VARIABILITY OF MARINE FOG ALONG THE CALIFORNIA COAST Maria K. Filonczuk, Daniel R. Cayan, and Laurence G. Riddle Climate Research Division Scripps Institution of Oceanography University of California, San Diego La Jolla, California 92093-0224 SIO REFERENCE NO. 95-2 July 1995 Table of Contents 1. Introduction ......................................................................1 2 . Climatological and Topographic Influences ................................2 2.1. General Climatology ...................................................2 2.2. Topography .............................................................3 3 . Background and Objectives ...................................................5 3.1 . Fog Mechanisms .......................................................5 3 .2. Objectives ...............................................................6 4 . Data .............................................................................6 5 . Climatology of West Coast Fog .............................................13 5.1. Seasonal Variability .................................................. 13 5.1.1. Coastal Station Fog Climatology .................................. 13 5.1.2. Marine Fog Climatology .............................................16 5.2. Interannual Variability ...............................................21 5.2.1. Coastal Stations .......................................................21 5.2.2. Marine Observations .................................................31 6 . Local Connections with Marine Fog ....................................... 31 6.1. Sea Surface Temperature ........................................ 31 6.2. Sea Surface-Air Temperature Difference ..........................36 6.3. Wind Directions and Speeds .......................................36 7 . Large-Scale Atmospheric Connections ....................................54 7.1. Seasonal Atmospheric Circulation ................................58 7.2. Daily Atmospheric Circulation ....................................58 8 . Spatial Patterns of Marine and Coastal Fog ...............................64 8.1 . Seasonal Fog Distribution ..........................................64 8.2. Spatial Coherence of Monthly Fog Anomalies .................... 69 8.3. Daily Fog Distribution ............................................... 73 9 . Summary and Conclusions .................................................75 Acknowledgments ...................................................................77 References ........................................................................... 78 Appendix A ........................................................................... 79 Appendix B ...........................................................................83 Appendix C ...........................................................................89 Appendix D ........................................................................... 93 Stratus and Fog at the Golden Gate Bridge Photograph of the Golden Gate Bridge showing stratiform clouds and fog covering portions of the bridge. The view is from approximately over Lime Point (north end of the bridge) looking south toward Fort Point and the City of San Francisco. This photograph illustrates the patchy nature of coastal fog, especially at the San Francisco Bay-North Pacific Ocean transition. Photograph courtesy of the late Dr. Albert Miller, Department of Meteorology, San Jose State University. 1 Introduction who pointed out that the San Diego area has neither cold, upwelled water nor strong SST Visibility in coastal regions has a significant gradients but still has a large number of impact on government, commercial, and dense fog occurrences. private sector activities. The primary phenomenon significantly affecting visibility Leipper ( 1948) defined three indicators of along the western United States coastal fog probability, and validated the occurrence regions is fog. Fog is a natural hazard to of fog on most days that the indicators were boating, commercial shipping, and other favorable. Local conditions that were waterway activities. identified as being favorable for Southern California fog development are: The West Coast of the United States has been identified as one of the major fog (I) a low temperature inversion (base <1 producing regions of the world. Present km); accuracy in predicting marine coastal fog and (2) a sea surface temperature cooler than low stratus clouds is limited. Although most the warmest air temperature below the weather forecasting has improved with recent inversion; and advances in atmospheric circulation models (3) a "least-fog" occurrence when sea and satellite observations, there is relatively surface temperature is more than 5°C little operational guidance for the prediction warmer than the late afternoon dew of marine and coastal fog. point temperature. Currently the forecasting of visibility relies Other local conditions found to be favorable mainly on the availability of local for fog formation in the Southern California observations and experience with local coastal area include: the dissipation phase of weather tendencies. National Weather Santa Ana conditions, a strong temperature Service and other forecasters typically focus inversion in the lower atmosphere, and the on selected local conditions which usually approach of an upper level trough off the presage fog formation and if a sufficient coast. Several of these factors are included number are present, they will forecast in Southern California operational possible fog for the region. However, this forecasting practices (NOCF, 1992; NOCD, level of information and experience is not 1993). Leipper's (1994) review of fog available in all coastal areas. Results in this studies on the United States West Coast study suggest that attention to the large-scale summarized factors useful in understanding circulation in addition to local conditions may West Coast fog including the formation of a lead to increased skill and accuracy in strong but relatively shallow inversion, the extended range forecasts. presence of cooler waters off the coast, and the depth of fog development. All of these Numerous studies have examined local factors are based on relationships between atmospheric conditions associated with fog physical variables, but have not been formation. Pettersen (1938) was one of the documented using extensive climatic data first to investigate the causes of fog along the sets. Several cases of synoptic scale fog California coast. He showed that fog is not development are presented in Appendix B. directly formed by cooling from below (due to the cold upwelled coastal water) but is Leipper (1948) devised a model qualitatively formed when the temperature inversion caps illustrating the evolution of fog during Santa the cool moist Pacific marine layer below Ana conditions. Fog generally develops at warm dryer air aloft. Results from this study the end of a Santa Ana condition because of indicate that sea surface temperature (SST) the formation of a low inversion, creating a connections are subtle and fog may develop thin marine layer with weak offshore flow. regardless of the presence of cold upwelled He later generalized this model to create a water. This is supported by Leipper (1994), four-phase sequence based on physical processes, and has applied it to other regions basic guidance to fog occurrences, but more along the West Coast (Leipper, 1994). In detailed wind and moisture observations the first phase, the Eastern North Pacific might permit more reliable forecasts. High is dominant over Northern California, causing an easterly (offshore) flow at the The large-scale structure and interannual surface, a condition known as a "Santa variability of fog have not been well Ana." Because of subsidence, the air described because data has been unavailable column (below 1 km) at the coast is warm and a large volume of data is required. In the and dry. In phase two, the easterly flow present analysis, several years of observa- weakens as the Eastern North Pacific High is tions from a set of coastal weather stations split over the mountains. An inversion and a set of comprehensive marine weather develops with its base below 250 m. Fog reports (primarily ships) is used to examine formation begins in a shallow layer offshore large-scale processes and local conditions below the inversion. The third phase begins which affect fog formation. A discussion of when northwesterly flow and the sea breeze fog observation criteria is given in Appendix return-then fog spreads inland along the A. In addition, evidence is provided by a coast. The inversion base in the third phase collection of observations of hours of fog- is between 250 and 400 m. In phase four, horn operations at several coastal sites over the marine layer expands above 400 m and 15 years, starting in 1950 (Appendix D). continues to deepen until stratus clouds form as the fog base rises above 45 m. A case The relationship between local and large- study of the development of fog along the scale conditions, and fog and stratus California Coast over the course of a Santa formation may yield improved predictability Ana event is shown in Appendix C. of fog. This study indicates that large-scale conditions, especially regional-hemispheric Results of marine fog studies conducted circulation, are a vital component of both during the 1970's were presented by high and low fog occurrences; moreover, Noonkester (1 979). Using surface-based atmospheric circulation in the Eastern North