MULTIBEAM TECHNIQUES FOR DERIVING WIND FIELDS FROM AIRBORNE DOPPLER RADARS

Two techniques for deriving horizontal and vertical air motions using vertically scanning airborne Doppler radar data are presented and disc ussed. These techniques make use of the scanning ability of the NOAA P -3 tail-mounted radar antenna to view a region of space from at least two vantage points during a straight-line night track. The scanning me thodology is termed the ''Fore/Aft Scanning Technique'' or FAST becaus e the antenna is alternately scanning forward and then aft of the flig ht track. The major advantages of FAST over flying two quasi-orthogona l flight tracks with the antenna scanning normal to the flight track a re that the data are collected in roughly half the time and the aircra ft does not have to execute a right-angle turn. However, accuracy of t he resulting wind field is compromised slightly because the beam inter section angle is reduced from 90 degrees to about 50 degrees. The redu ction of area covered because of large drift angles is also discussed. A three-dimensional wind field can be constructed using the dual-Dopp ler equations from FAST data using the two radial velocity estimates a nd vertical integration of the continuity equation with a boundary con dition of no vertical motion at cloud top and the Earth's surface. To keep errors in the calculated winds acceptably small, the elevation an gles are typically restricted to +/-45 degrees from the horizontal to minimize contamination of the horizontal wind by terminal fallspeeds. A different, and perhaps more believable vertical velocity, can be der ived using a second technique that utilizes two (or more) airborne Dop pler radar equipped aircraft each using FAST to observe the echo-top v ertical velocity at common point (e.g., two aircraft flying parallel n ight paths, or by using an L-shaped flight track with a single aircraf t). This technique results in 4 (or more) radial velocity estimates at each point (hence is called the ''quad-Doppler'' technique). Horizont al winds can be derived using either an overdetermined three-equation solution or an overdetermined dual-Doppler solution, whichever is more accurate. For the calculation of vertical velocity a new approach is proposed that utilizes the overdetermined triple-Doppler solution for vertical particle motion near cloud top, minus an estimate of terminal fallspeed, as a top boundary condition for the downward vertical dive rgence integration to derive vertical air velocity elsewhere in the do main. In addition, this approach allows measurements at steep elevatio n angles allowing for more depth of coverage for a given range. To sho w the utility of the method, analyses of data collected using FAST are compared to conventional dual-Doppler-derived wind fields constructed from data collected simultaneously by S-band ground-based Doppler rad ars. An example of the quad-Doppler technique is also presented from t he recently completed Tropical Oceans/Global Atmospheres Coupled Ocean /Atmosphere Response Experiment (TOGA/COARE). Comparisons of quad-Dopp ler vertical velocity are made with in-situ derived vertical air motio ns collected by the NASA DC-8 to judge the quality of the approach.