Atmos. Chem. Phys. Discuss., 13, 21079-21124, 2013
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Borneo vortex and meso-scale convective rainfall
S. Koseki3, T.-Y. Koh1,2, and C.-K. Teo3
1School of Physical and Mathematical Science, Nanyang Technological University, Singapore
2Earth Observatory of Singapore, Nanyang Technological University, Singapore
3Temasek Laboratories, Nanyang Technological University, Singapore

Abstract. We have investigated how the Borneo vortex develops over the equatorial South China Sea under cold surge conditions in December during the Asian winter monsoon. Composite analysis using reanalysis and satellite datasets has revealed that absolute vorticity and water vapour are transported by strong cold surges from upstream of the South China Sea to around the equator. Rainfall is correspondingly enhanced over the equatorial South China Sea. A semi-idealized experiment reproduced the Borneo vortex over the equatorial South China Sea during a "perpetual" cold surge. The Borneo vortex is manifested as a meso-α cyclone with a comma-shaped rainband in the northeast sector of the cyclone. Vorticity budget analysis showed that the growth of the meso-α cyclone was achieved mainly by vortex stretching. The comma-shaped rainband consists of clusters of meso-β scale rainfall patches. The warm and wet cyclonic southeasterly flow meets with the cold and dry northeasterly surge forming a confluence front in the northeastern sector of the cyclone. Intense upward motion and heavy rainfall result both due to the low-level convergence and the favourable thermodynamic profile at the confluence front. At both meso-α and meso-β scales, the convergence is ultimately caused by the deviatoric strain in the confluence wind pattern but is much enhanced by nonlinear self-enhancement dynamics.

Citation: Koseki, S., Koh, T.-Y., and Teo, C.-K.: Borneo vortex and meso-scale convective rainfall, Atmos. Chem. Phys. Discuss., 13, 21079-21124, doi:10.5194/acpd-13-21079-2013, 2013.
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