CASE STUDIES OF FOG AND LOW CLOUDS IN THE WASHINGTON, D. C. MESONETWORK AREA.

The spatial representativeness of surface observations at the U.S. Weather Bureau mesonetwork surrounding Washington, D. C. was investigated using autocorrelation coefficients. Data consisted primarily of ceiling, visibility, atmospheric transmission, and surface wind observations from five periods (cases) of low cloud and fog during three winter months. Closer-than-conventional station spacings, while generally too large for detailed analysis of visibility and transmission changes, permitted study of low-cloud formation, movement, and dissipation. Network data revealed air-flow disturbances including meso-scale lowpressure centers, windshift lines, and weak cyclonic vortices imbedded in larger-scale low-pressure troughs. Low cloud and fog, associated with some distrubances, were tracked across the network. Low-level wind divergence from time-space means were computed for two areas in each case; these divergence values were very useful for identifying and analyzing both moving and stationary distrubances. Level-of-saturation changes due to advective changes in temperature and specific humidity over sloping terrain, computed for one case, agreed reasonably well with observed ceiling changes. A canopylike structure along the forward edge of a moving lowcloud band, in one case, was attributed to vertical wind speed shear. Evidence was found of a reduction of wind speed in the Potomac River Valley south of Davison that significantly affects the behavior of low cloud and fog. Under bands or blankets of low clouds, highest station elevations experienced lowest visibilities and most persistent fog; the higher stations experienced less fog during periods of fog blanket within a shallow inversion. (Author)