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John D. Locatelli, Peter V. Hobbs, and Kumud R. Biswas


Cloud and precipitation processes in a stratocumulus cloud layer (∼1 km thick) were investigated by means of airborne, radar and ground observations for three situations: 1) the stratocumulus alone, 2) fallstreaks from altocumulus falling into the stratocumulus, 3) regions of stratocumulus not appreciably affected by fallstreaks but strongly affected by artificially seeded dry ice.

In case 1) the cloud was composed primarily of supercooled droplets. In cast 2) dendritic ice crystals in the fallstreaks increased their mass by riming as they passed through the stratocumulus; derived precipitation rates for this case were ∼0.02–0.08 mm h−1. In addition, it appeared that the dendrites provided a source for high concentrations of needle crystals in the stratocumulus; these crystals were estimated to give a precipitation rate of ∼0.01–0.03 mm h−1. In case 3) high concentrations of needle crystals were produced by the dry ice seeding and it was deduced that these also produced precipitation rates ∼0.01–0.03 mm h−1. Some implications of the results for areal artificial seeding experiments are discussed.

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Peter V. Hobbs, Nathan T. Funk, Richard R. Weiss Sr., John D. Locatelli, and Kumud R. Biswas


A 1960 35 GHz radar has been modernized through the use of solid state electronics, Dopplerization and improved data-display capabilities. Radars of this frequency are particularly useful for observing the internal structures of clouds and for detecting low concentrations of ice particles in the atmosphere.

The minimum effective radar reflectivity factor of a cloud of water drops that is measurable by this radar at a range of 1 km was estimated to be −36 ± 4 dBZ. Simultaneous airborne and radar measurements showed that the radar reflectivity factors for various water clouds determined from radar measurements were generally in good agreement with those derived from in situ measurements of the drop size spectra. These measurements also showed that the radar can detect clouds in which the diameters of the droplets do not exceed ∼27 μm provided there are sufficient concentrations of 10–15 μm diameter droplets. Clouds containing only 1 L−1 of 100 μm diameter ice crystals (corresponding to a mass concentration of ∼10−3 g m−3) are detectable by the radar.

Due to its narrow beamwidth (0.26°), reflectivity measurements with the 35 GHz radar can reveal more detailed structural information on clouds and precipitation than more powerful 5.5 GHz radars.

The 35 GHz radar has been Dopplerized using a new magnetron phase correlation technique that allows detailed measurements to be obtained of the velocities of particles down to ∼25 cm s−1. Some examples are presented of measurements of the spectra of the vertical velocities of cloud particles using this technique.

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Robert A. Houze Jr., Peter V. Hobbs, Kumud R. Biswas, and William M. Davis


Mesoscale rainbands (5–50 km in average width and hundreds of kilometers in length) have been found in eleven extratropical cyclones. Six types of rainbands have been identified (warm frontal, warm sector, cold frontal–wide, cold frontal–narrow, wave-like, post-frontal). Comparison with extratropical cyclone data from various parts of the world indicates that these rainband types occur rather generally in midlatitude cyclones.

The rainbands examined in this study contained small-scale areas of especially concentrated rainfall. These small-scale elements were 10–500 km2 in area, occurred in concentrations of 1–3 per 1000 km2, and moved with the wind between 850 and 700 mb.

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