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

AN ATTEMPT TO CALIBRATE THE UHF STRATO-TROPOSPHERICRADAR AT ARECIBO USING NEXRAD AND DISDROMETER DATAP. Kafando* and M. PetitdidierCETP/CNRS, Vélizy, France, monique.petitdidier@cetp.ipsl.frOn leave to Ouagadougou University, Burkina Faso (petronille.kafando@univ-ouaga.bf)1- IntroductionDuring the period, September 15th-October 16th 1998, a campaign devoted to thunderstormobservations took place at NAIC, Puerto Rico. Several instruments were deployed on theObservatory site, like a disdrometer. Other instruments, in the eastern part of the island, havebeen operated by the National Weather Service like the rain gauges, radiosondes and theNexRad Radar.The goal of this paper is to present an attempt to calibrate the backscattered signal of theUHF radar. Several studies reported such calibrations. They have been done with acollocated disdrometer, reflectivities being compared. The lowest reliable heights are below500m (Gage et al., 2000; William et al., 2000). The ST radar at NAIC observed from alowest altitude around 5.9km, that is above the melting layer (4.8km). As a consequence thecollocated disdrometer cannot be used to calibrate the radar. Another difficulty comes by thefact that the running parameters like the transmitted power and the attenuation, put in ordernot to saturate the receiver, may be changed at any time during the same experiment and arenot recorded with the data. Then, an attempt has been done to calibrate the UHF radar atArecibo by using the NexRad radar data located in Cayey (PR). But, one uncertainty may bethe NexRad data calibration. In order to avoid any assumption for the Z-R relationship that itis highly variable during one event, reflectivities are compared. Ulbrich and Lee (1999) haveshown that a systematic offset in radar constant explains the large discrepancies observedwhen rainfall amounts, deduced from NexRad reflectivity, are compared to the rain gaugeones. The authors found an offset around -5.4dB. Then the first step has been to compare theNexRad reflectivity with the reflectivity deduced from the disdrometer data. Once this workdone, the comparison between the reflectivity observed by the UHF radar and the NexRadradar is carried out. As the attenuation of the UHF backscattered signal varies as a functionof time to avoid the receiver saturation the gain of the different period has to be evaluatedusing the noise power. Then the backscattered signal is corrected by this factor and thecomparison is done in the common range.2- Data setDuring that period, there were 18 days out of 29 when rain was detected by the disdrometerbut only 7 with a rain amount larger than 10mm. Simultaneous measurements of disdrometer,and NexRad Radar occurred only on 3 days, September 18 and 30 and October 15. Amongthese 3 days, on two days only, September 30 and October 15, we have simultaneous ST andNexRad data during the thunderstorm activity. Finally, only on October 15 the UHF radarwas working, on September 30 there was only the VHF radar working.3- NexRad CalibrationCalibration set-upFirst, from the latitude and longitude of each instrument, using different geoide models wecalculate the distance and the azimuth of the disdrometer relatively to the NexRad radar.The distance between NexRad and the disdrometer is found equal to 75978m, with an anglebetween both instruments found equal to 289.86°. The final distance and azimuth wereadjusted.395