The Spin-up Process of a Cyclone Vortex in a Tropical Cyclone Initialization Scheme and Its Impact on the initial TC Structure

The structure of the tropical cyclones produced by Nguyen and Chen's (MWR, 2011) initialization scheme at the model initial time has been examined for storms over the western Pacific. A total of 9 storms at different stages of their life cycles were initialized using the WRF model. Through the spin-up process, the initial vortex is well adjusted to the storm environment and the model employed with intensity close to the best track data. Furthermore, the initial structure of the storm in the model initial conditions was compared with the observed structure of these storms from available radar and satellite microwave observations. For all cases considered, the scheme is capable of producing different types of initial storm structures consistent with observations, including storms that have an asymmetric structure and a double eyewall. With intensity close to the best track data and realistic storm structure at the model initial time, the initialization scheme not only improves the track and intensity forecasts but also the horizontal rainfall distribution after the storm makes landfall.

The preliminary results of this study have shown that the initial storm structure is significantly affected by the environment in which it is embedded. During the early season, storms embedded within the southwesterly monsoon flow have a tendency to exhibit a “9” type asymmetric structure with an upper level outflow channel extending southwestward from the southeastern quadrate of the storm. In low levels, the convergence area between the storm circulation and the southwesterly flow is the favorable location for the development of spiral rainbands. Late season storms have a tendency to produce a “6” type storm structure with an outflow channel extending northeastward from the northwestern part of the eyewall, especially when an upper-level cold low or trough is present to the northwest of the storm. In low levels, the convergence of the northeasterly monsoon flow and the cyclonic circulation of the storm are favorable for the occurrences of spiral rainbands. For intense storms that underwent an eyewall replacement cycle, the scheme also shows considerable skill in reproducing the double eyewall structure in the model initial conditions.