Forecasting Extratropical Transition of Tropical Cyclone Intensification via Projection Pursuit

The extratropical transition (ET) of tropical cyclones (TC), defined as a two stage process by Klein et al. (2000), ends either with dissipation or rapid intensification as a midlatitude cyclone. The interaction between TCs and midlatitude upper level troughs is a highly variable process and the end result is usually very difficult to predict accurately even with full-physics forecast systems, such as NOGAPS. Recent research has indicated that the end result of this poorly understood process is dependent more on the basic midlatitude structure and the phasing between the trough and the TC rather than the details of the TC structure.

In this paper we use single variable, 500-mb geopotential height, images of 85 ET cases from the years 1997-2004 to capture the basic midlatitude structure around the TC during ET. The ET cases are aligned around the ET time and the data is processed in a multistage projection pursuit technique in an effort to find the best projections of the data without losing the separability of two classes, dissipaters and intensifiers. The forecasting performance of the technique is tested on 2003-2004 storms, which are projected on the training set of 1997-2002 storms. Two different sets of results are presented, spatial and spatiotemporal. The performance of the spatial forecasts is found to be less accurate than that of NOGAPS. However, incorporating the temporal change of information in the spatiotemporal system improves the results considerably over the purely spatial system.