Reflectivity measurement of rain using a 94 GHz radar

As a result of Mie-scattering and very high levels of attenuation, 94 GHz would not typically be considered a suitable frequency for making Z-R measurements of rain. However, this kind of measurement could prove to be an extremely useful way of monitoring rainfall around the Soufriere Hills volcano on the island of Montserrat.

It is thought possible that at least one of the recent eruptions there may have been caused by heavy rainfall destabilising the dome, but the rainfall data currently available on the island is limited to that from a small number of tipping-bucket gauges and some radar coverage from Guadeloupe. The St Andrews Millimetre-wave Group (in conjunction with the University of Reading, Lancaster University and Montserrat Volcano Observatory) is in the process of deploying an autonomous 94 GHz FMCW radar to measure the changing topography of the volcano. It is intended also to task this system with the measurement of localised rainfall around the volcano. This could potentially also be useful in assessing the likelihood of lahars (flash floods) in the Belham Valley.

Z-R relationships appropriate to the frequency being considered are determined from assumptions about the DSD and are used to determine the rainfall rate. The attenuation along the path to each sample volume is determined from the reflectivities of the sample volumes along that path, and then corrected for prior to applying the relevant Z-R relationship to that sample volume.

This methodology is tested on data collected in the UK, and some initial field results from Montserrat are also presented.

While Z-R type rain measurement at millimeter-wave frequencies are unlikely to ever find broad use in the meteorological community, they do offer some advantages over their lower frequency counterparts. Specifically, they can offer spatial resolutions far higher than those achieved with more conventional radars. The radars can also be very compact, sufficiently compact to be man-portable. This small size also makes comparatively straightforward very high mechanical scan rates which, when combined with the high spatial resolution available can provide incredibly detailed rainfall maps of small areas.