Cessation of the 22–25 June 2006 Coastally Trapped Wind Reversal

Abstract

Coastally trapped wind reversals (CTWRs) occur periodically in the marine boundary layer off the western coast of the United States and dramatically change the low-level wind regime and coastal weather. Southerly flow becomes established with the passage of a CTWR along with cooler temperatures and a low stratus deck in a narrow band along the coast. CTWRs can propagate northward along the coast for hundreds of kilometers. A strong CTWR commenced in southern California on 22 June 2006 and moved north along the California coastline before stalling at Cape Mendocino on 24 June 2006. A transcritical Froude number differentiates the CTWR layer from the climatologically favored northern wind regime to the north of Cape Mendocino and indicates a barrier to the movement of a density current. A well-defined cloud boundary is present as detected by radar and satellite imagery and sharp gradients exist in the basic-state parameters as measured by instrumented aircraft. As the Pacific high migrates back offshore, the horizontal pressure field near Cape Mendocino becomes increasingly adverse to the continued northward movement of the CTWR layer and typical summertime conditions are reestablished. Observations and modeling results show that the cessation phase of this CTWR was characterized by a surprising lack of topographic blocking due to the mountainous coastal terrain and Cape Mendocino massif. The horizontal pressure field over the ocean to the north of the CTWR was the key impeding force to continued propagation of this event.