Impacts of coastal advection fog on the surface radiation regime

Clouds and other aerosols greatly alter the nature of the surface radiation budget, particularly in reducing the quantity and changing the quality of solar radiation incident upon the surface as well as enhancing down welling longwave radiation. This study aims to characterize the surface radiation budget regime in southwest San Francisco, CA, USA, and in particular to examine the radiative control and signature of the ubiquitous coastal Californian fog. Direct and diffuse solar radiation and thermal infrared radiation data were recorded at a monitoring station at San Francisco State University at 15-minute intervals over the period June 2005 to August 2008. Indices are developed to characterize the radiation signatures of different atmospheric conditions, principally summer advection fog, winter frontal clouds, air pollution and smoke. The resulting surface radiation budget produces a distinctive seasonal radiation regime that is unique to the narrow coastal strip, producing striking contrast in surface energy balances over small spatial scales. The energetic impacts are quantified as well as the condition of light (diffuse fraction) available for coastal plants. In addition, the use of radiation signatures for the detection of fog is particularly promising and allows us to build a robust climatology of fog occurrence for both day and night hours.