The impact of observations on Mesoscale Model forecasts of three-hourly rainfall accumulations

The dim of data-impact studies at the UK Met. Office is to investigate how, observations affect the accuracy of model forecasts. Results from such exp eriments provide useful evidence on which to base the design of observation al networks. This project, using a case study approach, investigated the re lative benefit of different observation types within The Met. Office's Meso scale Model domain on forecasts of three-hourly precipitation accumulation over the UK tip to 12 hours ahead The method used assesses the impact of as similating single observation types, or a limited combination of types, whe re impact is measured against a control forecast obtained after a dummy ass imilation using no observations. In experiments for 13 case studies, the ob servation types that most frequently provided a beneficial impact when pres ented alone to the assimilation were sonde data, surface data and data from the Moisture Observation Processing System (MOPS).