Observing systems experiments: relative model response to various FGGE datasets in the tropics

Eight experiments are summarized, all of which involved a 12-h data assimilation period based on the Newtonian relaxation procedure followed by a 36-h forecast. Predictions using all of the data produced very good forecasts of the June 1979 onset vortex over the Arabian Sea. The dropwindsonde data were found to be most responsible for this success, primarily because they resolve the rotational modes of the system and cover a significant depth of the troposphere. While the winds were more important, the dropsonde thermodynamic data were beneficial. All datasets, when tested individually, had a positive impact on the forecasts. When used in combination, however, some datasets became less important or even redundant. The influence of satellite winds was enhanced greatly by spreading the wind increments over a larger vertical depth. It is shown that the dynamical response of the model to the various distributions and amounts of new data is consistent with geostrophic adjustment theory and provides guidance for future observing system strategies. -from Authors