Sea fog off the California coast: Viewed in the context of transient weather systems

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Abstract

Widespread sea fog off the California coast occurred in conjunction with a transient weather disturbance in mid‐April 1999. In this paper we examine the larger‐scale influences that have bearing on the fog formation and its dissipation. Furthermore, this event is viewed in the context of the monthly changes of weather along the California coast. We compare and contrast the fog event with the precedent synoptic event that was fog‐free in an effort to understand the sensitivity of fog formation to large‐scale parameters. The principal results of the study are (1) the sea fog is characterized by long over‐water trajectories in persistent northwesterly flow. The sea surface temperatures (SSTs) are greater than the air temperature along these trajectories and gradual lowering of the stratus in the presence of strong subsidence eventually produces fog. (2) The synoptic‐scale subsidence associated with the anticyclone in these transient weather systems is a critically important factor in sea fog formation: a persistent, large‐magnitude subsidence favors fog by decreasing the depth of the marine layer while increasing the strength of the inversion. (3) Trajectories obtained by back‐tracking foggy and fog‐free air in the fog event of mid‐April 1999 make it clear that air with a path history over land is less likely to be associated with sea fog. In short, the demarcation between foggy and fog‐free zones is related to the differences in path history. (4) The occurrence of sea fog depends on the frequency of the transient weather systems, where an extended period of air mass transformation favors fog formation.

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