotropic in nature, with scatter from overhead generally being stronger thanscatter observed when off-vertical beams are used. The phenomenon is especially dominantin the stratosphere (e.g. Liu and Roettger, 1978; Gage and Green, 1978, Tsuda et al., 1986;Hocking et al., 1990). Discussions about this phenomenon often revolve aroundconsideration of specular reflectors, but anisotropic turbulence can also contribute to thedifferences in relative powers. Hocking and Hamza (1997) have discussed the differencesbetween specular reflections and anisotropic turbulence, and described some methods whichcan help distinguish the phenomena.Tropospheric observations of the aspect sensitivity have also been made. The troposphere isgenerally less stable than the stratosphere on average, and it is not unreasonable to supposethat tropospheric aspect sensitivity might be considered to be due mainly to anisotropicturbulence. However, most observations of anisotropy are only made over relatively shorttime frames, since the studies have often been considered to be primarily for curiosity-basedresearch.In this paper, we present data from a VHF ST radar system which routinely measures theaspect-sensitivity on a continuous basis. The radar works at a frequency of 52 MHz, andreceives its greatest scatter from Fourier scales of about 3 metres. This corresponds toturbulent eddies with typical depths of the order of 2 to 3 metres (see Hocking (1987), andreferences therein). The radar is located close to a large S-band radar which is sensitive toprecipitation, and the comparisons between the two radars form the basis of this paper.Method.The aspect sensitivity of radar scatter is most simply measured by comparing the signalstrengths determined on an off-vertical bean and a vertical beam. For a very narrow beam,the relative powers are of the formP ratio = exp{ -sin 2 θ/sin 2 (θ s )}444