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Abstract-Repeated changes associated with lightning have been observed with multiparameter radar in the echoes from the tops of Florida thunderstorms. These lightning-related radar signatures are interpreted as changes in the orientation of ice crystals being preferentially aligned parallel to the in-cloud electric field. The changes occur at intervals on the order of 10 s and are easily observed in the signatures of the differential propagation phase shift and the linear depolarization ratio which are sensitive to propagation effects caused by the oriented ice crystals. Orientation of ice crystals aloft has been previously observed using circularly polarized radar while the simultaneous differential phase shift and linear depolarization measurements reported here were obtained with a dual-linear polarized radar. The observations indicate crystal orientation angles greater than 45" and occasionally near vertical. In one case, the crystals were found to be oriented in a layer near radar cloud top spanning a 20-km range and 3 km in depth.

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SUMMARYDifferential radar reflectivity data are presented which indicate that some initial echoes of convective clouds arise from a small number of very large raindrops; these raindrops may be supercooled. Typically raindrop concentrations are three orders of magnitude lower than the average Marshall-Palmer drop size distribution found in mature clouds. An explnn;ition of these observations in terms of the swecp-out of cloud droplets by a low concentration of ultra-giant nuclei of radii between 30 and 100 pm is offered. A simple model of this process, initiated using the avcragc background levels of these nuclei, gives good agreement with the radar data. The model is not critically dependent upon the choice of collection efficiencies of the nuclei for the cloud droplets nor on the precise form of the ultra-giant nucleus spectrum. The ultra-giant nuclei seem capable of explaining the appearance of the raindrops without invoking complex mixing processes within clouds or appealing t o the stochastic nature o f the collisions hetween cloud droplets.

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