8807号登陆台风的数值研究：内核结构及能量水汽收支

使用PSU/NCAR研制的非静力中尺度模式MM5对登陆后维持较久的典型个例8807号台风(Bill)进行了数值模拟。模拟采用了网格距分别为18和6 km的两重双向嵌套网格。通过给定初始涡旋和选择合适的物理过程,模式不仅比较好地模拟了Bill的强度变化过程,而且再现了风暴的移动轨迹,对降水量的模拟也相当成功。文中利用细网格的模拟结果,分析台风登陆前后的内核结构特征和能量水汽收支,揭示Bill登陆后维持较久的可能原因。分析表明,Bill台风登陆后出现了中低层的稳定度特别是湿稳定度显著加大、表面热通量和水汽通量明显减小、摩擦耗散的动能显著增加等不利于台风维持的特征。但是台风登陆后眼墙结构长时间维持,在登陆初欺眼墙区的上升气流乃至较登陆前更强;台风登陆后通过低层辐合获得了大量水汽,眼墙区强劲的上升运动将低层辐合的水汽向中上层输送,在眼墙附近凝结产生大量的非绝热加热;非绝热加热不仅增暖气柱、增加位能还影响位能向动能的转换,虽然台风登陆后陆面摩擦显著加大,但气压梯度力所做的正功足以补偿摩擦耗散的动能,因而Bill能在登陆后长时间维持。

THE NUMERICAL STUDY OF LANDFALLING TYPHOON BILL (1988) : INNER CORE STRUCTURES AND BUDGETS OF ENERGY AND MOISTURE

Typhoon Bill(1988)-a typical long duration case after landfall,was simulated using the PSU/NCAR nonhydrostatic,two-way interactive nested grid mesoscale model MM5.In the control experiment,two domains with a grid spacing of 18 and 6 km were nested,and the Blackadar PBL parameterization,Betts-Miller convective parameterization,mixed phase ice explicit moisture scheme,Dudhia radiation scheme were adopted and an appropriate vortex was implanted in the initial fields.The model outputs show that not only the evolution of intensity changes of Bill including rapid intensification prior to landfall and long time maintenance after landfall is well simulated but also the track and the total precipitation are successively reproduced. The high resolution model outputs of the finer mesh of the control simulation were used in the analysis of inner core axisymmetric structures and the calculation of budgets of energy and moisture to explore the possible mechanism for Bill's long duration after landfall.The calculated results show that many features,such as significant increases in frictional dissipation and static stability,especially convective stability in the low and middle levels,and apparent decreases in surface heat and moisture fluxes,disadvantageous to the maintenance of typhoon appeared within Bill after landfall.However,Bill's eyewall and warm core structures remained for a long time after its landfall,and the updraft near the eyewall associated with the increase in low level strong convergence was even stronger soon after landfall than that prior to landfall.Large amount of moisture converged in the low level was carried upwards to the middle and upper levels by the strong updraft,and large amount of latent heat was released near the eyewall.The latent heating not only warmed the vortex,increased the potential energy of the vortex,and played a role in the maintenance of warm core structure,but also enhanced the energy transformation from potential energy to kinetic energy within the vortex.Bill could be maintained for a long time after landfall for the positive work done by pressure gradient force was large enough to compensate the loss of the frictional dissipation of kinetic energy.