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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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Arctic observations and numerical simulations of surface wind effects on Multi-Angle Snowflake Camera measurements
Abstract. Ground-based measurements of frozen precipitation are heavily influenced by interactions of surface winds with gauge-shield geometry. The Multi-Angle Snowflake Camera (MASC), which photographs hydrometeors in free-fall from three different angles while simultaneously measuring their fall speed, has been used in the field at multiple midlatitude and polar locations both with and without wind shielding. Here, we present an analysis of Arctic field observations – with and without a Belfort double Alter shield – and compare the results to computational fluid dynamics (CFD) simulations of the airflow and corresponding particle trajectories around the unshielded MASC. MASC-measured fall speeds compare well with Ka-band Atmospheric Radiation Measurement (ARM) Zenith Radar (KAZR) mean Doppler velocities only when winds are light (≤5ms-1) and the MASC is shielded. MASC-measured fall speeds that do not match KAZR-measured velocities tend to fall below a threshold value that increases approximately linearly with wind speed but is generally <0.5ms-1. For those events with wind speeds ≤1.5ms-1, hydrometeors fall with an orientation angle mode of 12∘ from the horizontal plane, and large, low-density aggregates are as much as 5 times more likely to be observed. Simulations in the absence of a wind shield show a separation of flow at the upstream side of the instrument, with an upward velocity component just above the aperture, which decreases the mean particle fall speed by 55 % (74 %) for a wind speed of 5 m s−1 (10 m s−1). We conclude that accurate MASC observations of the microphysical, orientation, and fall speed characteristics of snow particles require shielding by a double wind fence and restriction of analysis to events where winds are light (≤5ms-1). Hydrometeors do not generally fall in still air, so adjustments to these properties' distributions within natural turbulence remain to be determined.
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- Award ID(s):
- 1841870
- PAR ID:
- 10220638
- Date Published:
- Journal Name:
- Atmospheric Measurement Techniques
- Volume:
- 14
- Issue:
- 2
- ISSN:
- 1867-8548
- Page Range / eLocation ID:
- 1127 to 1142
- 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-14-1127-2021
