Forecasting and Adaptive Observing Issues in Extratropical Transition of Tropical Cyclones

Extra-tropical transition (ET) of tropical cyclones presents many forecast difficulties for the northeast Pacific and for much of Western Europe. In 2004 and 2005, several tropical cyclones underwent ET and brought strong winds and flooding rains to portions of the western United States and for northern regions of Western Europe. One goal of this study of ET is to examine locations for adaptive sampling to improve forecasts of transitioning storms before they dissipate or reintensify. To this, we compare the following adaptive sampling strategies: the Ensemble the Ensemble Transform Kalman Filter (ET KF) used by NCEP and Singular Vectors (SV's) used by NRL and ECMWF, both for the northern Atlantic. Another goal of our study is to examine storms that undergo ET where the forecasts may have been poor for different reasons. In 2004, Typhoons Nock-ten and Nanmadol caused problems in Japan as far as the forecast and outcome of the storms are concerned. Typhoon Nock-ten was forecast to reintensify as an extratropical storm, but it did not, and Nanmadol was forecast to reintensify, but did not to the extent of the forecast. These two goals - adaptive sampling and case comparison - should help improve forecasting of ET and provide a longer lead time for preparation in areas often impacted by ET.