台风生成阶段周期性对流爆发机制及辐射的影响

Mechanism of periodic convective bursts during tropical cyclogenesis and impact of radiation

最新观测统计分析表明台风生成前对流活动存在周期性特征,周期集中在18~26 h,峰值多出现在夜间。本文通过一系列理想的云分辨数值控制试验和辐射敏感性对比试验讨论了台风生成阶段周期性对流爆发的成因以及辐射日变化在此过程中产生的影响。结果表明,在给定无日变化的恒定太阳辐射情况下,台风生成过程中也会出现与观测一致的周期性对流爆发,说明辐射日变化不是该周期性形成的控制因素。对湿静力能扰动的收支分析发现边界层中湿静力能的周期性增减是导致周期性深对流爆发的关键因素。深对流爆发形成的冷池通过平流作用降低了边界层内的湿静力能,需要通过辐射与海表面通量过程使其恢复,才能再次触发对流。辐射日变化对于一些较弱的热带低压涡旋有调制作用,能使其周期性对流爆发的峰值时间相位改变并趋于在夜晚出现;但对于较强的涡旋,辐射的这种调制作用并不显著,涡旋会保持其最初对流爆发的周期和相位。本研究对观测到的台风生成阶段对流峰值出现在白天或夜间的情况都可以作出合理解释,也深化了对台风生成过程对流和辐射的作用和影响科学问题的理解。

The statistical analysis of observation shows that there is a periodic characteristic of convective activity before the formation of tropical cyclones,with periods concentrated between 18-26 hours and peaks mostly occurring at night.This article discussed the causes of periodic convective bursts during tropical cyclogenesis and the impact of daily variations in radiation during this process through a series of idealized cloud-resolved numerical control experiments and comparative experiments of radiation sensitivity.Results indicate that under a constant solar radiation without daily variation,periodic convective bursts may also occur during tropical cyclogenesis as in observation,indicating that diurnal radiation cycle is not only the controlling factor of the periodic convection.The analysis of the budget of moist static energy anomaly reveals that the periodic increase or decrease of moist static energy in the boundary layer is a key factor affecting the occurrence of periodic deep convection.The cold pool formed by the outbreak of deep convection reduces the moist static energy in the boundary layer through advection,which needs to be recovered through radiation and surface flux processes in order to trigger convection again.The diurnal variation of radiation has a modulating effect on some weaker tropical depressions,which can cause the peak time phase of periodic convective bursts to change and to appear at night;but for strong vortices,the modulation effect of radiation is not significant,and vortices will maintain their initial period and peak time phase of convective burst.This study can provide a reasonable explanation for the observed occurrence of convective peaks during the daytime or nighttime during the tropical cyclogenesis,and also advance the understanding of scientific issues on the role and impact of convection and radiation on tropical cyclogenesis.