Measuring Vertical Rainfall Velocity through Spaceborne Doppler Radar: Performance Analysis and System Requirements

In this paper we will present the results of the trade studies on the performances of a spacebome Doppler radar in measuring vertical rainfall velocity. Particular emphases will be placed on: 1) the choice of the PRF vs. antenna size ratio, 2) the choice of the observational strategy, 3) the choice of the operating frequency; and the 4) processing strategy. The results show that accuracies of 1 m/s or better can be achieved with the currently available technology and with careful selection the system parameters. Knowledge of the global distribution of the vertical velocity of precipitation is important in estimating latent heat fluxes, and therefore in the general study of energy transportation in the atmosphere. Such knowledge can only be acquired with the use of spacebome Doppler precipitation radars. Although the high relative speed of the radar with respect to the rainfall particles introduces significant broadening in the Doppler spectrum, recent studies have proven that the average vertical velocity can be measured to acceptable accuracy levels by appropriate selection of radar parameters and of the spectral moments estimator [ 1,2]. Furthermore, methods to correct for specific errors arising from NUBF effects [2,3] and pointing uncertainties have recently been developed [4]. In this paper the system performances are analyzed accounting for all the aforementioned sources of error and the use of the corresponding correcting techniques. Several configurations are analyzed within the category of single-antenna, crosstrack scanning, narrow-beam (Le., antenna 3dB beamwidth 8 j d ~ < 19, pulse Doppler radars mounted on a Low Earth Orbit spacecraft this cathegory will be referred to as DPR in the remainder of this paper. The shape of the Doppler spectrum observable from a DPR is determined by the velocity of all the raindrops within a volume of resolution relative to the radar. The high linear velocity of the spacecraft (v, 7 lads) is the main contribution to such relative velocity even for very small forwardaft angles (e.g., a non-moving target seen by DPR at an off-nadir angle of 8 = 0.05' and a zero azimuth with respect to the line of flight has an apparent velocity of -6 d s ) . For this reason when the raindrops are homogeneously distributed within the volume of resolution, the Doppler spectrum is determined mainly by the two-way antenna pattem [2,5,6]. In general it is well approximated by a