| 摘要(英) |
Rapid intensification (RI) of tropical cyclones (TCs) is a main source of intensity forecast errors and merits further study. This study first uses the Seasonal Genesis Parameter (SGP) to analyze the average dynamic and thermodynamic conditions over the western North Pacific during the active TC season (June–November) and the inactive TC season (December–May) over the past four decades. The results show that during the active season, the dynamic potential, which consists of relative vorticity, vertical wind shear, and Coriolis, is generally favorable for TC genesis, whereas the favorable area is greatly reduced during the inactive season. The thermodynamic potential, consisting of ocean energy, moist stability, and humidity, is largely positive south of 30°N in the active season, with maxima over the southeastern South China Sea, east and southeast of the Philippines. During the inactive season, positive thermodynamic potential contracts markedly and shifts south of 10°N, indicating that reduced TC activity north of 10°N is mainly due to insufficient thermodynamic support. To examine the climatological characteristics of RI on the seasonal scale, the SGP is modified by fitting its parameters to the observed RI frequency using the Trust-Region-Reflective Least Squares Algorithm, producing a new Seasonal RI Index. This index estimates the number of RI events occurring in different regions during a season and shows strong spatial correlation with observations. The results also reveal that vertical wind shear and moist stability play important roles within the dynamic and thermodynamic components, respectively, during the active season. In contrast, the Coriolis and ocean energy contribute more during the inactive season, while the contributions from relative vorticity and humidity are relatively small. |
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