Precipitation streaks as a cause of radar upper bands

Several workers have reported enhanced radar echoes, from regions well above the freezing level, in layer‐type rain clouds. Usually, there is a decrease with time in the height of these ‘upper bands,’ as Bowen has called such enhanced echoes. This paper describes an investigation into upper bands which has been made at Cambridge, using a 3‐cm radar with a vertically directed beam. The mean downward velocity of upper bands at a height 9,000 ft above the freezing level is found to be about 20 ft/sec, the velocity generally decreasing systematically with time. Such velocities are much greater than the rate of fall of the ice particles which are shown to be responsible for the upper‐band echoes, which, it is suggested, are produced by precipitation streaks in a region of vertical wind shear. A simple theory is worked out which gives the apparent rate of fall of an upper band in terms of vertical wind shear, the horizontal wind speed near the cloud top, and the terminal velocity of the ice particles forming the precipitation streak. Measured values of the fall speed of upper bands agree reasonably well with those predicted by this theory. A previous theory of upper bands (Bowen 1951) is shown to be untenable, though it is possible that the upper bands observed by Bowen in Australia have a different origin from those observed at Cambridge. It is proposed to make further observations to settle this question.