Research aircraft observations and numerical simulations of a warm front approaching Vancouver Island

The mesoscale structure of the low-level flow and the character of the turb ulence are investigated for a warm front as it approached the prominent ter rain of the Pacific Northwest in the vicinity of Vancouver Island. Flight-l evel and airborne Doppler radar measurements collected from a National Ocea nic and Atmospheric Administration P-3 research aircraft on 9 December 1995 during the Coastal Observation and Simulation with Topography experiment a nd high-resolution numerical simulations, performed with the Naval Research Laboratory's nonhydrostatic Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) model, have been used to document the warm front as it appr oached the coast and the structure of cool air trapped along Vancouver Isla nd. This air mass was capped by a flat, thin transition zone of enhanced st atic stability and vertical wind shear. The flow within the trapped air mas s was oriented parallel to the terrain of Vancouver Island, but appears to have been more an outflow from the Strait of Juan de Fuca than a barrier je t. Model sensitivity experiments suggest that the cold air that exited from the strait and the steep topography of Vancouver Island acted in concert t o impede the northward movement of the front and steepen it near the surfac e. A wind speed maximum near the exit of the Strait of Juan de Fuca was con sistent with transcritical expansion fan behavior. Both the observations fr om the gust probe system on the P-3, and output from COAMPS, indicate that the turbulent kinetic energy at low levels was relatively weak in the warm sector, moderate in the frontal zone, and strongest in the trapped flow nea r Vancouver Island.