Convectively Generated Hot, Vortical Towers During the Genesis of Typhoon Nari (2001)

Because of the absence of observations over the vast tropical oceans, our ability to predict and understand tropical cyclogenesis is very limited. In this work, we will present a successful 4-day cloud-resolving simulation of the genesis of Typhoon Nari (2001) using a two-way interactive, nested-grid (36/12/4/1.33 km) version of the MM5 with the finest grid size of 1.33 km. The model was initialized with the ECMWF analysis without any bogussed data 12 hours prior to the beginning of the best track (i.e., 1200 UTC 5 September). Analyses show that this typhoon developed in a near-barotropic environment. It deepened 6-8 hPa as moving northeastward in the first 36 h. Subsequently, the storm experienced significant deepening at an average rate of 0.5 hPa h-1. It became quasi-stationary for a period of 12 h before turning north- to northwestward and then southeastward under the influence of another typhoon Danas (2001). Nari's eye with organized deep convection in the eyewall began to emerge after 0000 UTC 8 September. Very encouragingly, the model reproduces the time series of the minimum sea-level pressure and maximum surface wind as well as the complicated track, despite the use of the ECMWF (synoptic-scale) analysis. The model also reproduces the development and evolution of Typhoon Danas, which appears to have important impact on the movement and evolution of Nari.

The cloud-resolving simulation results show quite different structures of deep convection during the genesis and mature stages. In the first 36 h, the model convection develops mainly along Nari's periphery, with multiple localized low-pressure centers associated with convectively generated vortices, and “bow” echoes. Significant temporal and spatial variations of deep convection are also evident during this genesis stage due likely to the generation of cold downdrafts and the subsequent recovery of the maritime boundary layer. As the storm contracts in radius, more convection begins to move toward the central portion of the cyclonic circulation, leading to the formation of an eye.

Vorticity budgets show that Nari's genesis occurs with its associated rotation amplified from the bottom upward. That is, the cyclonic vorticity is convectively generated in the lower troposphere as a result of intense convergence in the boundary layer, and then advected upward in convective cells into the midtroposphere. At the time of presentation, we will show how convectively generated vortices grow upscale in terms of potential vorticity, and how a lower-tropospheric cold anomaly are transformed to a warm core during the genesis, and the processes leading to variable intensity changes and complicated track.