Measurements of drop size distribution in the southwestern Amazon basin

Abstract

Simultaneous observations of an optical and an impact type disdrometer and their application in radar rainfall estimation are evaluated. The disdrometers and two collocated rain gauges were operated in the southwest Amazon region of Brazil in 1999 as part of a NASA Tropical Rainfall Measuring Mission (TRMM) field campaign and the hydro‐meteorological component of the Large Scale Biosphere–Atmosphere Experiment (LBA). During the experiment, we observed large drops with diameters greater than 5 mm. These large drops were not adequately detected by the impact disdrometer and resulted in differences in drop size distribution and integral rain parameters derived from the two sensors. Considering coincident observations, we calculated that the impact disdrometer recorded about 11% lower rainfall accumulations than the optical disdrometer. In addition, radar rainfall algorithms, which we derived from the impact and optical disdrometer measurements, showed instrument dependency. Out of four radar rainfall algorithms that we considered, rain rate derived from specific differential phase has the least dependency, while the rain rate derived from reflectivity at horizontal polarization and differential reflectivity combined exhibited the largest. We also observed the characteristics of rainfall and drop size distribution in two distinct wind regimes present during the TRMM–LBA field campaign. Rain was heavier in the easterly regime, with more large drops being present.

Differences in raindrop size distribution from southwest monsoon to northeast monsoon at Gadanki

“…As shown in Figures 1(a) and (b), at Gadanki the wind flow in the SW monsoon is over the continent, while in the NE monsoon it is over the ocean (Bay of Bengal). Therefore, the rainfall and DSD are mostly continental and oceanic in nature during SW and NE monsoon seasons, respectively, consistent with the observations of Kozu et al (2006) (Tokay et al, 2002;Rosenfeld and Ulbrich, 2003;Bringi et al, 2003;Ulbrich and Atlas, 2007). For instance, during moderate to heavy rain (R > 10 mm hr −1 ), 80% of the D m population at Gadanki is in the range 1.5-2.6 and 1.3-2.2 mm in the SW and NE monsoons, respectively.…”

“…Except for the first study mentioned above, all other studies used ensembles of rain DSDs collected from several geographical locations with an assumption that all oceanic (continental) DSD are influenced by similar microphysical processes. The study by Tokay et al (2002), similar to the present study, has shown that the DSD at a single location can be influenced by both oceanic and continental precipitating systems. Nevertheless, the range of D m or D 0 values in all these studies is nearly equal.…”

“…Several possible reasons for the observed differences in DSD are examined, including instrumental problems to geophysical mechanisms. From the observed wind field and similarities in the distribution (and the range) of D m during the SW (NE) monsoon and continental (oceanic) DSD elsewhere (Tokay et al, 2002;Bringi et al, 2003;Rosenfeld and Ulbrich, 2003;Ulbrich and Atlas, 2007), it is considered that the rain DSD at Gadanki in the SW and NE monsoons are, respectively, continental and oceanic in nature. Except for the first study mentioned above, all other studies used ensembles of rain DSDs collected from several geographical locations with an assumption that all oceanic (continental) DSD are influenced by similar microphysical processes.…”

Quality Control and Calibration of the Dual-Polarization Radar at Kwajalein, RMI

“…An impact type Joss-Waldvogel (Joss and Waldvogel 1967) ; b=0.9926, and c = -0.5138) with Z H in mm 6 m -3 , Z DR in dB, and K DP in deg km -1 . The polarimetric radar parameters of Z H , Z DR , and K DP , were calculated for each minute of DSD observations for an S-band radar (10.7 cm) and a temperature of 20 ˚C as shown in Tokay et al (2002). For drop shape, the mean axis ratios offered by Andsager et al (1999) were adopted for drops less than 4 mm in diameter and equilibrium drop shapes (Beard and Chuang 1987) for larger drops.…”

The Role of DSD and Radio Wave Scattering in Rain Attenuation

“…Marshall and Palmer, 1948, Joss and Waldvogel, 1968, Lakomá, 1971, Federer and Waldvogel, 1975, Li and Zhang, 1980 or more recently (e.g. Doelling et al, 1996, Hubert et al, 1999, Tokay et al, 1999, Tokay et al, 2002, Schönhuber et al, 2000, Bringi et al,2003 using different types of measurement technique (filter or glass catchment, electromechanical distrometer, optical distrometer, videodistrometer and others). Many DSD measurements have been made by the electromechanical distrometer the concept of which was developed by Joss and Waldvogel (e.g.…”