Determining winds aloft with a radar wind sensor: accounting for rain-drop fall speed

A radar to measure the winds aloft from a satellite, using Doppler shifts of hydrometeors, is called a RAdar Wind Sounder (RAWS). This radar has been studied at The University of Kansas for several years with preliminary studies reported previously. Such a radar can provide information on winds aloft in areas where other measurements are sparse. Combined, RAWS and the LAser Wind Sounder (LAWS), can make a major contribution to the data needed in studying global atmospheric circulation. The four-vector scan pattern considered earlier for wind retrieval was found unsuitable since the measurements at two vertical pointing angles are not independent. Thus, the authors have only two independent measurements of velocity, while a third, vertical component exists in rain. Though the vertical component is not significant for clouds, it cannot be neglected in areas of precipitation where the fall velocity of the rain particles may be sizeable (as large as 9 m/s, depending on the particle size). This paper shows a method of obtaining the fall velocity from the reflectivity. The rain rate is determined from the reflectivity, and empirical relations between rain rate and fall speed are then used to estimate the fall speed. Since the RAWS must have much more sensitivity for cloud measurements than that needed to measure rain rate, it can easily provide an estimate of the needed rain rate. The primary difficulty is in determining the fall speed from the rate, but data are available that give such estimates. The authors present error sensitivity of the wind retrieval method due to noisy measurements and errors in estimating the fall velocity