A Multiple-Case Analysis of Nocturnal Radiation-Fog Development in the Central Valley of California Utilizing the GOES Nighttime Fog Product

Abstract

Radiation fog in the Central Valley of California has received very little attention in terms of climatological research. This study uses the Geostationary Operational Environmental Satellite (GOES) nighttime fog product to develop a sequence of images and datasets that reveal patterns of nocturnal radiation-fog development in the Central Valley. Twenty long-lived, spatially extensive radiation-fog episodes, occurring from October through January, were selected for the period of 1997?2000. Mean hourly parameters for fog cover, fog development rate, and vertical development were calculated for the 20 episodes in the Central Valley. The study region is separated into five analysis divisions oriented from south to north for spatial comparisons within the valley. Large-scale radiation fog begins developing before 1800 LST, and rates of development vary widely from south to north. Radiation fog develops earlier and covers a larger area of the southern valley as compared with the central and northern portions of the valley. The horizontal extent of radiation fog is maximized at 0600 LST in the southern valley and near midnight in the central and northern parts of the valley. Vertical development reaches 300 m with regularity in the southern valley. Radiation-fog development of greater than 300 m occurs primarily in the early morning hours. Vertical development ?bursts? are also observed in the southern valley during the morning hours. Climatologically important conditions for radiation-fog development in the Central Valley include cool 1600 LST surface temperatures, moisture availability as reflected by 1600 LST dewpoint temperatures, early evening surface cooling trends, the rapidity with which mean relative humidity reaches 90%, and the presence of cool, dry air aloft (700?500 hPa).