Distributions of Radar Echoes Over the United States

Book excerpt: The frequency and probability of radar echoes of convective clouds over the United States are presented. Analysis of three years of observations from a 31-station WSR-57 weather radar network indicates that at all altitudes, radar echo probabilities are greatest over Florida and the Gulf coast, generally decreasing northward over the United States. Also, largest probabilities for most layers and locations occur in summer. Echoes have been reported up to at least 70 kft in May, June, and July, and up to 60 kft in winter. Diurnal variations reveal greatest probabilities between 1600 and 2100 LST in all regions. Largest mean monthly 3-hour values are 85 percent, and the maximum mean daily range is roughly 65 percent in the southeast during the summer months. The probability of an echo-free horizontal view near the earth's surface for a 100-mile range is also presented. Tabulations of echo-free sectors, as percent of the 360-degree radar scope, show that the probability of obstructions to a horizontal view increases generally from northwest to southwest during all seasons. The probability of having no echoes is greater in winter than in summer except along the Pacific coastal region. Diurnal variability is larger in July than in January. (Author)