Multi-scale vortex interaction during genesis of Hurricane Dolly (2008)

Through the cloud-resolving simulation with the Weather Research and Forecast (WRF-ARW) model, this study examines the key processes that led to the early development of Hurricane Dolly (2008). It is found that the initial development of Dolly consisted of three phases: (i) initial burst of convection, (ii) low- to mid-tropospheric moistening and adjustment, and (iii) reinvigoration of moist convection and rapid intensification. Since the enhancement of the low-level vorticity is mainly ascribed to the stretching effect associated with the surface-based moist convection, the genesis of Dolly is essential a “bottom-up” process.

During the development of Dolly, the cyclonic vortexes in the convective scale were very active. It is found that the accumulation and release of convective potential available energy and increase and decrease of relative humidity usually occurred alternatively in the evolution of the vortexes with long life period. This indicates that these cyclonic vortexes not only contributed to the enhancement of the system vorticity but also play an important role in the moist convection activity, which is critical to the strengthening of the large-scale convergent flow. As the small-scale positive vorticity anomalies were wrapped into the storm center by the convergent flow, the convergence and accompanying aggregation of vorticity anomalies projected the vorticity into larger scales and a system-scaled vortex with well-defined structure were finally developed. Along with the burst of convection, the anticylonic vortex was also prominent. 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.