. Compendium of meteorology. Meteorology. Fig. 8.—Photograph of RHI scope showing tall (snow) shower with abrupt shear below the altitude of 9000 ft. Lack of shear in showers more distant from the radar can best be explained by nonuniformity of the wind field. Storms are ap- proaching the radar; shear therefore indicates decreasing wind velocity with altitude (in the plane of the picture) up to the point where echo signal edges become nearly vertical. Actual slope of shower in shear zone about 80° from the verti- cal (X-band radar). { Weather Radar Research.) precipitation echo signal is
. Compendium of meteorology. Meteorology. Fig. 8.—Photograph of RHI scope showing tall (snow) shower with abrupt shear below the altitude of 9000 ft. Lack of shear in showers more distant from the radar can best be explained by nonuniformity of the wind field. Storms are ap- proaching the radar; shear therefore indicates decreasing wind velocity with altitude (in the plane of the picture) up to the point where echo signal edges become nearly vertical. Actual slope of shower in shear zone about 80° from the verti- cal (X-band radar). { Weather Radar Research.) precipitation echo signal is vertical, the cell is embedded in air moving with constant velocity and direction. Where the echo signal slopes, the precipitation par- ticles are descending into air moving at a different velocity along the direction of the radar beam. This difference may be due to a velocity differential with height, a direction differential, or a combination of both. The slope of the precipitation is a function of the fall velocities of the particles giving the echo and the wind velocities and directions involved. Unless the fall velocity is known, only relative wind velocities and directions may be calculated from observations of this type [21, 41]. In Fig. 8 it will be noted that in the actual storm the trajectory of the particles was almost hori- zontal in the shear zone; the 10:1 vertical expansion increases the slope on the scope by this factor. With increasing use of narrow-beam radars for weather observation, hitherto unsuspected phenomena have been observed in conjunction with warm-front precipitation. Precipitation is occasionally observed to form in narrow, closely spaced parallel bands as shown in Fig. 9. The formation is suggestive of billow clouds formed by shear waves, and is in all likelihood partially due to the same action as that which causes clouds of this type. The phenomena are rather transient, usually persisting for only ten or fifteen minutes in any par- ticular a
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