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.