Water droplet calibration of the Cloud Droplet Probe (CDP) and in-flight performance in liquid, ice and mixed-phase clouds during ARCPAC
Water droplet calibration of the Cloud Droplet Probe (CDP) and in-flight performance in liquid, ice and mixed-phase clouds during ARCPAC
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DOI：
10.5194/amt-3-1683-2010
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发表时间：
2010-06
影响因子：
3.8
通讯作者：
S. Lance;C. Brock;D. Rogers;J. Gordon
中科院分区：
文献类型：
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作者：
S. Lance;C. Brock;D. Rogers;J. Gordon
抽象的。云滴探测器（CDP）样本面积和液滴大小的实验室校准使用已知大小的水滴，以已知的速度产生。尽管使用PSL和玻璃珠的校准与理论仪器响应一致，但液体水滴校准不一致，并且需要制造商校准中的2 μm偏移。我们表明，这种响应的转变可能是由于相对于激光束的轴的光学器件的不对准。在飞行中的操作过程中与一个独立的测量液态水含量（LWC）的比较表明，更大的偏差，液滴的大小和/或液滴浓度测量的CDP比预期的实验室校准的基础上。由于CDP-LWC中的偏差强烈依赖于浓度，我们假设这种差异是巧合的结果，当两个或更多个液滴在很短的时间内通过CDP激光束时。重合误差，最常见的是由一个液滴外部和一个内部的仪器样品区在同一时间内通过，是在“扩展样品区”（SAE），在该区域中，个别液滴可以影响尺寸检测器，而不必在限定器上注册。SAE用标准化水滴校准，并用于蒙特-卡罗模拟以估计重合对测量的液滴尺寸分布的影响。模拟结果表明，即使水滴浓度低至200 cm−3，扩展的重合误差对CDP也很重要，这些误差对于解释在影响北极气候的气溶胶、辐射和云过程（ARCPAC）研究期间在液体和混合相云中观察到的计算和测量LWC之间的趋势是必要的。我们从模拟中估计，当液滴浓度超过400 cm−3时，可能会出现60%的尺寸过大误差和50%的计数不足误差。目前正在探索CDP的光学设计的修改，以努力减少这种重合偏差。
Abstract. Laboratory calibrations of the Cloud Droplet Probe (CDP) sample area and droplet sizing are performed using water droplets of known size, generated at a known rate. Although calibrations with PSL and glass beads were consistent with theoretical instrument response, liquid water droplet calibrations were not, and necessitated a 2 μm shift in the manufacturer's calibration. We show that much of this response shift may be attributable to a misalignment of the optics relative to the axis of the laser beam. Comparison with an independent measure of liquid water content (LWC) during in-flight operation suggests much greater biases in the droplet size and/or droplet concentration measured by the CDP than would be expected based on the laboratory calibrations. Since the bias in CDP-LWC is strongly concentration dependent, we hypothesize that this discrepancy is a result of coincidence, when two or more droplets pass through the CDP laser beam within a very short time. The coincidence error, most frequently resulting from the passage of one droplet outside and one inside the instrument sample area at the same time, is evaluated in terms of an "extended sample area" (SAE), the area in which individual droplets can affect the sizing detector without necessarily registering on the qualifier. SAE is calibrated with standardized water droplets, and used in a Monte-Carlo simulation to estimate the effect of coincidence on the measured droplet size distributions. The simulations show that extended coincidence errors are important for the CDP at droplet concentrations even as low as 200 cm−3, and these errors are necessary to explain the trend between calculated and measured LWC observed in liquid and mixed-phase clouds during the Aerosol, Radiation and Cloud Processes Affecting Arctic Climate (ARCPAC) study. We estimate from the simulations that 60% oversizing error and 50% undercounting error can occur at droplet concentrations exceeding 400 cm−3. Modification of the optical design of the CDP is currently being explored in an effort to reduce this coincidence bias.