Accurate Determination of Vertical Air Velocities in Rain by Doppler-Radar

Abstract

Vertical air velocity in rain is calculated from the minima in the reflectivity profile of the Doppler velocity spectra between succeeding range cells. The reflectivity minimum is converted to air velocity, assuming a negative exponential dropsize distribution. The scatter in the observed reflectivity minima can be lowered considerably by representing the reflectivity profile around the minima with a best-fit second order polynomial. A statistical method is developed to express the observed scatter in true fluctuations of the air velocity and a measurement error. In this way the accuracy of the method could be calculated and compared to other methods. Experimental verification has been made during a steady rain with a vertically directed FM-CW Doppler radar. This type of radar is very suitable for this kind of measurements because of its high sensitivity and range resolution. High quality pulse Doppler radars might also be used. Accuracies in the mean vertical air velocity of a few cm s?1 have been obtained. However, just below the bright band larger errors were found, due to deviations from the assumed dropsize distribution by some large, partly melted snowflakes. Turbulent fluctuations of the vertical air velocity in a space volume of 60 ? 20 ? 20 m and a sampling time of 3 s were determined with a rms accuracy of 0.11 m s?1. Even in the stratiform rain this measurement error appeared much smaller than the matter in the true air velocity and the errors that should be made by estimating the air velocity from the mean fall velocity. It is expected that these differences will be even more pronounced in turbulent precipitation.