Modification of the Boundary Layer over the South China Sea during a Winter MONEX Cold Surge Event

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

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Jeffery R. Zimmerman Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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Abstract

Aircraft dropwindsonde and conventional sounding data have been used to document the boundary layer structure over the South China Sea during the period of a 3-day moderate cold surge that occurred during the December 1918 Winter Monsoon Experiment (Winter MONEX).

Throughout the cold surge event, equatorward deepening of the cumulus layer is observed over the northern South China Sea (to the south of a coastal cloud-free region) as cold air streaming of the coast encounter substantial surface, sensible and latent heat fluxes. Following the onset of the cold surge, the cumulus-layer depth increases over the central South China Sea. However, this depth decreases over the northern South China Sea at a distance of 200–300 km from the China coast. The depth and strength of the cumulus (or subsidence) inversion are greatest new the China coast (∼600–700 m, 4°–6°C) and weaken toward the central South China Sea (∼200 m, 1°C), whew they eventually become undetectable. Possible explanations for the observed variations are suggested.

Abstract

Aircraft dropwindsonde and conventional sounding data have been used to document the boundary layer structure over the South China Sea during the period of a 3-day moderate cold surge that occurred during the December 1918 Winter Monsoon Experiment (Winter MONEX).

Throughout the cold surge event, equatorward deepening of the cumulus layer is observed over the northern South China Sea (to the south of a coastal cloud-free region) as cold air streaming of the coast encounter substantial surface, sensible and latent heat fluxes. Following the onset of the cold surge, the cumulus-layer depth increases over the central South China Sea. However, this depth decreases over the northern South China Sea at a distance of 200–300 km from the China coast. The depth and strength of the cumulus (or subsidence) inversion are greatest new the China coast (∼600–700 m, 4°–6°C) and weaken toward the central South China Sea (∼200 m, 1°C), whew they eventually become undetectable. Possible explanations for the observed variations are suggested.

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