Winter Monsoon Convection in the Vicinity of North Borneo. Part II: Effects on Large-Scale Fields

Richard H. Johnson Department of Atmospheric Science, Colorado State University, Fort Collins 80523

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David L. Priegnitz Institute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City 57701

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Abstract

An observational study of the thermodynamic and kinematic structure and properties of monsoon convective systems and their large-scale environment over the southern South China Sea during the field phase (December 1978) of the Winter Monsoon Experiment (WMONEX) has been carried out. The primary observations used are from three Soviet research vessels positioned in a triangular array off the north coast of Borneo during the period 6–28 December. Computations of thermodynamic fields, divergence and vertical motion have been made for the duration of the field phase based on six-hourly rawinsonde releases at the ship sites.

Analysis of the data indicates that the degree of convective activity over the southern South China Sea is modulated by long-period synoptic forcing (monsoon surges, easterly waves) and also significantly by diurnal forcing (land-sea breeze circulations). A diurnal cycle of convection persists in the region whether the synoptic-scale forcing is weak or strong. Convection over water to the north of Borneo regularly evolves on a diurnal basis from a small group of cumulonimbus clouds into a uniform mesoscale precipitation area having the characteristic structure of those observed in recent years over the tropical eastern Pacific and Atlantic oceans. In their mature stage the precipitation systems contain mesoscale anvil clouds commonly extending from near 500 mb to the tropopause covering a 104–105 km2 area. The ship observations provide direct evidence of mesoscale updraft motion within the anvil clouds and mesoscale down-draft below extending to near the surface.

Abstract

An observational study of the thermodynamic and kinematic structure and properties of monsoon convective systems and their large-scale environment over the southern South China Sea during the field phase (December 1978) of the Winter Monsoon Experiment (WMONEX) has been carried out. The primary observations used are from three Soviet research vessels positioned in a triangular array off the north coast of Borneo during the period 6–28 December. Computations of thermodynamic fields, divergence and vertical motion have been made for the duration of the field phase based on six-hourly rawinsonde releases at the ship sites.

Analysis of the data indicates that the degree of convective activity over the southern South China Sea is modulated by long-period synoptic forcing (monsoon surges, easterly waves) and also significantly by diurnal forcing (land-sea breeze circulations). A diurnal cycle of convection persists in the region whether the synoptic-scale forcing is weak or strong. Convection over water to the north of Borneo regularly evolves on a diurnal basis from a small group of cumulonimbus clouds into a uniform mesoscale precipitation area having the characteristic structure of those observed in recent years over the tropical eastern Pacific and Atlantic oceans. In their mature stage the precipitation systems contain mesoscale anvil clouds commonly extending from near 500 mb to the tropopause covering a 104–105 km2 area. The ship observations provide direct evidence of mesoscale updraft motion within the anvil clouds and mesoscale down-draft below extending to near the surface.

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