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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/amt-2020-296
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-296
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  03 Sep 2020

03 Sep 2020

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This preprint is currently under review for the journal AMT.

Numerical simulations and Arctic observations of surface wind effects on Multi-Angle Snowflake Camera measurements

Kyle E. Fitch1,2, Chaoxun Hang3, Ahmad Talaei1, and Timothy J. Garrett1 Kyle E. Fitch et al.
  • 1Department of Atmospheric Sciences, University of Utah, Salt Lake City, 84112, USA
  • 2Department of Engineering Physics, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio, 45433, USA
  • 3Department of Civil Engineering, Monash University, Clayton, 3168, Australia

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 mid-latitude and polar locations both with and without wind shielding. Here we show results of computational fluid dynamics (CFD) simulations of the airflow and corresponding particle trajectories around the unshielded MASC and compare these results to Arctic field observations with and without a Belfort double Alter shield. 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). MASC-measured fall speeds compare well with Ka-band Atmospheric Radiation Measurement (ARM) Zenith Radar (KAZR) mean Doppler velocities only when winds are light (< 5 m s−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 < 0.5 m s−1. For those events with wind speeds < 1.5 m s−1, hydrometeors fall with an orientation angle mode of 12° from the horizontal plane, and large, low-density aggregates are as much as five times more likely to be observed. 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 (< 5 m s−1). Hydrometeors do not generally fall in still air, so adjustments to these properties' distributions within natural turbulence remain to be determined.

Kyle E. Fitch et al.

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Kyle E. Fitch et al.

Model code and software

Mascpy code for "Graupel Precipitating from Thin Arctic Clouds with Liquid Water Paths less than 50 g m-2" Fitch, K. E. and Garrett, T. J. https://doi.org/10.7278/S50DVA5JK2PD

MATLAB code for "Numerical simulations and Arctic observations of surface wind effects on Multi-Angle Snowflake Camera measurements" Fitch, K.E., Hang, C., Talaei, A., and Garrett, T. J. https://doi.org/10.7278/S50DQTX9K7QY

Kyle E. Fitch et al.

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Short summary
Snow measurements are very sensitive to wind. Here we compare airflow and snowfall simulations to Arctic observations for a Multi-Angle Snowflake Camera to show that measurements of fall speed, orientation, and size are accurate only with a double wind fence and winds below 5 m s−1. In this case, snowflakes tend to fall with a nearly horizontal orientation, and the largest flakes are as much as 5 times more likely to be observed. Adjustments are needed for snow falling in naturally turbulent air.
Snow measurements are very sensitive to wind. Here we compare airflow and snowfall simulations...
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