High-Resolution Rainfall Measurements Using a Portable X-Band Radar system

Abstract

This thesis describes the operation and development of a portable, non-coherent, X-band weather radar operated by the Department of Physics, University of Auckland. This system has been deployed throughout New Zealand in a range of synoptic conditions. The suitability and limitations of this system in providing accurate, quantitative measurements of rainfall is assessed. The philosophy adhered to throughout this work is to operate the radar at a single elevation angle, which maximises the temporal resolution possible. This allows some further averaging to improve the accuracy of estimates and minimises the influence of storm development and motion between scans. The approach and methodology used to calibrate the system is described, along with methods which have been developed to monitor the quality of rainfall estimates and where necessary correct for known biases. These methods consist of a new means of limiting divergent attenuation estimates and novel applications of vertical profile and shielding corrections, which act to assess the suitability of a particular site. These methods are geared to short-range radar measurements and utilise the exceptional spatial (150m) and temporal resolution (5s) achieved. The procedures rue described and their ability to correct biased data is evaluated. It is not implied that X-band is a viable alternative for conventional radar operating at longer wavelengths, as the range of operation is of primary concern. However, when the range of operation is limited (<20km). accurate estimates of rainfall are shown to be possible at scales sensible for hydrological applications (500m and <5 min.). Beyond this, it is shown that estimated rainfields may be represented at a temporal resolution of l0 seconds with negligible influence from measurement errors. The factors biasing measurements may be identified and successively corrected for in all but the most severe cases. These findings suggest that low-cost X-band radar systems are a useful scientific and hydrological tool when operated at short-range in a temperate climate such as New Zealand's. The resolution achievable at close range allows for more accurate estimates of rainfall than may be achieved by the much slower sampling rates typical of weather radar even when operating at modest range (50km-l00km).