Dual-polarisation X-band radar estimates of precipitation assessed using a distributed hydrological model for mountainous catchments in Scotland

Observing, in a quantitative and robust way, the dynamic space-time pattern of precipitation in mountainous terrain presents a major challenge of great practical importance. The difficulties of this task are further exacerbated in mid to high latitudes where the typical melting layer for precipitation (i.e. the 0°C isotherm) is often close to the surface during winter months. One way to address this challenge is by improving observations made using networks of weather radars. Quantitative Precipitation Estimates (QPEs) derived from these instruments have many applications, for example as input to a hydrological model to simulate river flow for flood forecasting purposes. 

Here, a set of QPEs - obtained from an observation campaign using the National Centre for Atmospheric Science’s mobile X-band dual-polarisation Doppler weather radar (NXPol) in a mountainous area of Northern Scotland - are assessed with reference to observed river flows. Each form of QPE is used as an input to Grid-to-Grid (G2G), a distributed hydrological model used for flood forecasting across Great Britain, and the simulated river flows compared to observations. The location of the radar was specially chosen to infill an area of reduced coverage in the existing C-band radar network for the British Isles.

Assessments of radar QPE often only examine a final precipitation “best estimate” product and typically with reference to raingauges at specific locations. Here, we exploit the processing capabilities of NXPol and the hydrological modelling framework to investigate the benefits of ten separate processing methods that increase with complexity and make differing use of dual-polarisation variables. The role of the radar beam elevation and distance from the radar is investigated, and NXPol QPEs are compared to that provided by the radar network. Additionally, a preliminary investigation is carried out into the role of the drop-size distribution on the relationship between radar-reflectivity and rain-rate using disdrometer data.

The hydrological assessment reported on here has the benefit of integrating the precipitation over space and time which serves to complement and extend a previous meteorological assessment using raingauge data alone. The assessment proves to be insensitive to issues affecting both raingauges (e.g. representativity, wind-induced under-catch) and local artefacts in the space-time radar-rainfall field. It facilitates a direct assessment of whether potential benefits in the new QPEs are carried forward to an end-use such as flood forecasting, providing fresh insights for the development of new dual-polarisation radar QPE methods.