DROP SIZE AND RADAR STRUCTURE OF A PRECIPITATION STREAMER

David Atlas Air Force Cambridge Research Center

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Abstract

A detailed analysis is presented of the structure of an isolated precipitation streamer and of its drop-size history observed at the ground. The pattern aloft and the reflectivity structure through the melting zone are consistent with the release of fully grown graupel (equivalent drop diameters up to 2 millimeters) from a generating cloud some 2500 feet in depth. A sharp reflectivity increment in the lower layers is shown to exceed that which can occur as a result of raindrop growth in cloud. Instead, the increment is attributed to a horizontal variation in the particle sizes released by the generator and manifested as a vertical reflectivity gradient when carried into the radar beam from the side. Particle trajectories and drop sizes provide auxiliary information regarding the extent, orientation and motion of the parent cloud. Differences in the characteristics of the drop-size spectra from previously established empirical relations are also discussed. A discussion of the “bright band” theory of melting graupel is included.

Abstract

A detailed analysis is presented of the structure of an isolated precipitation streamer and of its drop-size history observed at the ground. The pattern aloft and the reflectivity structure through the melting zone are consistent with the release of fully grown graupel (equivalent drop diameters up to 2 millimeters) from a generating cloud some 2500 feet in depth. A sharp reflectivity increment in the lower layers is shown to exceed that which can occur as a result of raindrop growth in cloud. Instead, the increment is attributed to a horizontal variation in the particle sizes released by the generator and manifested as a vertical reflectivity gradient when carried into the radar beam from the side. Particle trajectories and drop sizes provide auxiliary information regarding the extent, orientation and motion of the parent cloud. Differences in the characteristics of the drop-size spectra from previously established empirical relations are also discussed. A discussion of the “bright band” theory of melting graupel is included.

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