Simulations of Cold Surges over Oceans with Application to AMTEX'75

Abstract

Comparative numerical experiments and observational verifications for cold surges over the ocean are conducted using a primitive equation planetary boundary layer (PBL) model. The comparative experiments illustrate the importance of large-scale environmental conditions on air mass transformation. Such factors as baroclinity, static stability, moisture content, upwind inversion strength and height exert strong controls on the downwind evolution of the PBL and clouds. Most results are obtained for clouds in the cumulus mode, but two comparisons are made with simpler stratocumulus simulations. The PBL flow is influenced by the synoptic geostrophic wind distribution and the PBL depth is also sensitive to large-scale vertical velocity. The responses of this advective PBL to stronger wind are found to be different from that of a horizontally homogeneous PBL. A simulation of an intense cold air outbreak observed during the Air Mass Transformation Experiment, 1975 (AMTEX'75) is performed. Downwind variations of dynamic and thermodynamic PBL properties and cloud distribution and type are well reproduced. The steep sea-surface temperature gradient produces strong PBL baroclinity and a strongly divergent PBL flow. The simulated large cross-isobar angle associated with intense cold air advection and vigorous momentum mixing is in favorable agreement with both observation and theory.