Aircraft observations of a coastally trapped wind reversal off the California coast

The summertime marine atmospheric boundary layer off the California coast is normally characterized by northerly winds associated with the Pacific High. This pattern is occasionally (1-2 times per summertime month) disturbed by episodes of southerly winds and a finger of fog or low stratus adjacent to the coastline extending approximately 100 km offshore. These events propagate northward along the coast with speeds between 5-12 m s^-1 and have a lifespan of several days. Such phenomena have been referred to as coastally trapped wind reversals (CTWRs), coastally trapped disturbances, or southerly surges. These have been interpreted as Kelvin waves, topographically trapped density currents, internal bores, and as a response to a synoptically driven pressure gradient reversal that produces a trapped ageostrophic response. While there have been studies utilizing both observational data such as sounder and buoy data, and also numerical simulations, few in-situ airborne measurements of CTWRs exist. This was in part motivation for the Dynamics and Microphysics in Marine Stratocumulus field project conducted in May and June 2006. During this field campaign, one CTWR event occurred and was measured extensively.

The CTWR event of 22-25 June 2006 was a strong propagating event that initiated from the California Bight and traveled north until it stalled and decayed south of Cape Mendocino. The synoptic features were typical for a propagating event. The Pacific High tracked to the northeast and the resulting pressure field supported significant and persistent low-level offshore flow across the California coastline. The offshore flow implies subsidence as the air moves across the Sierra and coastal ranges and lee troughing became established. This was associated with record warm temperatures that occurred over the region.

This CTWR event was explored by the University of Wyoming King Air research aircraft to document the characteristics of the wind reversal in attempt to infer the forcing mechanisms responsible for the propagation. Flights from the morning and afternoon on 23 June are presented that are representative of a CTWR during its mature stage. At the time of the observations, the CTWR was located near Point Arena. Flight strategies included sawtooth maneuvers to measure the vertical structure of the CTWR and isobaric legs to measure the horizontal pressure gradient force (PGF) within the CTWR in both alongshore and cross-shore directions. Numerous legs were conducted over various scales with cross-shore legs reaching as close as 10 km to the shoreline.

These observations showed a general deepening of the CTWR layer in an alongshore direction to the south. The inversion layer is shown to vary substantially throughout the day. The first sawtooth leg of the morning flight depicted a uniform deepening of the CTWR layer with a constant vertical gradient of potential temperature within the inversion layer. By contrast, the final sawtooth leg in the late afternoon depicts clear dynamic destabilization within the inversion layer at the head of the CTWR and a suggestion of a secondary inversion developing near the surface due to the substantial convergence within the inversion layer. A PGF is present at the head of the CTWR that is directed northward. This is in contrast to the strong (nearly five times greater) PGF directed southward north of the CTWR. No significant PGF was detected in the cross-shore direction, nor were there large changes in the MBL depth or characteristics. This is confirmed by numerous cross-shore legs taken over this day which all indicate a lack of any significant PGF directed normal to the coast within the CTWR and no damming against the topography. This is in contrast to the significant variations seen between the CTWR region and the region outside the CTWR which represent the typical summertime low-level jet.

Cloud-top heights derived from the Wyoming Cloud Radar (WCR) reflectivities show a thin cloud layer beginning at the surface and extending up to a height of ~250 m within the CTWR. Little vertical wind shear can be inferred from these reflectivity images and this is confirmed by the horizontal dual Doppler velocities. WCR measurements suggest that drizzle is present and that it dominates the radar reflectivity. Reflectivities also emphasize the point that clouds are often not a good indicator of CTWR boundaries.