Comparison of drop size distribution measurements by impact and optical disdrometers

Simultaneous observations made with optical- and impact-type disdrometers w ere analyzed to broaden knowledge of these instruments. These observations were designed to test how accurately they measure drop size distributions ( DSDs). The instruments' use in determining radar rainfall relations such as that between reflectivity and rainfall rate also was analyzed. A unique se t of instruments, including two video and one Joss-Waldvogel disdrometer al ong with eight tipping-bucket rain gauges, was operated within a small area of about 100 x 50 m(2) during a 2-month-long field campaign in central Flo rida. The disdrometers were evaluated by comparing their rain totals with t he rain gauges. Both disdrometers underestimated the rain totals, but the v ideo disdrometers had higher readings, resulting in a better agreement with the gauges. The disdrometers underreported small- to medium-size drops, wh ich most likely caused the underestimation of rain totals. However, more me dium-size drops were measured by the video disdrometer, thus producing high er rain rates for that instrument. The comparison of DSDs, averaged at diff erent timescales, showed good agreement between the two types of disdromete rs. A continuous increase in the number of drops toward smaller sizes was o nly evident in the video disdrometers at rain rates above 20 mm h(-1). Othe rwise, the concentration of small drops remained the same or decreased to t he smallest measurable size. The Joss-Waldvogel disdrometer severely undere stimated only at very small drop size (diameter less than or equal to 0.5 m m). Beyond the Joss-Waldvogel disdrometer measurement limit were very large drops that fell during heavy and extreme rain intensities. The derived par ameters of exponential and gamma distributions reflect the good agreement b etween the disdrometers' DSD measurements. The parameters of fitted distrib utions were close to each other, especially when all the coincident measure ments were averaged. The low concentrations of very large drops observed by the video disdrometers did not have a significant impact on reflectivity m easurements in terms of the relationships between reflectivity and other in tegral parameters (rain rate, liquid water content, and attenuation). There was almost no instrument dependency. Rather, the relations depend on the m ethod of regression and the choice of independent variable. Also, relations hips derived for S-band radars and Tropical Rainfall Measuring Mission (TRM M) precipitation radar (PR) differ from each other primarily because of the higher reflectivities at the shorter PR wavelength at high rain-rate regim e.