4C.4
Observational Analysis and Numerical Simulation of Tropical Cyclone Energy Dispersion and Associated Cyclogenesis in the Western North Pacific
Tim Li, University of Hawaii, Honolulu, HI; and B. Fu, X. Ge, B. Wang, Y. Zhu, and Y. Wang
Using the QuikSCAT and TMI data, we identified tropical cyclone (TC) genesis processes during the western North Pacific (WNP) typhoon seasons in 2000 and 2001. Our particular focus is on synoptic wave forcing scenarios associated with the Rossby wave energy dispersion of a pre-existing TC and easterly waves. Among 34 TCs during the two seasons, there are 7 TC genesis cases associated with easterly wave forcing and 6 cases associated with pre-existing TC energy dispersion. Not all TCs have a clear Rossby wavetrain pattern in their wakes. It seems that the existence of the Rossby wave train depends largely on TC intensity and background flows.
The TC induced Rossby wavetrain is investigated in a 3-D high-resolution model. While a simulated TC moves northwestward in a resting environment due to the beta drift, its energy propagates southeastward, forming a synoptic-scale wavetrain with alternating regions of anticyclonic and cyclonic vorticity perturbations. The maximum intensity of the energy dispersion is found in the lower troposphere. Experiments with both explicit and mass-flux convective heating schemes show that cyclogenesis cannot occur in the Rossby wavetrain of a preexisting TC under the resting environment.
An anomaly baroclinic model was developed to understand the role of the mean monsoon flow on cyclogenesis. In the model a time-independent basic state similar to the WNP summer mean flow is specified. In the presence of such a mean flow, the model simulates TC formation in the wake of a pre-existing TC. Due to the influence of the mean flow, vortex-scale perturbations within the cyclonic circulation of the Rossby wavetrain are organized and intensified, leading to vortex-scale convection-circulation feedback. The simulated new TC has realistic dynamic and thermodynamic structures such as spiral rain bands, an upper-tropospheric warm core, and an eye wall. A vorticity budget is conducted to understand the cause of the PV generation, particularly in the earlier development stage when diabatic heating is relatively small.
.Session 4C, tropical cyclogenesis IV
Monday, 3 May 2004, 3:45 PM-5:15 PM, Napoleon II Room
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