Sea fog along the California coast in response to synoptic forcing

During the cool season, fog events along the California coast are governed by the transient weather systems that move from the oceanic areas onto the continent. The frequency of the frontal passages, the strength of subsidence associated with the anticyclonic component of the system, the air mass characteristics of invading air, and the trajectory of the air with respect to the underlying sea surface, are key factors that are germane to the occurrence or absence of fog. Furthermore, we believe it is imperative that a fog episode during this season be viewed in the context of the sequence of synoptic events, i.e., the precedent as well as the current event has bearing on the fog episode. The precedent event serves to precondition the marine layer and establish a basic state that is conducive to fog. We have chosen to examine fog along the California coast during the month of April 1999. Two fog episodes occurred during this month, one covering a 36-h period (April 14-15), the other a 12-h period (April 18). This is close to the April average of 72 h of fog at California?s coastal stations. We pay particular attention to the synoptic events of 8 - 12 April (event A) and 12 - 15 April (event B), where event A is fog-free, and event B is associated with fog as mentioned above. We show that synoptic event A preconditions the marine layer and sets the stage for fog formation during synoptic event B. Processes that lead to fog in event B can only be understood if a Lagrangian viewpoint is adopted, i. e., the trajectories of the marine air must be taken into account rather than viewing the process at a fixed point in space (Eulerian view). The air associated with fog in the Point Conception area (southern California) came from an area far to the north. The air moved from waters off the California - Oregon border to the California Bight ( the wide bay at the southern extreme of California) over a 2 - 3 day period. It has been argued that large - scale forcing from the atmosphere (subsidence) and the ocean (sea surface temperature structure) are crucial to fog formation in this case.