Improving Weather Radar Rainfall Estimates over the Mooi River Catchment, South Africa

Weather radars are very important instruments in understanding rainfall and rain rates. South Africa has a state-of-the-art network of weather radars across the country. The network was upgraded between 2009 and 2012 from C-band weather radars to S-band radars. The network includes nine single-polarised S-band radars, one dual-polarised S-band radar, two mobile dual-polarised X-band radars and 5 C-band radars. In a developing country like South Africa, the operation and maintenance of these radars is a challenge. With a very small infrastructural footprint these instruments can measure rainfall over very large areas up to a 300km radius from the radar. Due to the high variability of rainfall events in the Mooi River catchment new innovative methods need to be developed to accurately measure rain rates using weather radars. One of these parameters that can be changed to improve the accuracy and reliability of rain rates is the Z-R relation. This is the relationship between the Z, reflectivity measured by the radar, and the R, rain rate on the ground. Currently the Marshall and Palmer Z-R relation is used for all weather radars in South Africa. Due to the big differences in storm structure between convective and stratiform rainfall and also the high variability within storms using only one relation can create discreptencies in data. For this study the Irene weather radars was used. Firstly to develop unique Z-R relation we needed to understand the storm structure and variability of convective and stratiform storms in the catchment. Unique Z-R relation was developed for each storm type using the Parsivel disdrometer which measured the reflectivity and rain rate on the ground simultaneously. Secondly the performance of the radar also needed to be evaluated to determine whether bias measurements needed to be implemented. For this the Z measured on the ground by the Parsivel and the Z measured by the radar in the air was compared. Radar data improved significantly when the custom Z-R relations was used combined with the bias measurements calculated using the Parsivel data. Especially with convective events which is the dominant rainfall type in the catchment improvements was observed. It is recommended that a more convective Z-R relation must be used for rainfall estimation in the Mooi River catchment.