Evaluation of the Met Office super-resolution C-band radar rainfall product over the Greater London Area

Urban catchments are characterised by high spatial variability and fast runoff processes and are therefore very sensitive to the spatial and temporal variability of precipitation. Consequently, high resolution precipitation estimates are required in order to properly represent urban runoff processes. A high spatial resolution (100 m / 5 min) radar rainfall rate product has been developed and trialled over the Greater London Area in order to meet this requirement as part of the North-West Europe Interreg IVB funded project RainGain. Quantitative precipitation estimates (QPEs) were generated using data from two operational dual polarisation C band radars operating in “short-pulse” mode with a pulse length of 0.5 microseconds (or 75 m) and half power beam width of 1 degree. QPEs from each radar were interpolated onto a 100 m resolution grid every 5 minutes, and the use of advection and rain-gauge merging techniques to improve rainfall accumulation estimates was investigated.

The quality and benefit of these high resolution QPEs for urban hydrological applications is evaluated using as case study 6 storm events observed in a small urban catchment in North East London for which dense rain gauge and sewer flow records, as well as a recently-calibrated high-resolution urban drainage model were available. Results show that the high-resolution radar product performs well in terms of accuracy and captures the small-scale structure of rainfall fields. When comparing the use of higher spatial resolution radar QPEs and the 1 km / 5 minute operational Met Office radar QPEs as input for urban hydrological modelling, improvements are only tangible in the case of highly spatially variable storms. Further work is required to better understand the interaction and dependence between rainfall input and the resolution of hydrological models.