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

At this range from the radar, the central beam is a thousand meters above the surface and iswell below the freezing level around 4.8km for all thunderstorms. The beamwidth of theNexRad radar is relatively broad.Disdrometer dataThe data were collected with a Disdromet Joss-Waldvogel disdrometer (Joss and Waldvogel,1967). One-minute samples of DSDs were collected for 29 days from September 15 th toOctober 15 th , 1998. Rain was observed on 18 days, and the total number of DSDs equals1298 samples.The JWD records the number of raindrops, nd i , of diameter D i , in each of its 20 diameterchannels, for a one-minute sampling period.From the DSD data the radar reflectivity is deduced:Z=109 ∞∫06D N(D)dDZ is expressed in mm 6 m -3 with D, dD in m and N(D) m -4 .The drawback of the instrument is in presence of strong rains.NexRad dataThe Doppler weather radar, NexRad, is installed atop Cerro Baldios, around 8.15 km east ofthe city of Cayey, in the south-western part of the island. The WSR-88D units are powerful10cm wavelength radars with approximately 1° beam width by 250m range resolution andvolume scan sampling frequency of 5 or 6mn according to the mode selected (Crum et al.,1993). The discrete values of the elevations provide a relatively poor vertical resolution from1.26 km to 1.76 at 16km. For the elevation greater than 10°, the vertical resolution goes from2.8 to 4km. Just above the disdrometer, the radar beamwidth is about 1.3km.According to the weather conditions, a specific scanning mode is selected. On October 15,the precipitation mode, VCP11, selected consists of volume scans at 14 elevations from 0.4°to 19.5°. The time resolution is 5mn with a pulse length of 1.57µs. On September 30, aprecipitation mode, VCP21, was also operating but with only 9 elevations. The timeresolution is 6mn.ComparisonFrom the relation (3), the reflectivity is deduced for the three days, September 18 and 30, andOctober 15. Figure 1 presents the variation of the reflectivity, Z d , observed on October 15,superimposed and indicated by a star the values of the NexRad reflectivity, Z Nx .Due the height difference between the disdrometer and the central beam axis above thedisdrometer, i.e. 1050m, the time delay between the precipitation located into the NexRadbeam and the observed one by the disdrometer varies according to the mean diameter from 2to 4mn. Due to the beamwidth that is quasi perpendicular to the cloud and intercepts a rangeof 1.3 km around a mean altitude of 1.4km MSL. the time delay could not be precise.Gage et al. (2000) found a difference of about 2mn between the UHF reflectivity at 327mand the disdrometer one.The first result points out no significant difference between the computed reflectivity and theobserved one by the radar in the event observed. At least, if it exists it will be relativelysmall.4- UHF calibrationCalibration set-upDuring this experiment the measurements were carried out simultaneously at UHF (430MHz)and VHF (46.7MHz) with a 2µs pulse, the inter pulse period (IPP) being 1ms. Therelationship to compute the mean height of the first sample or gate takes into account thezenith angle, and various delays (Cho, Private communication, 1994). In UHF, the first gate396