Analysis of the Mesoscale Structure and Vortex Rossby Waves in High-Resolution Numerical Simulations of Typhoon Morakot (2009)

Typhoon Morakot, which made landfall over Taiwan during August 2009, is now well-known for the destruction that it caused in that nation, through the extreme rainfall it produced and the subsequent flooding and landslides. However, in many aspects it was also an interesting and unusual system from a dynamical point of view, due to its enormous size, complex mesoscale structure and interaction with the terrain of Taiwan.

Here, we present the results of simulations of Morakot utilizing a high resolution mesoscale model, the Advanced Regional Forecast System (ARPS) with finest grid scale of 3 km. Following a simple initialization procedure using analyses and 3 hour forecasts from the operational Global Forecast System (GFS) model for initial fields and boundary conditions, the simulations closely reproduced the track and intensity of Morakot during the 48 hour period encompassing its passage over Taiwan. Satellite and ground-based radar imagery from Taiwan showed that the model also captured important aspects of the mesoscale structure and evolution as well. There appeared to be two main processes affecting the asymmetric structure of Morakot during the period of study. Firstly, the typhoon was embedded in a moderate to strongly sheared environment and its deep convection had become highly asymmetric as it approached Taiwan, being almost completely confined to the southern semi-circle. This was reproduced in the ARPS runs, and analysis showed that the asymmetric up-motion and deep convection was primarily confined to the downshear-left part of the storm, which is consistent with other findings regarding tropical cyclones in vertically sheared environments. Later in the period, as the system moved over Taiwan, the vertical shear reduced with time, and in response the system became more symmetric, with the wavenumber (WN) 1 asymmetries in vertical motion and deep convection becoming weaker. The other important process was the development of relatively strong wave structures within the outer eyewall of the system, near the radius of maximum winds. Although the WN 1 asymmetry was dominated by the effects of vertical shear, analysis showed that the structure of WN 2 and 3 waves were consistent with that of Vortex Rossby Waves (VRWs), which developed rapidly in the barotropically unstable annulus of cyclonic vorticity associated with Morakot's large radius of maximum winds. A potential vorticity budget analysis of these waves yielded results consistent with earlier studies, with the vortex “beta” term (the effects of the radial gradient of vorticity in the symmetric vortex) tending to oppose the advection of the waves by the swirling flow, and leading to upstream propagation. Diabatic heating terms were found to be significant and sometimes dominant sources, particularly the asymmetric heating term, and this appeared to affect the propagation speed of the waves where they interacted with the high terrain of the Taiwan Central Mountain Range.

Current ideas implicate the effects of VRWs in inducing the mixing of vorticity (or potential vorticity) between the eyewall and eye, and the possibility of this process leading to short term intensity changes. This process was also observed in the simulations of Morakot, with the onset of strong mixing coinciding with a movement of a strong VRW into a position such that the asymmetric flow associated with it added to the storm relative southerly flow at low levels. The mixing event led to a flux of potential vorticity into the eye, and at least a partial transition from the annular structure of vorticity to a monopole, with the maximum in vorticity located near the center. Coincident with this was a slight weakening in the near surface wind speeds, although the central pressure remained virtually unchanged. With the transition away from the barotropically unstable annular structure, VRW activity weakened, although much more so in the runs with no terrain. Following this period, the strong asymmetric diabatic heating forced by upslope flow over the Taiwan mountains appeared to become a source for the re-emergence of WN 2 and 3 VRWs in the runs with terrain. Previous work has highlighted several processes that were important to the extreme rainfall Morakot produced. These include the very large size of the typhoon, its slow motion near Taiwan, the persistent area of convergence between the monsoon southwesterlies and the circulation of Morakot, and the upslope flow into the Central Mountain Range. Here we present evidence of an additional process, that of enhanced vertical motion ahead of propagating VRWs near the radius of maximum winds of Morakot, which led to the repeated development of zonally orientated deep convective band further north than the large scale convergent area associated with the monsoon flow. These were noted in both the model simulations and the ground based radar data from Taiwan.