Numerical Study of the Diurnal Cycle along the Central Oregon Coast during Summertime Northerly Flow

Abstract

A triply nested mesoscale atmospheric numerical model is used to study the dynamics of the diurnal cycle of the summertime lower atmosphere along the central Oregon coast. Simulations of four consecutive days in September 1998, during which the winds were strong and northerly, are analyzed. Comparisons with profiler observations suggest that the model performed well enough to provide a useful estimate of the diurnal circulation. During the four days of interest, the low-level wind pattern has a broad maximum between Cape Blanco and Cape Mendocino, with a large north?south gradient along the Oregon coast. The low-level jet undergoes diurnal horizontal and vertical displacements, which partially resemble previous observational and modeling results along the California coast. In both the model and the profiler data, there is a minimum in northerly wind between 1500 and 1800 UTC (0700 and 1000 local time), and a double maximum in offshore flow above the marine boundary layer, with peaks near 0700 and 1600 UTC. At the jet core height, the advection of alongshore momentum is an important component of the alongshore force balance. After 2100 UTC, this advection is the main term balancing the pressure gradient force. Thus, in contrast to the previous results for the California coast, the diurnal circulation is fundamentally three-dimensional in the coastal zone, for several hundred kilometers alongshore and as far as 100 km offshore. The blocking effect of coastal terrain has a strong influence on the diurnal circulation.