Proceedings with Extended Abstracts (single PDF file) - Radio ...

MOVEABLE UHF/S-BAND PROFILER/DISDROMETERSYSTEMS AS A CALIBRATION STANDARD IN RAINY PLACESWallace L. Clark 1,2 , Kenneth S. Gage 2 , Christopher R. Williams 1,2 , Paul Johnston 1,2 ,and David Carter 21. CIRES/University of Colorado, Boulder, CO, 80309-0216, USA2. NOAA Aeronomy Laboratory R/AL3, Boulder, Colorado, 80305-3328, USAIntroductionA critically important task for many types of radar work is reflectivity calibration. Theobvious, and probably most thorough, method of calibrating is to find the radar constant bybringing the reflectivity observed from a standard calibration target into agreement with thevalue expected. However, the fixed and more or less vertical orientation of profiler/MSTradar beams makes the use of such artificial targets quite difficult. Fortunately, in locationswhere it rains, raindrops can serve as the calibration target. The relation between drop-sizedistribution (DSD) and reflectivity is well developed and may be measured for any givenperiod of time by a collocated surface disdrometer. The near vertical orientation of profilersand MST radars is nearly optimum for such a comparison, although it is required that a fullyrecovered range gate in the far field of the antenna be less than a few hundred meters abovethe surface. Commercial disdrometers are readily available that are rugged, stable overperiods of years, and thus suitable as a field reference standard. This technique has beenused many times over the years for scanning radar calibration (Joss, J. and Pittini, A., 1991),even though the geometry between scanning radars and a surface disdrometer is generally notso ideal.Since small profilers (Gage, K. S., Williams, C. R. et al., 2000) such as the AL (NOAAAeronomy Lab) UHF/disdrometer systems are quite moveable, they can serve as travelingreference standards for larger systems for which the low range gate requirement is notfulfilled. When collocated with such systems, the calibrated profiler becomes a transferstandard to the larger system provided echoes (clear air or rain) can be simultaneouslyobserved in nearly coincident range gates. Differing sizes of range gates, of course, wouldrequire some more sophisticated analysis.Over the last few years the AL has, along with studies of obtaining profiler basedDSDs ( Williams, C. R., Kruger, A. et al., 2000), been field-testing this disdrometer methodof calibrating profilers during the ground validation campaigns of the NASA/NASDATropical Rainfall Measuring Mission (TRMM). The results of these campaigns suggest thatcalibration accuracies within a dBZ or so are obtainable, and the technique has been used forverification, and in at least one case, correction of scanning radar observations. It has alsobeen found easy and beneficial to use this calibration procedure to monitor the health of radarsystems, thus allowing timely correction of occasional system component failures.Calibration SetupThe inset in Figure 1 shows a sketch, more or less to scale, of the geometricalrelationship of the disdrometer/profiler combination. The rest of the Figure shows an actualfield implementation for calibration of wind-profilers with disdrometers during the TRMM-LBA campaign. In this case there are two profilers of different wavelengths (10.6 and 32.8361