Cloud and Convection Frequencies over the Southeast United States as Related to Small-Scale Geographic Features

Abstract

Visible and infrared data from the GOES West satellite were collected at 0700 CST and at each hour from 1000 CST to 1700 CST during summer 1986. Use of relatively high spatial and temporal resolution satellite data allowed study of local area cloud variations over broad regions in many new ways. Cloud frequency charts were computed for the area from Mississippi cast to Georgia and the Gulf of Mexico north to Tennessee for each of the nine hours as well as convection frequency charts of four convection intensifies as defined by the temperature of the cloud top. Strong diurnal cloud variations were observed. These new data analyses show an average maximum cloud frequency near 45 percent over the land areas at about 1400 local time. The maximum of deep convection, about 8 percent, was one hour later. Cloudiness and deep convection were at a maximum during the nocturnal hours over the Gulf of Mexico. Cloud frequency shows a strong relationship to small terrain features. Small fresh water bodies have cloud minima relative to the surroundings in the afternoon hours. The higher, steep terrain shows cloud maxima and the adjacent lower terrain exhibits afternoon cloud minima due to divergence caused by the valley to mountain breeze. The sea breeze-induced convergence causes relative cloud maxima over Gulf of Mexico near-shore land areas with the stronger maxima and greater areal coverage over peninsulas. Peninsulas that are of a similar scale or larger as compared to that of the convective cells show a late afternoon maxima of deep convection. Small scale geographical features such as small coastal islands and reservoirs show no relationship to deep convection in the frequency analysis.