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Volume 11, issue 3 | Copyright
Atmos. Meas. Tech., 11, 1377-1384, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Mar 2018

Research article | 08 Mar 2018

Raindrop fall velocities from an optical array probe and 2-D video disdrometer

Viswanathan Bringi1, Merhala Thurai1, and Darrel Baumgardner2 Viswanathan Bringi et al.
  • 1Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado, USA
  • 2Droplet Measurements Technologies, Longmont, Colorado, USA

Abstract. We report on fall speed measurements of raindrops in light-to-heavy rain events from two climatically different regimes (Greeley, Colorado, and Huntsville, Alabama) using the high-resolution (50µm) Meteorological Particle Spectrometer (MPS) and a third-generation (170µm resolution) 2-D 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 and several squall lines that passed over the site with high wind speeds/gusts. As a proxy for turbulent intensity, maximum wind speeds from 10m height at the instrumented site recorded every 3s were differenced with the 5min average wind speeds and then squared. The fall speeds vs. size from 0.1 to 2 and  > 0.7mm were derived from the MPS and the 2DVD, respectively. Consistency of fall speeds from the two instruments in the overlap region (0.7–2mm) gave confidence in the data quality and processing methodologies. Our results indicate that under low turbulence, the mean fall speeds agree well with fits to the terminal velocity measured in the laboratory by Gunn and Kinzer from 100µm up to precipitation sizes. The histograms of fall speeds for 0.5, 0.7, 1 and 1.5mm sizes were examined in detail under the same conditions. The histogram shapes for the 1 and 1.5mm sizes were symmetric and in good agreement between the two instruments with no evidence of skewness or of sub- or super-terminal fall speeds. The histograms of the smaller 0.5 and 0.7mm drops from MPS, while generally symmetric, showed that occasional occurrences of sub- and super-terminal fall speeds could not be ruled out. In the supercell case, the very strong gusts and inferred high turbulence intensity caused a significant broadening of the fall speed distributions with negative skewness (for drops of 1.3, 2 and 3mm). The mean fall speeds were also found to decrease nearly linearly with increasing turbulent intensity attaining values about 25–30% less than the terminal velocity of Gunn–Kinzer, i.e., sub-terminal fall speeds.

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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...