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 turbulence are investigated for a warm front as it approached the prominent terrain of the Pacific Northwest in the vicinity of Vancouver Island. Flightlevel and airborne Doppler radar measurements collected from a National Oceanic and Atmospheric Administration P-3 research aircraft on 9 December 1995 during the Coastal Observation and Simulation with Topography experiment and 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 approached the coast and the structure of cool air trapped along Vancouver Island. This air mass was capped by a flat, thin transition zone of enhanced static stability and vertical wind shear. The flow within the trapped air mass 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 jet. Model sensitivity experiments suggest that the cold air that exited from the strait and the steep topography of Vancouver Island acted in concert to impede the northward movement of the front and steepen it near the surface. A wind speed maximum near the exit of the Strait of Juan de Fuca was consistent with transcritical expansion fan behavior. Both the observations from 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 near Vancouver Island.