热力和动力环境因子对与台风“海燕”路径相似的两类台风强度的相对贡献

Relative contributions of thermodynamic and dynamic environmental conditions to the intensification of tropical cyclones with similar tracks to Haiyan(2013)in the western North Pacific

本文分析了热力、动力环境因子对与百年一遇超强台风“海燕”有相似路径的热带气旋的强度的影响。为了定量分析环境因子的相对重要性,将相似路径下的台风分为超强台风和一般台风两类。结果表明,相较于动力因子,热力因子对生成超强台风更为重要。根据BDI(Box Difference Index)指数的排序,选择925 hPa湿静力能MSE(Moist Static Energy)、950 hPa比湿、900 hPa温度作为重要预测因子来判断是否生成超强台风。此外,海洋热容量和台风移动速度在两类台风中有着明显差异,也可以作为辅助预测因子。最后,用WRF模式(Weather Research and Forecasting)设计理想数值试验,揭示环境温度和水汽垂直剖面对台风强度影响的相对重要性。敏感性试验结果表明,环境水汽和温度的相对贡献比约为1∶4,它们的共同作用使得相似路径下的一般台风可以发展成为超强台风。

Numerical prediction models often encounter difficulties in predicting the intensity changes of tropical cyclones(TCs),particularly in the case of super typhoons and the progression of TC intensification.This study aims to investigate the large-scale environmental factors influencing the generation and development of super typhoons in the western North Pacific(WNP).We analyze the effects of thermodynamic and dynamic environmental factors on the intensity of TCs with tracks similar to Super Typhoon Haiyan(2013),the strongest TC in the past century in the WNP.Typhoons with tracks resembling Haiyan(2013)are categorized into two groups:super TCs and regular TCs.The Lanczos method is utilized to filter out synoptic-scale disturbances and typhoon disturbances,and mean composite differences are used to compare the composite environmental factors of the two groups.Observational analysis results indicate that,compared to dynamic factors,thermodynamic factors play a more significant role in strengthening TCs,such as higher temperatures and more abundant vapor in the lower troposphere,along with higher relative humidity in the middle troposphere.This configuration facilitates the release of a large amount of latent heat,which is beneficial for ascending motion.Meanwhile,low-level Ekman pumping enhances TC strength through positive feedback.To quantitatively assess the relative importance of environmental conditions in regulating TC intensity,a box difference index(BDI)analysis method is used to rank the environmental factors.Accordingly,925 hPa moist static energy(MSE),950 hPa specific humidity,and 900 hPa temperature are selected as the top-3 predictors for estimating TC intensity.Furthermore,we calculate the values of ocean heat content(OHC)and TC moving speed for each case,which are found to be larger for super TCs compared to regular TCs and may contribute to an increase in TC intensity.As the typhoon moves faster,the cold water upwelling caused by the typhoon leads to a smaller cooling effect,allowing the OHC to increase and facilitating typhoon intensity strengthening.Additionally,OHC and TC moving speed can also serve as additional predictors.Lastly,idealized numerical model experiments with the Weather Research and Forecasting(WRF)model are conducted to reveal the relative importance of environmental temperature and moisture vertical profiles in affecting TC intensity.The control experiment,identified as CTRL,and a set of sensitivity experiments,identified as Axel_SH,Axel_T,and Axel_SH+T,respectively,are designed.The sensitivity experiments Axel_SH,Axel_T,and Axel_SH+T indicate that the vertical profile of the area-averaged specific humidity,temperature,and specific humidity plus temperature of Typhoon Alex is replaced by that of Tyhpoon Haiyan.The sensitivity experiment results indicate that the relative contribution of environmental moisture and temperature profiles is 1∶4,suggesting that the environmental static stability parameter is the most important factor regulating super TC formation.Overall,the study confirms the significant impact of thermodynamic factors on typhoon intensity with tracks similar to Typhoon Haiyan(2013),providing quantitative insights into the contributions of different environmental factors.Given the challenges in predicting TC intensification,numerical modeling efforts should focus on understanding the interaction with the entire upper-ocean column and improving physical parameterization.