(Silvio Davolio)
Accurate quantitative forecasting of precipitation, especially during severe weathe episodes, is one of the most challenging tasks of meteorological modelling and has a direct impact on the management of such events in areas at risk. The frequent assimilation of variables directly related to the formation of precipitation and the water cycle may contribute to a better definition of model moisture, divergence (vertical velocity) and latent heating, and could therefore lead to an improvement in short-range precipitation forecasts. Over recent years, increasing attention has been focused on the assimilation of precipitation data into meteorological models.
Recently, rainfall assimilation has become an attractive prospect not only in tropical areas, but also in midlatitudes, where there is a need to improve the initial conditions and the balance of the hydrological cycle, especially in the first few hours, in order to obtain better estimates of precipitation on a regional scale.
Precipitation itself cannot be directly introduced into weather prediction models because it is not a prognostic variable, but an end result of very complex dynamical and microphysical processes. However, rain observations can be used to correct humidity and/or temperature profiles (the latter as a consequence of latent heat release) in order to obtain simulated precipitation closer to reality.
Even if somewhat empirical, the nudging technique is a straightforward, physically based method that can solve this data assimilation problem. The choice of a nudging technique mainly stems from the fact that it is computationally much cheaper than variational methods and easily allows for operational implementation. In spite of its simplicity, nudging has proved to be efficiently applicable to many synoptic-scale and mesoscale cases.
A rainfall assimilation technique, based on specific humidity nudging, has been developed for application to satellite derived precipitation fields, with the aim of improving the short range forecasts of BOLAM mesoscale model.

Further info

S. Davolio and A. Buzzi 2003: Assimilation of precipitation into a regional model. 4th Plinius Conference on Mediterranean Storms, October 2-4 2002, Mallorca (Spain).
S. Davolio and A. Buzzi 2003: Application of a nudging procedure for rainfall assimilation to the 2001 Algeria flood. 5th Plinius Conference on Mediterranean Storms, October 1-3 2003, Ajaccio, Corsica (FR).
S. Davolio and A. Buzzi, 2003: A nudging scheme for the assimilation of precipitation data into a mesoscale model. Wea. Forecasting, 19, 855-871.