Four-Dimensional Data Assimilation Experiments in the Southern Hemisphere

Abstract

An attempt has been made to isolate some of the problems likely to be associated with the practical implementation of four-dimensional data assimilation schemes in the Southern Hemisphere. In particular, the requirement for a reference-level specification over the Southern Hemisphere, and the importance of assimilation frequency (i.e., the period between data insertions) are investigated. The assimilation scheme used consists of three components: a ?sigma? surface analysis model to ?insert? data as they become available, an initialization module of the Nitta-Hovermale type to remove high-frequency inertio-gravitational oscillations, and a multi-level primitive equation model to ?advect? the assimilated atmosphere state forward in time. In all experiments, real data consisting of both the conventional and satellite-derived type are used. Verifications concentrate on the synoptic verisimilitude of the assimilation process, and where possible, the impact of various assimilation procedures on subsequent numerical prognosis. Results underline the critical importance of reference-level pressure in the scheme evaluated. There is also some suggestion of improved performance when the assimilation frequency is increased.