Cumulus Cloud Suppression

Late Morning Gulf Coast Cumulus
Figure1

This intriguing visible image from GOES 8 (Channel 1) at 16:45 UTC on May 7, 1997 (Figure 1) shows several of the major rivers in the Gulf Coast states as dark gray lines on a semi-white background. The zoomed image in Figure 2 (below) shows a close-up of southern Alabama and Mississippi and northern Florida where the effect is more pronounced than elsewhere. At the time of the image, most surface observation stations in the area were reporting conditions within a relatively narrow range. Horizontal visibilities were from 7 to 10 miles, temperatures ranged from 23.9 C (75 F) at Destin, FL near the coast to 28 C (82 F) at Pensacola and Eglin Air Force Base, and winds were calm or from the southeast at 4 to 9 knots. Dew points were also reported in a narrow range, generally from 13 C (55 F) to 18 C (64 F). The primary difference between observations was in the reported cloudcover. The reason why these differences occurred and their impact on the satellite image is explained below.

Skies were mostly clear in the eastern Gulf Coast states during the early morning hours of May 7, 1997, and temperatures and dew points were identical, or nearly so. Usually this condition portends fog development, but in this case this did not happen, primarily because a light breeze at most locations caused just enough mixing to prevent it from occurring.

After the sun rose, intense solar radiation began warming the land and parcels of air began to rise and cool. Eventually these parcels and their accompanying moisture rose high enough and cooled enough to form cumulus clouds. These are the puffy, "cotton-ball" clouds which are common on many summer days over much of the U.S. Though other weather mechanisms can induce their formation, the most common is through the process just described. On satellite photos, cumulus clouds show up as areas that look somewhat stippled as exhibited in Figure 1. Though the smallest individual cumulus clouds are normally smaller than the resolution capacity of the satellite, as the number and size of these small clouds increases over a given area, the contrast between cloud and land increases. Because of the distribution of small cumulus, terrain features such as rivers or lakes can be identified.

The biggest difference in the surface weather observations on May 7 is in cloudcover reports, and this offers the clue as to why the river valleys are dark in the image. Stations nearest the coast (Destin and Tallahassee, FL, Lake Charles, LA, and both Mobile WSO and Mobile Brookley (BFM), AL) reported clear. More inland stations and on the east side of Florida reported clouds with most bases reported from 3000 feet at Gulfport to 5500 at Jacksonville. (Some higher bases were reported at a few stations). From the satellite image, we know that these are cumulus clouds forming in the late morning hours over the land areas warmed by the early May sunshine.

Nearby river valleys remain somewhat cooler by the presence of river waters which are slower to respond to the sunshine than the nearby land areas. In this case, the difference in temperature was just enough to result in cumulus clouds over the land areas, whereas over nearby river valleys, cumulus development was suppressed and the valleys remained clear. From the satellite's perspective, the clear river valleys are darker than the partially cloud-covered land areas. A specific example of this process can be found by examining surface weather observations from Mobile WSO and nearby Mobile Brookley (BFM) field. At 11:00 AM CST (17:00 UTC), both stations reported clear skies. An hour later, Mobile WSO reported a few clouds with bases at 4800 feet, whereas Brookley Field, 185 feet lower in elevation and closer to the cooler Mobile Bay remained clear.