Numerical Study of a Cold Air Outbreak over the Ocean

Abstract

A cold air outbreak over the East China Sea was simulated by using a high-resolution mesoscale planetary boundary layer model developed at Purdue University. The model includes the major physical processes such as surface fluxes of heat and moisture, condensation and evaporation, and vertical turbulent transfer of heat, moisture, and momentum. The distribution of specific humidity during the passage of the front reveals that moisture is pushed upward along the frontal surface as observed. The cold dry air behind the cold front is quickly modified by strong convection over the warm water surface, especially over the Kuroshio. A cloud-free region exists near the coast where the air is very cold and dry. As air moves over the water and undergoes modification, a layer of stratocumulus forms downstream from the cloud-free region. The depth of the convective boundary layer increases toward the Kuroshio. In the horizontal momentum equations, the diffusion term almost balances the ageostrophic force in the convective layer. The numerical results compare favorably to the AMTEX '74 observation.