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 ($\le \mathrm{5}\mathrm{m}{\mathrm{s}}^{-\mathrm{1}}$) 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 $<\mathrm{0.5}\mathrm{m}{\mathrm{s}}^{-\mathrm{1}}$. For those events with wind speeds $\le \mathrm{1.5}\mathrm{m}{\mathrm{s}}^{-\mathrm{1}}$, 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 ($\le \mathrm{5}\mathrm{m}{\mathrm{s}}^{-\mathrm{1}}$). Hydrometeors do not generally fall in still air, so adjustments to these properties' distributions within natural turbulence remain to be determined.

中文翻译：

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多角度雪花相机测量的表面风影响的北极观测和数值模拟

地面降水对冻结降水的测量受到地表风与量规护罩几何形状相互作用的很大影响。多角度雪花相机（MASC）可以从三个不同的角度拍摄自由落体的水流星，同时测量其坠落速度，已在具有和不具有防风罩的多个中纬度和极地地区使用。在这里，我们对北极场观测结果进行了分析（带或不带Belfort双Alter防护罩），并将结果与​​非屏蔽MASC周围的气流和相应粒子轨迹的计算流体力学（CFD）模拟进行了比较。MASC测量的下降速度与Ka波段大气辐射测量（ARM）天顶雷达（KAZR）的平均多普勒速度只有在风很弱时才能很好地比较（$\le \mathrm{5}\mathrm{米}{\mathrm{s}}^{--\mathrm{1个}}$），并且MASC被屏蔽。MASC测得的下降速度与KAZR测得的速度不匹配，往往会下降到阈值以下，该阈值随风速近似线性增加，但通常$<\mathrm{0.5}\mathrm{米}{\mathrm{s}}^{--\mathrm{1个}}$。对于那些有风速的事件$\le \mathrm{1.5}\mathrm{米}{\mathrm{s}}^{--\mathrm{1个}}$，^水凝物以与水平面成12 orientation的定向角模式落下，大型，低密度的聚集体观察到的可能性高5倍。在没有防风罩的情况下进行的模拟显示，仪器上游侧的气流是分开的，向上的速度分量恰好在孔径上方，对于风速为的情况，平均颗粒落下速度降低了55％（74％）。5 ms ^-1（10 m s ^-1）。我们得出的结论是，精确的MASC对雪颗粒的微物理，方向和下降速度特征的观测需要通过双层防风罩来屏蔽，并且必须将分析限制在轻风事件下（$\le \mathrm{5}\mathrm{米}{\mathrm{s}}^{--\mathrm{1个}}$）。水凝物通常不会掉入静止的空气中，因此在自然湍流中对这些特性的分布的调整仍有待确定。