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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/amt-2017-401
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
09 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).
Raindrop Fall Velocities from an Optical Array Probe and 2D-video Disdrometer
Viswanathan Bringi1, Merhala Thurai1, and Darrel Baumgardner2 1Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado, USA
2Droplet Measure ments Technologies, Longmont, Colorado, USA
Abstract. We report on fall speed measurements of rain drops in light-to-heavy rain events from two climatically different regimes (Greeley, Colorado, and Huntsville, Alabama) using the high resolution (50 microns) Meteorological Particle Spectrometer (MPS) and a 3rd generation (170 microns resolution) 2D-video disdrometer (2DVD). To mitigate wind-effects, especially for the small drops, both instruments were installed within a 2/3-scale Double Fence Intercomparison Reference (DFIR) enclosure. Two cases involved light-to-moderate wind speeds/gusts while the third case was a tornadic supercell that passed over the site with high wind speeds/gusts. As a proxy for turbulent intensity, maximum wind speeds from 10-m height at the instrumented site recorded every 3 s were differenced with the 5-min average wind speeds and then squared. The fall speed versus size from 0.1–2 mm were derived from the MPS data and the 2DVD was used for sizes > 0.7 mm. Consistency of fall speeds from the two instruments in the overlap region (0.7–2 mm) gave confidence in the data quality and processing methodologies. Our results indicate that under light-to-moderate wind gusts, the mean fall speeds agree well with fits to the terminal velocity measured in the laboratory by Gunn and Kinzer from 100 microns up to precipitation sizes. In the supercell case the very strong gusts and inferred high turbulence intensity caused a significant broadening of the fall speed distributions with the mean fall speeds about 25–30 % less than the terminal velocity of Gunn-Kinzer, i.e. sub-terminal fall speeds.

Citation: Bringi, V., Thurai, M., and Baumgardner, D.: Raindrop Fall Velocities from an Optical Array Probe and 2D-video Disdrometer, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2017-401, in review, 2017.
Viswanathan Bringi et al.
Viswanathan Bringi et al.
Viswanathan Bringi et al.

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Short summary
Raindrop fall velocities are important for rain rate estimation, soil erosion studies and in numerical modelling of rain formation in clouds. The assumption that the fall velocity is uniquely related to drop size is made inherently based on laboratory measurements under still air conditions from nearly 68 years ago. There have been very few measurements of drop fall speeds in natural rain under both still and turbulent wind conditions. We report on fall speed measurements in natural rain shafts.
Raindrop fall velocities are important for rain rate estimation, soil erosion studies and in...
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