Quantified precipitation measurements such as rain rate and drop size distribution are obtained from the observed Doppler spectral moments of reflectivity and radial velocity. In addition, utilizing the observed Doppler spectral width and the observed vertical structure, it is also possible to classify observations into categories such as stratiform, convective, or mixed.
Of course it is necessary to absolutely calibrate the profiler reflectivities in order to quantify the reflectivity dependent precipitation parameters. To this end a Joss-Waldvogel disdrometer was collocated with the profilers used in each of the TRMM Ground Validation Field Campaigns. The addition of this instrument as a reference standard has proven a powerful addition to the sites, enabling semi-continuous monitoring of the profiler absolute calibration. In addition, having this ongoing reference has been extremely useful in the early detection and identification of occasional hardware malfunctions.
This paper focuses on several issues that arise in the calibration of profilers using impact disdrometers. We show results of inter-comparisons of profiler reflectivities with disdrometer reflectivities calculated from surface drop-size distributions and direct attention to sampling issues inherent in the small sample size of the disdrometer and other issues that can cause disdrometer and profiler reflectivities to disagree.