Simulation on reintensification of Typhoon Winnie (1997) during extratropical transition

The landed typhoon Winnie (1997) on eastern China experienced a reintensificatio n process during its extratropical transition (ET). The process was numerically simulated with the mesoscale model MM5, and its evolution characteristic and phy sical mechanism were analyzed also. The analysis with isentropic potential vorti city (IPV) showed that Winnie had experienced two stages during its reintensifyi ng process: (1) Upper disturbance intensifying: in which the northward moving tr ansformed cyclone maintained a low PV in the upper level, then when the upstream PV trough moved eastward its PV gradient increased. It was shown that the uppe r disturbance had intensified. (2) Baroclinic development of transformed cyclone : in which the transformed cyclone developed when the lower baroclinic band coup led with the upper positive PV anomaly that intensified in the first stage. Simu ltaneously, during reintensification of Typhoon Winnie, the upper wind speed in creased and grew into an upper jet. It was results of the dynamical balance of t he upper disturbance and the transportation of lower baroclinic kinetic energy. The evolution of the upper disturbance and the upper jet implied that the trans formed cyclone itself might play a key role in its reintensification. By separ ating the initial cyclone, some numerical experiments to examine the effects of the transformed cyclone and its relative physical factors were performed. It was shown that the lower transformed cyclone Winnie intensified the upper disturbance at first through the latent heat process and then developed baroclinically when it moved northward, and phase locked with the intensified upper disturbance . The evolution of Winnie demonstrated the significant effect of the cyclone itself, in which, the effect of the initial cyclone circulation was the most import ant, then was the initial moisture, and the last was the initial baroclinicity. The cause for the feeble effect of baroclinicity was that the baroclinical band could be reconstructed in the circumstance of cyclone circulation and high humidity. Then the cyclone could continue to develop baroclinically when it interacte d with the upper disturbance. Thus, the baroclinicity was still the direct facto r for the transformed cyclone to reintensify. It was a special characteristic for Winnie that both the frontogenesis during transformation stage and the mainten ance of baroclinicity during reintensification stage related closely to the latent heat process accompanied with rainfall over the cyclone.