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

cm) positioned side by side to allow study of the effects of clear air scatter on the calibration.The details of the profilers shown are given in Table 1 below.For pure calibration work, only one profiler, of either wavelength, is necessary. The S-band is less sensitive to the clear-air scatter relative to the precipitation signal, and istherefore a better choice if the system is devoted to hydrometeor work, especially for smalldrop-size rain. On the other hand, the longer wavelength system allows some clear-air work.However, even this system is limited in clear air as its sensitivity must be limited to avoidsaturation in the lower range gates during hard rain. These close in range gates are a must forcalibration work in order to ensure that the disdrometer and the radar are observing the samerain and drop-size distribution.TRMM-LBA,Amazonia, , BrazilJanuary-February 1999915 MHz Profiler2835 MHz ProfilerJoss WaldvogelDisdrometerFigure 1: A typical configuration for calibration of a profiler by disdrometer. Thesketch in the left panel insert (upper left corner) shows, more or less to scale, thebasic calibration geometry. The Ji Parana, Brazil site photograph shows a typicalsetup, including the clutter shrouds surrounding the antennas for two side-by-sideprofilers of different wavelength, the control building, and the Joss-WaldvogelDisdrometer (Distromet RD80) located a little over 10 m from the profiler antennason a 1 m high tripod. Both profilers were calibrated using the disdrometer as areference standard, though special care had to be taken with the 915 MHz data toensure clear-air echo was not biasing the results. The right panel shows a close upof the disdrometer.The right panel of Figure 1 shows a close up of the JWD (i.e., Joss-WaldvogelDisdrometer). Falling raindrops impact the Styrofoam head, and the momentum transfer isconverted into drop size. The binning of a 60 second sample of these drop sizes forms a dropsize distribution, or DSD. From the definition of the reflectivity factor Z, it is then easy totransform the DSD into the value of Z that a profiler probing the sampling volume of thedisdrometer should see.Statistical Calibration362It cannot be emphasized enough that calibration using DSDs and radar reflectivities isstatistical in nature. A single one-minute comparison can be very misleading. This isinherent in the stochastic nature of rainfall (Jameson, A. R. and Kostinski, A. B., 2001), andprecise results require the sampling of very large numbers of drops to achieve stability. Thenature of this problem and its solution is best shown by example. Figure 2 Left shows the