Simulation and Diagnostics of a Near-Equator Tropical Cyclone

On 27 Dec 2001, tropical cyclone Vamei landed on the southern tip of the Malay Peninsula, bringing heavy rain and widespread flooding in the southern Malaysian state of Johor and nearby Singapore. While tropical cyclones are no strangers in some countries in Southeast Asia, e.g. the Philippines, the occurrence of Vamei at the very low latitude of 1.5°N is virtually unheard of before (Chang et al. 2003). We are interested in this rare case of extreme weather for two reasons: (i) numerical simulation of extreme weather is difficult in most circumstances, but in data-sparse Southeast Asia, it is particularly challenging; (ii) tropical cyclogenesis near the equator where the Coriolis force is almost absent is itself a meteorological puzzle.

The aim of this work is to address the question: why does the tropical depression, which is often seen in the Borneo region during the northeast monsoon season, intensify to form the tropical cyclone Vamei? The strong winter monsoon surge in the week before Vamei's genesis no doubt provided the synoptic backdrop that favors enhanced cyclonic circulation over the South China Sea. Strong low-level horizontal convergence is likely to play an important role in generating vertical absolute vorticity. But another possibility is the conversion of vertical shear to vertical vorticity by the tilting of vortex tubes. We attempt to clarify the issue through a vorticity budget diagnostic.

To investigate the generation of vertical absolute vorticity in tropical storm Vamei, the atmospheric module of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS^TM) was used to make a high-resolution simulation, initialized with Navy Operational Global Atmosphere Prediction System (NOGAPS) analyses. Observational data are assimilated every 12h, without "bogusing". We used three nested grids at 54:18:6 km resolution: the outmost grid spans the whole Southeast Asia, the middle grid spans Sumatra - Malay Peninsula - Borneo and the innermost grid covers the southern reaches of the South China Sea east of Singapore. The 3D velocity outputs were interpolated into pressure coordinates and an analysis of the absolute vorticity budget of an atmospheric column (defined as from the surface to 300mb) was carried out. We may conceptualize the change in vertical absolute vorticity (VAV) averaged over a vertical column as follows:

Change in VAV = 3D Convergence of VAV-flux + Generation/Destruction by Vortex Stretching/Compression + Generation/Destruction by Vortex Tilting + Destruction by surface turbulent drag

Our results show that waves of absolute vorticity perturbation grow over the north side of the Vamei vortex and interact, eventually leading to the rapid intensification of the cyclone. The convergence of VAV-flux serves mainly to redistribute VAV among the atmospheric columns, while its impact on the VAV budget is generally π/2 out-of-phase with vortex tilting. Strong vortex stretching is the primary cause of the increase in VAV. Surface turbulent drag has negligible effects on the VAV of an atmospheric column.

References

[1]C.-P. Chang, C. H. Liu and H. C. Kuo, Geophy. Res. Lett. 30(3), 1150 (2003).

NB. COAMPS is a registered trademark of US Naval Research Laboratory.