Initial trials have used a data assimilation system run with hourly cycling either in 3D-Var or 4D-Var mode and additional techniques of latent heat nudging of radar-derived precipitation rates and humidity nudging based on 3D cloud cover analysis. For the trials the model domain covers southern England and Wales with boundary conditions provided by a 4km resolution forecast with a domain covering the whole of the UK.
Our work is focussed on the assimilation of high resolution, novel observation types to enhance this nowcasting system. These observation types include Doppler radar winds, radar reflectivity, radar-derived rain rates and in the future may include radar refractivity. Assimilation of Doppler radar radial winds have been trialled in the system using 3D-Var and have shown a positive impact on forecasts of precipitation out to 5 hours.
The NWP-based nowcasting system is currently being upgraded to use boundary conditions from the 6 hourly 1.5 km variable resolution UK model (Lean et. al., 2011) forecast which uses 3 hourly 3D-Var. Trials will be conducted to assess the benefits from assimilating Doppler radar winds in the 4D-Var system and to see whether the latent heat and humidity nudging can be replaced by direct use of radar-derived surface rain rates and 3D cloud cover analysis in 4D-Var.
This talk will provide a summary of the latest developments in the UK Met Office's NWP-based nowcasting system.
Reference
Lean, H., F. J. Bornemann, Y. Tang, M. Mittermaier, N. Roberts, and M. Trueman., Experiences with a 1.5km version of the Met Office Unified Model for short range forecasting, Extended Abstract 4B.3 from AMS 24th Conference on Weather and Forecasting/20th Conference on Numerical Weather Prediction, Jan. 2011.
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