地面风对多角度雪花照相机测量影响的数值模拟和北极观测
Numerical simulations and Arctic observations of surface wind effects on Multi-Angle Snowflake Camera measurements
摘要：
基于地面的测量冻结降水的地面风与仪表屏蔽几何形状的相互作用的影响很大。多角度雪花照相机从三个不同的角度拍摄自由下落的水凝物，同时测量其下落速度，已在多个中纬度和极地地点实地使用，有风屏蔽和没有风屏蔽。在这里，我们显示的计算流体动力学（CFD）模拟的气流和相应的粒子轨迹周围的无屏蔽MASC和比较这些结果与北极地区的观测和没有贝尔福双Alter屏蔽。在没有挡风板的情况下进行的模拟显示，在仪器的上游侧存在气流分离，孔径上方有向上的速度分量，风速为5 m/s（10 m/s）时，平均颗粒下落速度降低了55%（74%）。只有在风小（<5米/秒）和MASC有屏蔽的情况下，MASC测量的下降速度才能与Ka波段大气辐射测量（ARM）天顶雷达（KAZR）的平均多普勒速度相比较。与KAZR测量速度不匹配的MASC测量的下落速度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<0.5 m/s。对于那些风速<1.5米/秒的事件，水凝物以与水平面成12度的方向角模式落下，而大的低密度聚集体被观测到的可能性要高出五倍。我们的结论是，准确的MASC观测雪粒子的微物理，方向和下降速度特性，需要屏蔽的双重风栅栏和限制的事件，风轻（<5米/秒）的分析。水凝物一般不会在静止的空气中降落，因此在自然湍流中对这些属性分布的调整仍有待确定。
英文摘要：
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.
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