Topographic Control on Cloud Cover Variations Along the West Coast

Although cloud cover can be notoriously patchy, investigation of historical cloud cover at coastal stations in California, Oregon and Washington, reveals considerable variability that is coherent at 100's of km along the coastal margin. The variability between months is considerable, with monthly fractional cloud cover ranging from about 20% to more than 75%. Depending on season, between 25 and 50% of the variance in cloud fraction over the Western U.S during 1973-1995 can be explained by one pattern, with the first empirical orthogonal function (EOF) spanning the entire west coast United States. When cloudiness variations are examined in finer grained detail, it is found that there is a distinct topographic influence. Using a 2km resolution set of GOES satellite measurements from four years between 2003 and 2006, significant spatial coherence exists over a large horizontal span of the California coastal region, especially concentrated to the west of the first major coastal mountain front. The topographic influence is strongest in summer and weakest in winter, and is clearly important in determining how eastern North Pacific stratus cover affects the landscape over the coastal lowlands, and to some extent bounding the stratus from the interior. In the warm season months, there is a striking diurnal variability, which in turn is modulated by synoptic scale weather events. These shorter period cloudiness fluctuations, at time scales of a day to a week, are strongly affected by North Pacific/western North America weather patterns, but these, in turn, are evidently modulated by larger scale climate anomalies. Seasonally, the first few patterns of monthly cloudiness variability are correlated to Pacific climate modes, amongst them the El Nino/Southern Oscillation (ENSO) in the sense that warm phases of ENSO favor higher cloudiness.