The BMRC High-Resolution Tropical Cyclone Prediction System: TC-LAPS

Abstract

A new Tropical Cyclone Limited Area Prediction System has been developed at the Australian Bureau of Meteorology Research Centre. The features of the new prediction system can be summarized as follows: first, a 12-h, coarse-resolution data assimilation is used to define the outer structure and environment of the storm and to provide initial conditions for coarse mesh prediction. Then, synthetic data are generated to define a storm?s circulation, consistent with observed location, size, intensity, and past motion. The method involves a definition of the environmental flow by filtering of the misplaced tropical cyclone circulation in the (old) objective analysis, the generation of a new, correctly located and intense symmetric vortex, and the construction of vortex asymmetries by requiring that the observed motion be the vector sum of the environmental flow and the asymmetric flow. The subsequent initialization for fine-mesh prediction is carried out using 24 h of diabatic, dynamical nudging through 6-hourly, high-resolution objective analyses, which include the synthetic vortex. During this phase the vorticity and surface pressure fields are largely preserved, while infrared satellite cloud imagery is used to reconstruct the vertical motion field. Finally, numerical prediction is carried out with a high-resolution version of the operational limited-area model of the Australian Bureau of Meteorology, which includes high-order numerics and advanced physical parameterizations. The very encouraging quality of the forecasts is demonstrated in numerous case studies of tropical cyclone events, including improved prediction for some situations when the official forecasts were poor. Average track errors at 24 and 48 h are 115 and 259 km, respectively. These are significantly smaller than corresponding errors in the official and in climatology-persistence forecasts. Given the uncertainties in estimated central pressures, forecasts of intensity are also encouraging. General discussion focuses on system characteristics and some remaining, unresolved issues.