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 experiment, reveal the mesoscale structure of an intense frontal system while it was well offshore and as it approached within 20 km of the Oregon coastline. During the offshore stage, a portion of the narrow coldfrontal rainband was characterized by deep convective cores. Pseudo-dual-Doppler analyses characterize the kinematic and precipitation structure of the deep convection. Pseudo-dual-Doppler analyses describe the subsequent evolution of the narrow cold-frontal rainband as it approached to within 20 km of the Oregon coast. Deformation of the frontal zone appeared to cause the dissipation of one of three precipitation cores contained within the dual-Doppler area. The precipitation cores and the strong convergence zone associated with the front conformed to some degree to the shape of the coastline near Cape Blanco, Oregon, as the 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. Comparison of pseudo-dual-Doppler-derived velocity profiles with idealized numerical model calculations suggests that the nearshore evolution of the frontal rainband was significantly affected by an upstream influence of the coastal orography.