A New Method of Observed Rainfall Assimilation in Forecast Models

Abstract

A method to assimilate observed rain rates in the Tropics for improving initial fields in forecast models is proposed. It consists of a 6-h integration of a numerical forecast model; the specific humidity at every time step at each grid point is modified (nudged) in such a way that the total model precipitation 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 specific humidity in the lower troposphere in proportion to the difference between the model and reference specific humidity profiles. The modified values depend on the difference between the model and target precipitation. The depth of the atmospheric column in which the humidity is changed is proportional to the target rain rate. Quality criteria of a rain assimilation procedure 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 nudging procedure was used for assimilation of the rain produced in the control experiment. The following experiments were performed: control (C)?without nudging, ?simulated nudge? (S)?nudging to the 6-h accumulated rainfall from the C experiment, and ?satellite nudge??nudging to the 6-h accumulated satellite-retrieved (observed) rainfall. Each experiment consisted of a 6-h forecast (first guess), analysis, next 6-h forecast (first guess), next analysis, 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 nudging procedure were determined in such a way that the assimilation procedure 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 assimilation procedure. The high sensitivity of forecast fields to the quality of rain retrieval is demonstrated.