THE NATURE AND DETECTABILITY OF CLOUDS AND PRECIPITATION AS DETERMINED BY 1.25-CENTIMETER RADAR

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Abstract
A survey is made of the clouds and precipitation detected at a wavelength of 1.25 centimeters. The frequencies of various cloud types and their typical echo characteristics are presented. Within 89 periods during which echoes were received, slightly less than 50 per cent of the echoes could be classed as echoes from internationally defined clouds. Detailed correlation of cloud echoes with the visual clouds present show that, provided each cloud type observed is assigned equal weighting, about 47 per cent of the visual clouds were detectable. The radar characteristics of common cloud types and the conditions favoring radar return from individual types are discussed. With most types, cloud temperature is indicated as being a critical factor governing detectability. This fact and the frequent occurrence of ice-crystal streamers indicate that the presence of ice crystals favors detectability. The detection of certain non-visible clouds is cited, and explanations for their presence are offered. The detectability of water clouds as a function of drop size and liquid-water content is discussed quantitatively. Radar theory and radar calibrations are used to determine altitude limits to which common clouds as sampled by Diem would be detected. It is found that the 1.25-cm radar used would detect only about 15 per cent. of these clouds at a range of 1 mile or less. “Clouds” detected at ranges greater than 10,000 feet must, therefore, be comprised either of relatively large water particles or ice crystals. The characteristic features of stratiform precipitation are discussed, and a preliminary correlation is made to determine the conditions necessary and sufficient to the release of such precipitation. Cloud echo-top temperatures between −10 and −20C and cloud echo-thicknesses in excess of 10,000 ft are especially favorable to the onset of precipitation. The ice phase is also found to be critical to the formation of strati-form precipitation (other than drizzle) in New England (although indications are that some showers may form entirely in the water phase). The ice crystals generally occur in the form of streamer-like trails, which may either fall into lower decks acting as natural seeding elements, or may themselves constitute the major portion of the precipitation. The streamers originate in cumuliform cells which are frequently of limited vertical extent.

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