Simulation of formation of a near-equatorial typhoon Vamei (2001) |
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Authors: | C R S Chambers T Li |
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Institution: | (1) International Pacific Research Center, Department of Meteorology, University of Hawaii, Honolulu, Hawaii, USA |
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Abstract: | Summary A community mesoscale model is used to simulate and understand processes that led to the formation and intensification of
the near-equatorial typhoon Vamei that formed in the South China Sea in December, 2001. The simulated typhoon resembles the
observed in that it had a short lifetime and a small size, formed near the equator (south of 2° N), and reached category-one
intensity. The formation involved the interactions between the scales of the background cyclonic circulation (the Borneo Vortex
of order ∼100 km) and of mesoscale convective vortices (MCVs, in the order ∼10 km). Before tropical cyclone formation MCVs
formed along a convergent, horizontal shear vorticity line on the eastern edge of an exceptionally strong monsoonal northerly
wind surge.
The typhoon genesis is marked by three rapid intensification periods, which are associated with the rapid growth of potential
vorticity (PV). A vorticity budget analysis reveals that the increases in low-level vorticity during the rapid intensification
periods are attributed to enhanced horizontal vorticity fluxes into the storm core. The increase of the horizontal vorticity
flux is associated with the merging of areas of high PV associated with MCVs into the storm core as they are advected by background
cyclonic flows. The increases in PV at upper levels are associated with the evaporation of upper level stratiform precipitation
and increases of vertical potential temperature gradient below the maximum stratiform cloud layer. It appears that two key
sources of PV at upper and lower levels are crucial for the build up of high PV and a deepening of a cyclonic layer throughout
the troposphere. |
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