Operational experience with assimilation of rainfall datain the Met Office Mesoscale model

Summary

The Met Office introduced the assimilation of radar-derived rainfall data into its operational mesoscale model in April 1996. Here we report on subsequent experience, involving impact studies to measure the benefit of rainfall assimilation and sensitivity of forecasts to the temporal frequency of the rainfall data. It is found that on a monthly timescale, impact from the rainfall assimilation can be detected at a forecast range of 12 hours in some months, and an exceptional impact was found after 15 hours in one case. Increasing the frequency of rainfall data from 3-hourly to hourly improves the first 6 hours of the forecast. There are plans to extend the coverage of radar data available to the model, and to take better account of the error characteristics of the surface precipitation rate estimates.