"1D+4D-Var" assimilation of NCEP Stage IV radar and gauge hourly precipitation data.

Title"1D+4D-Var" assimilation of NCEP Stage IV radar and gauge hourly precipitation data.
Publication TypeMiscellaneous
Year of Publication2006
AuthorsLopez, P, Bauer, P
Secondary TitleTechnical Memorandum

The "1D+4D-Var" assimilation method currently run in operations at ECMWF with rain-affected radiances from the Special Sensor Microwave/Imager is used to study the potential impact of assimilating NCEP Stage IV analyses of hourly accumulated surface precipitation over the United States (US) main land. These data are a combination of rain gauge measurements and observations from the high-resolution Doppler NEXtgeneration RADars (NEXRAD). Several "1D+4D-Var" experiments have been run over a month in spring 2005. First, the quality of the precipitation forecasts in the control experiment is assessed. Then, it is shown that the impact of the assimilation of the additional rain observations on global scores of dynamical fields and temperature is rather neutral, while precipitation scores are improved for forecast ranges up to 12 hours. Additional "1D+4D-Var" experiments in which all moisture-affected observations are removed over the US, demonstrate that the NEXRAD data on their own can clearly be beneficial to the analyses and subsequent forecasts of the moisture field. This result suggests that the potential impact of precipitation observations is overshadowed by the influence of other high-quality humidity observations, in particular radiosondes. It also confirms that the assimilation of precipitation observations has the ability to improve the quality of moisture analyses and forecasts in data sparse regions. Finally, the limitations inherent in the current assimilation of precipitation data, their implications for the future and possible ways of improvement are discussed.