IMPROVED HURRICANE TRACK FORECASTING FROM THE CONTINUOUS ASSIMILATIONOF HIGH-QUALITY SATELLITE WIND DATA

Despite recent improvements in the accuracy of hurricane track forecas ts, mean position errors still remain unacceptably large. For example, recurvature is captured poorly by forecast models and can produce exc essively large position errors. This study addresses the problem of hu rricane track forecasting in three ways. First, the initial conditions for the forecast model are augmented by a dense coverage of high spat ial and temporal resolution satellite-derived wind vectors. Second, to gauge the extent to which this additional four-dimensional detail of the atmospheric structure can he exploited, three distinct types of da ta assimilation methods are examined. These are 1) conventional (inter mittent, cycled) 6-h assimilation, 2) nudging over a 12- or 24-h perio d up to the initial time, and 3) recently developed barotropic and fou r-dimensional variational assimilation schemes, also over a 12- or 24- h period. The nudging and variational methods are continuous assimilat ion procedures and incorporate satellite-derived winds, typically at 6 -h frequencies, but up to hourly frequencies in trials over the Austra lian region. Over the Atlantic basin, only a 6-h frequency of high-den sity satellite-derived wind vectors was available for the 1995 season. Third, a very high-resolution (15 km) semi-implicit, semi-Lagrangian model provided forecasts out to 72 h. The present study describes the direct application to the Atlantic basin, for die specific case of Hur ricane Opal, of data assimilation and prediction procedures developed for tropical cyclones over the Australian Pacific basin. A series of f orecasts was made, from the two initial times 0000 and 1200 UTC 2 Octo ber 1995, respectively. In these cases, where CLIPER (climatology and persistence) and other conventional forecast guidance was poor, the nu dging and variational assimilation procedures, which were those that b est utilized the high spatial and temporal resolution satellite-derive d winds, produced greatly improved forecasts.