论文标题
对多角雪花摄像机测量的表面风影响的数值模拟和北极观察
Numerical simulations and Arctic observations of surface wind effects on Multi-Angle Snowflake Camera measurements
论文作者
论文摘要
地面降水的地面测量受到表面风与仪表屏风几何形状的相互作用的严重影响。多角度雪花摄像机（MASC）从三个不同角度拍摄自由落体的水直觉者，同时测量其跌落速度，在多个中纬度和极性位置都在田野中使用。在这里，我们显示了空气流量的计算流体动力学（CFD）模拟的结果以及未屏蔽的MASC周围的相应粒子轨迹，并将这些结果与带有和没有Belfort双重变化屏蔽的北极场观测值进行了比较。在没有风盾的情况下，模拟显示在仪器上游的上游一侧的流动分离，在孔径上方的向上速度成分，对于5 m/s（10 m/s）的风速，平均颗粒降速降低了55％（74％）。 MASC测量的秋季速度与KA波段大气辐射测量（ARM）Zenith Radar（KAZR）平均多普勒速度相比，只有在风中（<5 m/s）并且MASC被屏蔽时。不匹配KAZR测量速度的MASC测量降速往往低于阈值，该阈值与风速大约线性增加，但通常<0.5 m/s。对于那些<1.5 m/s的风速的事件，水平平面的方向角度模式下降，水平面的方向角度模式下降，并且观察到大的低密度聚集体的可能性高出五倍。我们得出的结论是，对雪颗粒的微物理，方向和跌落速度特征的准确观察需要双风围栏屏蔽，并限制分析对风（<5 m/s）的事件的限制。氢凝血器通常不会落入静态空气，因此对自然湍流中这些特性分布的调整仍有待确定。
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.
