Aircraft Observations of a Coastally Trapped Wind Reversal off the California Coast

During summertime months the low-level airflow over the eastern Pacific is dominated by the circulation associated with the Pacific High typically situated about 1000 km off the west coast of the United States. Subsidence above the Pacific High establishes a sharp temperature inversion at the top of the well-mixed marine boundary layer (MBL) with its depth increasing offshore to the west. A low-level coastal jet (CJ) is commonly found embedded within the broad northerly winds in response to this sloping inversion (e.g., Burk and Thompson 1996). The northerly wind regime is disrupted at times by coastally-trapped wind reversals (CTWRs, also referred to as coastally trapped disturbances or southerly surges) characterized by southerly wind and fog or stratus adjacent to the coastline that extends westward about 100 km (e.g., Ralph et al. 1998 and Nuss et al. 2000). Propagating CTWRs have a common synoptic signature as described in Mass and Bond (1996) and Nuss (2007). From nearly zonal geopotential heights at 500 hPa, a ridge over the eastern Pacific amplifies and tracks slowly to the east. The surface high tends to strengthen about two days before the event and track northeast while the existing trough over central California amplifies toward the coast. Low-level offshore flow develops advecting the warm continental air offshore, which is further warmed as it subsides down the elevated coastal topography. As a result, the MBL is lowered and this is often associated with a fairly weak horizontal pressure gradient force (PGF) near the coast. A number of conceptual models of CTWRs have been proposed, but the primary mechanism that causes and maintains this reversal is not currently widely accepted. Coastal mountains with heights about 300-800 m exist along the west coast with varying topographic features including a significant gap near San Francisco. The terrain is generally above the MBL, providing a vertical boundary. This two-layer system with a lateral