A new method of observed rainfall assimilation in forecast models

A method to assimilate observed rain rates in the Tropics for improving ini tial fields in forecast models is proposed. It consists of a 6-h integratio n of a numerical forecast model; the specific humidity at every time step a t each grid point is modified (nudged) in such a way that the total model p recipitation accumulated during this integration becomes very close to that observed. An increase in the model precipitation is achieved by moistening the lower troposphere above a grid point with prescribed supersaturation; a decrease in the model rainfall is brought about by decreasing the specifi c humidity in the lower troposphere in proportion to the difference between the model and reference specific humidity profiles. The modified values de pend on the difference between the model and target precipitation. The dept h of the atmospheric column in which the humidity is changed is proportiona l to the target rain rate. Quality criteria of a rain assimilation procedur e are proposed. The quality of the assimilation method was verified using a test in which precipitation generated by a forecast model without nudging ("control" experiment) was considered to be "quasi-target" data and the nud ging procedure was used for assimilation of the rain produced in the contro l experiment. The following experiments were performed: control (C)-without nudging, "simulated nudge" (S)-nudging to the 6-h accumulated rainfall fro m the C experiment, and "satellite nudge"-nudging to the 6-h accumulated sa tellite-retrieved (observed) rainfall. Each experiment consisted of a 6-h f orecast (first guess), analysis, next 6-h forecast (first guess), next anal ysis, and 24-h forecast. Nudging was applied during the two successive 6-h calculations of the first guess over the tropical belt. Parameters of the n udging procedure were determined in such a way that the assimilation proced ure converged quickly and simulated the observed precipitation very closely . The difference in forecast fields between the S and C experiments after a 24-h forecast turned out to be small, indicating high quality of the assim ilation procedure. The high sensitivity of forecast fields to the quality o f rain retrieval is demonstrated.