Evolution of Multi-scale Vortex in the development of Hurricane Dolly (2008)

Through the cloud-resolving simulation with the Weather Research and Forecast (WRF-ARW) model, this study examines evolution of multi-scale vortex in the development of Hurricane Dolly (2008). The high-resolution simulation indicates that the cyclonic vortex in the convective scale were very active during the development of Dolly. Some of the vortexes tended to be dissipated after the local burst of convection. However, many vortexes last a relatively long time and made a contribution to the enhancement of the vorticity of the storm. The long life period of these vortexes is found to be ascribed to not only the aggregation of vortex but also to the new burst of convection. Both vortexes traced in this work indicate that the accumulation and release of convective potential available energy and increase and decrease of relative humidity usually occurred alternatively during the evolution of these vortexes. The macro effect of the release of latent heating played an important role in the strengthening of the large-scale convergent flow. In addition, along with the burst of convection, the vortex with negative vorticity was also prominent. It is found that these vortexes underwent an inward propagation under the effect of the large-scale convergent flow and then dissipated gradually. The expulsion of the negative vorticity suggested by the previous works on the tropical cyclone was not significant in the development of Dolly.

Along with the aggregation of the vortex in convective scale, the mesoscale vortex also underwent noticeable evolution. At the earlier stage, four to six mesoscale vortexes were active in the storm center area. Although the intensity of these vortexes was different and varied with time, no one was seemed to be dominant. Following the development of the storm, two mesoscale vortexes were remarkably strengthened and combined together to form a dominant vortex. By ingesting the vortexes around and then axisymmetrization, the dominant vortex finally developed into a strong storm with well-defined structure. During this process, the negative vorticity trapped in the center area of the storm was dissipated gradually instead of being expulsed outside. In addition, it is found that the intensification of the mesoscale vortex usually occurred in the west side of the storm. This could be attributed to the effect of the largescale vertical wind shear.