Widespread dense fog can have large impacts on transportation systems, particularly aviation. However, our knowledge and ability to accurately predict the onset and duration of fog is limited. This study focuses on fog development associated with low clouds and precipitation, one of the least understood types of fog. A widespread dense fog case which developed in association with low clouds and precipitation, but with radiation processes clearly important, will be presented. Dense fog developed over much of Illinois during the early morning hours of November 7, 2007, in association with the passage of a low pressure system. During the nighttime hours preceding the onset of fog, the surface conditions were first characterized by a descending cloud base and then a period of light precipitation. Analyses of WSR-88D radar data indicated that evaporating precipitation from aloft played an important role in moistening the lower atmosphere and lowering of the cloud base. Immediately preceding fog formation, the near-surface atmosphere was nearly saturated, cloud bases were below 1000m, and weak warm air advection was observed in the region. Satellite imagery indicated that the deep clouds associated with the low pressure system departed most of Illinois by the early morning hours as precipitation ended in the region, but low clouds remained in place. Fog formed in an arc behind the departing deep clouds, suggesting that radiational cooling at the top of the cloud layer was instrumental in promoting early morning dense fog. A climatology of surface and prevailing weather conditions associated with dense fog events at Peoria, IL for the period 1970-1994 indicates that although fog cases with precipitation and low clouds have been little documented, such cases may be common.
|Original language||English (US)|
|State||Published - 2008|