Performance of 4D‐Var NWP‐based nowcasting of precipitation at the Met Office for summer 2012

The Met Office developed and demonstrated an hourly cycling 1.5 km resolution NWP-based nowcast system (0–6 hour forecasts) using 4-dimensional variational data assimilation (4D-Var). This was known as the Nowcasting Demonstration Project (NDP), and was principally for prediction of convective storms for flood forecasting. The NDP was run in real-time from March 2012 to April 2013 to cover the period of the London Olympics 2012. The system was run on a domain covering southern England and Wales nested in the UK variable resolution model (UKV). The UKV used a UK-wide 1.5 km domain with 3-hourly cycling 3-dimensional variational data assimilation (3D-Var) and produced 36 hour forecasts every 6 hours. The NDP 4D-Var included standard observations, Doppler radar radial winds, humidity derived from a 3D cloud cover analysis and geostationary satellite upper-tropospheric water vapour radiances not contaminated by cloud. This was used in combination with latent heat nudging of radar-derived precipitation rates. Example case studies compare the NDP precipitation forecasts to both the operational extrapolation/merged nowcast system and the UKV forecasts. Objective comparison of fraction skill score for the period June to August 2012 shows that the NDP skill was greater than the latest UKV forecasts, available to forecasters at the same time as the NDP, for the whole 6 hour forecast period. The skill of the NDP was greater than the operational extrapolation/merged nowcast beyond T+2 hours.