AIRBORNE DUAL-DOPPLER OBSERVATIONS OF AN INTENSE FRONTAL SYSTEM APPROACHING THE PACIFIC-NORTHWEST COAST

Airborne Doppler radar data, collected off the Pacific Northwest coast by a NOAA WP-3D Orion aircraft over an 8-h period on 8 December 1993 during the Coastal Observations and Simulations with Topography experi ment, reveal the mesoscale structure of an intense frontal system whil e it was well offshore and as it approached within 20 km of the Oregon coastline. During the offshore stage, a portion of the narrow cold-fr ontal rainband was characterized by deep convective cores. Pseudo-dual -Doppler analyses characterize the kinematic and precipitation structu re of the deep convection. Pseudo-dual-Doppler analyses describe the s ubsequent evolution of the narrow cold-frontal rainband as it approach ed to within 20 km of the Oregon coast. Deformation of the frontal zon e appeared to cause the dissipation of one of three precipitation core s contained within the dual-Doppler area. The precipitation cores and the strong convergence zone associated with the front conformed to som e degree to the shape of the coastline near Cape Blanco, Oregon, as th e front neared the coast. Changes in the prefrontal flow that occurred as the front approached the coast were qualitatively consistent with theoretical and numerical studies of upstream orographic influence. Co mparison of pseudo-dual-Doppler-derived velocity profiles with idealiz ed numerical model calculations suggests that the nearshore evolution of the frontal rainband was significantly affected by an upstream infl uence of the coastal orography.