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

while for a beam of finite width, the parameter θ s is found according to the expressionθ s = arcsin{√ [sin 2 θ t /{ln(P(0)/ln(P(θ t )} – sin 2 θ 0 ]}where θ t is the tilt of the off-vertical beam, θ 0 is the 1/e half-width of the beam, P(0) andP(θ t ) are the powers received on the vertical and off-vertical beams respectively (e.g.Hocking, 1988; Hooper and Thomas, 1995). The parameter θ s is small when the scatterersare highly stretched horizontally relative to their vertical extent, and θ s is large when thescatterers are close to isotropic. Hocking and Hamza (1997) have described how θ s relates tothe length-to-depth ratio of the scatterers in greater detail.The parameter θ s is a standard output from the McGill VHF windprofiler radar. Thisinstrument has been described by Campos and Hocking (this issue). A typical height-timediagram of this parameter is shown in fig. 1. Notice in particular that there are frequentoccurrences where θ s exceeds 20 o . The transition is often quite sharp, changing rapidlybetween values of the order of 9 o or less and values of over 20 o within less than one hour.Fig. 1. θ s parameter plotted vs height and time from 1200 on 15 September to 1200 on17 September, 2003.An S-band Doppler radar operates within close proximity to the windprofiler radar. It scansout to distances of typically 240 km, and is very sensitive to the occurrence of precipitation.Early observations suggested that when the radar detected precipitation, the VHF radarshowed its largest values of θ s , indicating greater isotropy. We have therefore carried out adetailed comparison between the θ s parameter measured with the VHF radar and theoccurrence of precipitation determined with the S-band radar. An example of a typicalprecipitation map seen with the S-band radar is shown in fig.2, and this map coincides withthe occurrence of strong isotropy seen in fig. 1.In order to compare the two radar sets, we decided to develop simple parameters to representeach data type. For the VHF radar, we examined the aspect sensitivity parameter within anygiven hour, and found the percentage of the height range between 1 km and 10 km(approximately the tropopause) in which θ s exceed 20 o . We assigned a value of 1 if thisnumber was less than 25%, 2 if it lay between 25% and 50%, 3 if it lay between 50% and75%, and 4 if it exceeded 75%. This parameter will be referred to as the “isotropy index”.We then performed a 5-point running mean of this parameter, in order to introduce a smalllevel of smoothing. At the same time, we examined the hourly precipitation maps andsearched for signs of precipitation within 200 km of the S-band radar. If precipitation existedin at least 10% of the viewing area, a number 1 was assigned to that hour. If no precipitation445