WIND ESTIMATION FROM GEOSTATIONARY-SATELLITE PICTURES

Abstract

The motion of properly selected clouds derived from photographs taken by the geostationary Advanced Technology Satellites provides estimates of the wind at cloud level. A large number of low- and high-cloud motions near rawin stations are compared to the balloon-derived winds to determine the levels at which these wind estimates might be used for map analyses and to assess their accuracy at those levels. For this sample, velocities of lower clouds correspond best to winds at 3,000 ft, while upper cloud velocities correspond best to winds at 30,000 ft. The median vector deviation of the cloud velocities from observed winds is 9 and 17 kt at 3,000 and 30,000 ft, respectively. Direction deviations are modest; therefore, these wind estimates are representative of the actual flow patterns in the lower and in the upper troposphere. Causes contributing to deviations are (1) uncertainty of cloud height, (2) nonadvective cloud motion, (3) photograph-measurement errors, (4) tracking errors, and (5) unrepresentative rawinsonde observations. The principal sources of cloud-velocity deviation from observed winds are ranked in order of their significance to identify the critical areas needing improvement. It appears that cloud-height uncertainty is the most significant cause of deviations, particularly about upper cloud heights. Work aimed at reducing this uncertainty is just beginning. To point out the unique problems involved, we discuss the technique of selecting and classifying cloud targets. The procedure is a subjective skill dependent upon the analyst's meteorological judgment. Despite the problems?many as yet unsolved?the single earth-synchronous satellite, with its immense areal coverage and high frequency of coverage, provides an important new source of data from remote regions.