Four-Dimensional Data Assimilation in the Monsoon Region. Part I: Experiments with Wind Data

Abstract

A limited primitive equation model has been used to study the feasibility of four-dimensional data assimilation in the monsoon region and, further, to study the applicability of several assimilation techniques currently being employed in global models. The two fundamental objectives of this research are (i) to understand how the model atmosphere responds to the insertion of asynchronous data and its impact on the assimilation-prediction cycle, and (ii) to determine what assimilation strategies work best for limited-area models in the tropics. A sequence of ten assimilation experiments are performed using different update procedures; all insertions are carried out with only the wind observations. The model is initialized with the ECMWF FGGE level III-b data for the onset vortex case of 17 June 1979, and assimilations are carried out using the summer MONEX level II-b data during the first 12 hours. From these assimilated states, 24-h forecasts are then made. The results lend support to the premise that the required initial conditions can be obtained by the process of four-dimensional updating of the prognostic variables. The results also clearly demonstrate the superiority of the continuous assimilation approach via Newtonian nudging over that by indirect insertion. Furthermore, the insertion shocks are significantly minimized by assimilating only the rotational component of the winds. On the other hand, the application of noise control measures only marginally alleviate the insertion shocks accompanying continuous indirect insertion.