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Raindrop shapes and fall velocities measured by 2-dimensional video disdrometer are presented for 2 high-wind/turbulent events. The shapes were reconstructed using a relatively new technique. 10m height wind sensor data are used to derive proxy-indicators for turbulent intensities. Our results show strong gusts, directional wind shifts (i.e. shear) and/or inferred high turbulence intensity are correlated with reduced fall speeds, reaching values ~25 %–30% less than the expected values, i.e. sub-terminal fall speeds. Significant percentage (20 %–35 %) of asymmetric drops (>2 mm) deviating from the most probable axisymmetric shapes were also detected for some events with high turbulent intensities.
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Raindrop fall velocities from an optical array probe and 2-D video disdrometer
Abstract. We report on fall speed measurements of raindrops in light-to-heavyrain events from two climatically different regimes (Greeley,Colorado, and Huntsville, Alabama) using the high-resolution(50 µm) Meteorological Particle Spectrometer (MPS) anda third-generation (170 µm resolution) 2-D videodisdrometer (2DVD). To mitigate wind effects, especially for thesmall drops, both instruments were installed within a 2∕3-scaleDouble Fence Intercomparison Reference (DFIR) enclosure. Two casesinvolved light-to-moderate wind speeds/gusts while the third casewas a tornadic supercell and several squall lines that passed overthe site with high wind speeds/gusts. As a proxy for turbulentintensity, maximum wind speeds from 10 m height at theinstrumented site recorded every 3 s were differenced withthe 5 min average wind speeds and then squared. The fall speedsvs. size from 0.1 to 2 and >0.7 mm were derived from theMPS and the 2DVD, respectively. Consistency of fall speeds from thetwo instruments in the overlap region (0.7–2 mm) gaveconfidence in the data quality and processing methodologies. Ourresults indicate that under low turbulence, the mean fall speedsagree well with fits to the terminal velocity measured in thelaboratory by Gunn and Kinzer from 100 µm up toprecipitation sizes. The histograms of fall speeds for 0.5, 0.7, 1and 1.5 mm sizes were examined in detail under the sameconditions. The histogram shapes for the 1 and 1.5 mm sizeswere symmetric and in good agreement between the two instrumentswith no evidence of skewness or of sub- or super-terminal fallspeeds. The histograms of the smaller 0.5 and 0.7 mm dropsfrom MPS, while generally symmetric, showed that occasionaloccurrences of sub- and super-terminal fall speeds could not beruled out. In the supercell case, the very strong gusts andinferred high turbulence intensity caused a significant broadeningof the fall speed distributions with negative skewness (for drops of1.3, 2 and 3 mm). The mean fall speeds were also found todecrease nearly linearly with increasing turbulent intensityattaining values about 25–30 % less than the terminalvelocity of Gunn–Kinzer, i.e., sub-terminal fall speeds.
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- Award ID(s):
- 1901585
- PAR ID:
- 10533483
- Publisher / Repository:
- Copernicus
- Date Published:
- Journal Name:
- Atmospheric Measurement Techniques
- Volume:
- 11
- Issue:
- 3
- ISSN:
- 1867-8548
- Page Range / eLocation ID:
- 1377 to 1384
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/amt-11-1377-2018
