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Understanding the Mechanisms Leading to the Rapid Intensification of Typhoon Megi (2010)
Typhoon,rapid intensification,convective heating,convective burst,secondary circulation,potential vorticity,inertial stability,warm core structure,
|Publication Year :||2014|
|Abstract:||在颱風路徑預報有長足進展的近30年間，颱風強度預報進步卻相對有限，其中一個困難在於無法準確預報快速增強(Rapid Intensification, RI)的現象所致，因此研究此現象背後的物理機制有其必要。
While TC track forecasts have been improved remarkably during the past 20 years, progress in intensity forecast has lagged significantly behind. One subset of intensity change, rapid intensification (RI), is particularly difficult to predict. Therefore, the objective of this study is to investigate the key mechanisms that lead to the RI of typhoon Megi (2010).
This study uses Weather Research and Forecasting Model (WRF) to simulate the RI process associated with Megi. The RI process of typhoon Megi (2010) is simulated reasonably well (by comparing with observations). Furthermore, we carry out a series of sensitivity experiments using different microphysical schemes to evaluate the uncertainty of RI with different model physical processes. Comparisons of different experiments indicate that RI TC has greater potential vorticity (PV), inertial stability (I2), axisymmetricity at mid-upper level. In addition, warmer core located at higher altitude, more active convection near TC center, stronger secondary circulation and more significant interaction between eye and eyewall that can also be identified. These features may be the precursors of RI.
The PV budget is conducted to gain more physical insights. Results show that when the convective bursts (CBs) are active, the simulated PV tendency is significantly greater. In addition, horizontal PV advection may play a role in increasing the mid-upper level PV, and this may be a result from the secondary circulation triggered by the heating of CBs. The Sawyer-Eliassen model is utilized to diagnose the balanced response of heating and it shows that when CBs are active, the enhanced latent heat strengthens the secondary circulation and the PV advection due to secondary circulation is greater. In addition, we also found that the strong secondary circulation is beneficial to the increased potential temperature in the eye.
In conclusion, this study suggests propose a possible new path leading to RI: more active convection near the TC center which could be triggered by high entropy air transported from the eye which will generate greater latent heat and initiating stronger secondary circulation. The stronger secondary circulation is favorable for the increased PV and I2 at mid-to-upper level, which also facilitate the formation of warm core at higher level. The greater I2 at mid-to-upper level could also sustain the warm core structure at higher level. The development of warm core at higher level induces the surface pressure dropping effectively, greater I2 at mid-to-upper level also enhances the heating efficiency; these can help lead to the onset of RI.
|Appears in Collections:||大氣科學系|
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