Combining C- and X-band Weather Radars for Improving Precipitation Estimates over Urban Areas

The topic of this thesis is weather radar precipitation measurements. Measuring the spatial and temporal variations of the precipitation by weather radars has significant advantages compared to point measurements from rain gauges within urban drainage applications. Knowledge on how the rainfall is distributed between the rain gauges is in itself valuable for retrospective analysis of urban drainage systems. In addition, on-line distributed weather radar precipitation measurements facilitate radar based now-casts of the precipitation. As the radar detects the precipitation before it reaches the urban area of interest, the radar measurements can be used in the process of predicting the near future loads and state of the urban drainage system. Such information is a necessity for real-time control decision support. The action possibilities within real-time control rely greatly on accurate prediction of future system state. Accurate and reliable weather radar measurements are, therefore, important for future developments and achievements within urban drainage.

This PhD study investigates two types of weather radars. Both systems are in operational use in Denmark today. A network of meteorological C-band weather radars provides a basic coverage of almost the entire country. In addition, the larger cities are also covered by small Local Area Weather Radars (LAWR). Whereas the large C-band network is operated and owned by the Danish Meteorological Institute (DMI), the smaller urban radars are operated individually and owned by local water utility companies.

Although the two radar systems use similar working principles, the systems have significant differences regarding technology, temporal resolution, spatial resolution, range and scanning strategy. The focus of the research was to combine the precipitation measurements from the two radar systems into a single radar data product, which utilises strengths and compensates for weaknesses of the individual radar system. The core of the research has thereby been to investigate the consequences of the differences of the two radar systems in order to understand the strengths and weaknesses for the best possible combination strategy.

The project has resulted in developments and methods, which improve the radar based precipitation measurements from both radar systems. Moreover, a better understanding has been gained on how the radar measurement represents the precipitation at the ground and how proper comparisons are conducted. It was discovered, that the LAWR (prior the project) was performing poorly and the low performance challenged any meaningful combination. This finding was unexpected, and great effort was made in order to improve the LAWR performance. The improvements have made LAWR measurements comparable with C-band measurements, thus meaningful combination is in principle possible today. The LAWR is, however, still the Achilles’ heel of the combination. A temporal interpolation method was developed for increasing the temporal resolution of the C-band measurements, with the result of significant improvements. It is concluded that temporal interpolated C-band measurements perform significantly better than the combination of the two radar systems.