Evolution of the Large Scale Circulation, Cloud Structure and Regional Water Cycle Associated with the South China Sea Monsoon during May-June, 1998

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概要

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• This paper studies the evolution of the South China Sea (SCS) monsoon during May-June 1998, to elucidate relationships among the large scale circulation, organization of convection, cloud structures, and fluctuations of the regional water cycle of the SCS. Primary data used include field observations from the South China Sea Monsoon Experiment (SCSMEX), and the satellite rain products from the Tropical Rainfall Measuring Mission (TRMM). Prior to the onset of the SCS monsoon, enhanced convective activities associated with the Madden and Julian Oscillation were detected over the equatorial Indian Ocean in early May while the SCS was under the influence of the West Pacific Anticyclone with prevailing low level easterlies and suppressed convection. Subsquently, a bifurcation of the MJO convection near 90★E led to the development of strong convection over the Bay of Bengal, which spawned low-level westerlies across Indo-China and contributed to the initial build-up of moisture and convective available potential energy over the northern SCS. The onset of the SCS monsoon occurred around May 18-20, and appeared to be triggered by the equatorward penetration of extratropical frontal disturbances, originating from the continental regions of East Asia. Analysis of TRMM microwave and precipitation radar data revealed that during the onset phase, convection over the northern SCS consisted of squall-type rain cells embedded in meso-scale complexes similar to extratropical systems. The radar Z-factor intensity indicated that SCS clouds possessed a bimodal distribution, with a pronounced signal (>30 dBz) at a height of 2-3 km, and another one (>25 dBz) at the 8-10 km level, separated by a well-defined melting level signaled by a bright band at around 5-km level. The most convectively active phase of the SCS monsoon, as measured by the abundance of convective and stratiform hydrometeor types, inferred from the radar vertical profile, was found to occur when the large scale vertical wind shear was weakest. The fluctuation of the water cycle over the northern SCS was found to be closely linked to the large-scale dynamical and SST forcings. Before onset and during the break, the northern SCS was relatively warm and served as a moisture source (E - P > 0) to the overlying atmosphere. During the active phase, the northern SCS was cooled, providing a strong sink (E - P < 0) for atmospheric moisture, with the primary source of moisture coming from regions further west over Indo-China and the eastern Indian Ocean. Vigorous water recycling by convective systems in the northern SCS occurred during the mature phase of the SCS monsoon, with precipitation efficiency (defined as the ratio of the surface precipitation to the sum of large scale moisture convergence and surface evaporation from the ocean) approaching 96%. Westward transport of moisture from Indo-China into, and northward transport out of, the northern SCS provided the main source of moisture for the torrential rain over the YRV in mid-June 1998. The present results suggest that the SCS may play an important role in regulating the SCS monsoon active and break cycles through charge and discharge of moisture, and convective available potential energy.

• 社団法人日本気象学会の論文
• 2002-10-25

著者

• Lau K‐m

  Laboratory For Atmospheres Nasa

• LAU K.-M.

  Laboratory for Atmospheres, NASA

• LI Xiaofan

  Goddard Space Flight Center

• WU H.T.

  NOAA

• Lau K.

  Laboratory for Atmospheres, NASA

• Wu H.

  NOAA

関連論文

• Evolution of the Large Scale Circulation, Cloud Structure and Regional Water Cycle Associated with the South China Sea Monsoon during May-June, 1998
• 東アジアにおける夏のモンスーンの2年周期変動と熱帯における海面水温

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