Comparison of Raindrop Size Distribution Measurements by Collocated Disdrometers

Abstract

An impact-type Joss-Waldvogel disdrometer (JWD), a two-dimensional video disdrometer (2DVD), and a laser optical OTT Particle Size and Velocity (PARSIVEL) disdrometer (PD) were used to measure the raindrop size distribution (DSD) over a 6-month period in Huntsville, Alabama. Comparisons indicate event rain totals for all three disdrometers that were in reasonable agreement with a reference rain gauge. In a relative sense, hourly composite DSDs revealed that the JWD was more sensitive to small drops (,1 mm), while the PD appeared to severely underestimate small drops less than 0.76 mm in diameter. The JWD and 2DVD measured comparable number concentrations of midsize drops (1-3 mm) and large drops (3-5 mm), while the PD tended to measure relatively higher drop concentrations at sizes larger than 2.44 mm in diameter. This concentration disparity tended to occur when hourly rain rates and drop counts exceeded 2.5 mm h 21 and 400 min 21 , respectively. Based on interactions with the PD manufacturer, the partially inhomogeneous laser beam is considered the cause of the PD drop count overestimation. PD drop fall speeds followed the expected terminal fall speed relationship quite well, while the 2DVD occasionally measured slower drops for diameters larger than 2.4 mm, coinciding with events where wind speeds were greater than 4 m s 21 . The underestimation of small drops by the PD had a pronounced effect on the intercept and shape of parameters of gamma-fitted DSDs, while the overestimation of midsize and larger drops resulted in higher mean values for PD integral rain parameters.

Overview: Precipitation characteristics and sensitivities to environmental conditions during GoAmazon2014/5 and ACRIDICON-CHUVA

“…Raindrops larger than 5 mm were eliminated from the dataset to best match the co-located rain gauge accumulated rainfall, and a complementary filter was applied as described by Giangrande et al (2016). The drop size distribution (DSD) and all respective rainfall rates (RRs, in mm h −1 ) and mass-weighted mean diameters (D m , in mm) were obtained in 5 min intervals for periods during which RR ≥ 0.5 mm h −1 , as suggested by Tokay et al (2013).…”

Correction of raindrop size distributions measured by Parsivel disdrometers, using a two-dimensional video disdrometer as a reference

“…Parsivels bin drops into classes of velocity and diameter and record the number of drops measured per class over an integration time. Parsivel disdrometers have been shown to be susceptible to errors in the recorded drop concentrations, particularly for small and large drops (Krajewski et al, 2006;Tokay et al, 2013). The Parsivel measurement technique assumes properties of the precipitation that are far more appropriate for rain than for solid precipitation; for example, that particles will be spheroidal, have a horizontal orientation of their major axis, and that only one particle will be in the beam at once (Yuter et al, 2006;Battaglia et al, 2010).…”

A comparison of rainfall measurements from multiple instruments

“…The 2DVD and OTT PARSIVEL disdrometer had close agreement in DSD parameters when R was less than 20 mm h −1 , while the mass-weighted mean diameter, the standard deviation of the mass spectrum, and the rain rate of PARSIVEL were higher than those of 2DVD when rain rate was particularly above 30 mm h −1 (Thurai et al, 2011). A combination of the JWD and 2DVD can meet the requirement for accurate sampling of the entire drop size spectrum (Tokay et al, 2013).…”