  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(10 m/s).
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) 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. For
those events with wind speeds <1.5 m/s, hydrometeors fall with an orientation
angle mode of 12 degrees 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).
Hydrometeors do not generally fall in still air, so adjustments to these
properties' distributions within natural turbulence remain to be determined.
