Radar Applications in Northern Scotland: RAINS

The Scottish Environment Protection Agency (SEPA) has a statutory responsibility to provide flood warning across Scotland. SEPA achieves this through an operational partnership with the Met Office by applying meteorological forecasts to a national distributed hydrological model, Grid- to- Grid (G2G), and catchment specific lumped probability distributed models (PDM). Both of these model types rely on observed precipitation input for model development and calibration, and operationally, for historical runs to generate initial conditions.

Scotland has an average annual rainfall of 1430mm per annum (1971-2000), but the spatial variability in rainfall totals is high, predominantly in relation to the topography which poses different challenges to both radar and point measurement methods of observing of rainfall. Also, the high elevations mean that in winter a significant proportion of precipitation falls as snow. For the operational forecasting models, observed precipitation data is provided in near real-time (NRT) from SEPA’s network of ~260 telemetered TBR gauges and 4 Met Office C-band radars. Both data sources have their strengths and weaknesses, particularly concerning the orography and spatial representativeness, but estimates of rainfall from the two methods can vary widely.

As part of SEPA’s Flood Warning Strategy (2012-2016), SEPA committed to increasing their understanding of the spatial representation and real-time measurement of rainfall. Activities associated with this include reviewing the UK's weather radar network and its suitability for flood warning provision and making recommendations for future improvements (including new and temporary installations to help increase the probability of detection). To meet this strategic objective, SEPA partnered with the NCAS and the University of Leeds' School of Earth and Environment to conduct a pilot study entitled “Radar Applications in Northern Scotland” (RAINS).

Northern Scotland, particularly near Inverness, is a comparatively sparse part of the radar network. In addition, the Northern Western Highlands and Cairngorms mountain ranges significantly impact precipitation totals and distribution in this area are determined by, which also have a negative impact on the radar observations. In recognition of this issue, the NCAS mobile X-band dual-polarisation Doppler weather radar (NXPol) was deployed in this area between February and August 2016 to collected high-resolution data to help answer:

1) What are the benefits of high temporal and spatial resolution radar precipitation estimates for flood forecasting in Northern Scotland?

2) How do dual-polarisation radar observations improve rainfall estimates used in flood forecasting in Northern Scotland?

Here we make a comparison of rainfall estimates for the Inverness and Moray Firth region generated from the operational radar network, the TBR network, and the NXPol. Quantitative precipitation estimates (QPEs) from both sources of radar data are compared to point estimates of precipitation with a particular emphasis on the advantages of the benefits of various polarimetric precipitation relations. The comparison will include the discussion of improvements provided from QPE based on a novel hydrometeor classification algorithm applied to the NXPol observations.

To show the impact of this, the QPEs are applied to operational PDM models to compare the effect on the simulated runoff on communities in Northern Scotland. The results highlight the hydrological significance of uncertainty in observed precipitation. From these results, recommendations for future investigations are made to improve the estimate of radar QPEs through improvement of the correction for orography and correction for different precipitation types.