Digital signal processing of data from conventional weather radar : the displace method

Abstract

The ability of radar to probe large volumes of the atmosphere at high resolution, providing real-time data at a radar site, has advantages in studies relating to hydrology and meteorology. The information obtained from radar is comparable to that from a X-ray image as the radar beam penetrates clouds, seeing the precipitation that reveals their inner structures. The advantages of using radar to measure rainfall over large areas has been demonstrated during recent devastating floods in South Africa. During the last decade, the South African Weather Bureau (SA WB) has made a large investment in a network of weather radars. Radar also plays an important role in the National Precipitation Research Programme's (NPRP) rainfall stimulation research funded by the Water Research Commission (WRC) and the SAWB. The emphasis of this research is moving away from the relative radar measurements between seeded and non-seeded storms of randomized seeding experiments. The focus is now on absolute radar measurements needed in area experiments. The continuous upgrading of the equipment, improvements in methods used to process the data and calibrate the equipment are crucial to optimise the benefits of this investment. The latest addition to the weather radar infrastructure in South Africa has been the acquisitioning of an MRL-5 dual-wavelength radar by the WRC in 1994. This thesis describes the development, testing and implementation of a new, improved method to process the output from a weather radar's logarithmic receiver. The processing method, called DISPLACE, has proven to have many applications, and is computationally efficient and accurate. Its applications include the processing of digitized logarithmic receiver output in order to simulate different receiver transfer functions, the processing of multi-parameter radar measurements and the filtering of ground clutter. It facilitates the computation of CAPPI' s (Constant Altitude Plan Position Images) and radar-rainfall accumulation. The thesis also deals with the upgrading of South African weather radars since about 1990 through the in-house developed radar data acquisition system and the procedures established to ensure accurate calibrations. In addition, the hydrometeorological infrastructure deployed in the Bethlehem research area is used in an integrated manner to verify data obtained using the new method. This work is well timed to address the needs that are now emerging in South Africa and clearly illustrate the role the NPRP (National Precipitation Research Programme) played in reviving radar meteorology. The DISPLACE method is proving that the potential of conventional weather radar has not been fully exploited. It has also stimulated the interest of young technicians and scientists in the field of radar meteorology. This augurs well for the future use of weather radar in South Africa, both in the field of rainfall stimulation and as an integral part of systems designed to forecast and to help manage the effects of severe weather conditions.