The compactness of typhoons in the western North Pacific

This study presents a composite analysis of compact typhoons in the western North Pacific. In this study, a typhoon is considered to be compact, if (1) the radius of the maximum wind (RMW) is smaller than what is expected for a general TC for a given maximum tangential wind speed and (2) the tangential wind speed decreases at a greater rate than that for a general TC outside the RMW. A structure parameter S is defined to provide a quantitative measure of the compactness of a typhoon. The QuikSCAT 10-m wind data are used to calculate Vmax , RMW, radii of specific wind speeds, and then the S parameter. The computed S parameters are used to classify all cases as compact or incompact according to the 33% and 67% percentiles.

Results show that when a TC becomes compact, the intensity tends to increase greatly. The composites of brightness temperature show that a compact typhoon has highly symmetric convective structure with strong convection concentrated in a small region near the center. Through the analysis of wind profiles crossing the center for the compact and incompact TCs at different intensity stages, it can be expected that the intensification mechanism is different between these two types of typhoon. With forecasting issues in mind, an objective identification of compact TCs in an operational setting could help forecasters to better predict future intensity changes for these tropical cyclones, and likely reduce the overall intensity forecast errors. On the other hand, when compact TCs make landfall, the heavier rainfall accompanied by strong gust wind would be expected.