CEDAR: Polar Mesospheric Cloud Research Using the Sondrestrom, Greenland Lidar

研究人员将研究极地中间层云（PMC）粒子的非球面性质以及PMC粒子与中间层钠层的相互作用。PMC颗粒的形状影响与可见云形成相关的生长-沉降-升华循环。例如，PMC颗粒的下落速率取决于其形状以及其大小;非球形颗粒经历不同的空气动力，这可能导致其在过饱和区域中停留更长时间，从而可能促进其生长。PMC的非球面性质可以影响PMC粒度分布的表征和PMC微观物理的解释。最后，PMC的存在可能会影响钠的浓度和更大的表面积与体积比的非球形颗粒可能会增强PMC颗粒与中间层钠物种的相互作用。由于PMC颗粒的亚微米尺寸和用于探测它们的可见波长，Mie-Rayleigh散射理论主要用于研究PMC。然而，这些研究假设PMC颗粒是球形的--这是一个基本假设，几乎没有实验支持。非球面粒子在散射时使入射的线偏振光去偏振。激光雷达观测表明，非零PMC消偏振比激光雷达测量从挪威。最近，格陵兰岛的Sondrestrom激光雷达系统也测量了可见光波段的非零消偏振比。结果表明，PMC颗粒有时可以是非球面的。本研究的目的是评估在Sondrestrom的PMC的非球面性质。另一个新的和正在出现的研究领域是研究PMC与中间层金属物种的相互作用。Sondrestrom钠和瑞利激光雷达的测量表明，在钠层的下边界处的钠密度减少时，PMC的存在。这些新的观察结果表明PMC和钠之间的联系，涉及冰存在下的非均相化学或现有冰粒对钠的物理吸收。研究人员将研究高纬度地区钠的季节性变化，包括冰对钠分布的影响，方法是使用Sondrestrom激光雷达测量的七年数据库和钠化学模型。该研究计划包括分析PMC退偏振测量的Sondrestrom激光雷达在2003年，2004年和2005年的夏天，以解决有关的PMC的非球面性质的问题。研究人员将开发一个基于非球面粒子散射的模型，以解决PMC形状和大小的问题。此外，还将汇编六年的PMC和钠密度测量结果，并详细说明PMC发生与钠密度耗竭的关系。此外，新的观测1064 nm PMC后向散射测量在2005年和2006年的夏天将被使用。532 nm和1064 nm反向散射测量与532 nm处的去偏振测量的组合将用于更好地约束PMC模型并改善形状和尺寸参数估计。激光雷达观测到的钠层的季节性变化将用于测试钠化学模型。自2003年以来的PMC去极化测量和建模工作将提供一个合理的数据库，将用于表征基本的PMC属性。PMC被认为是全球变化的指标。因此，重要的是要了解它们形成的物理和化学机制。

The investigators will study the aspherical nature of polar mesospheric cloud (PMC) particles and the interaction of PMC particles with the mesospheric sodium layer. The shape of PMC particles affects the growth-sedimentation-sublimation cycle associated with visible cloud formation. For example, the fall rate of a PMC particle depends on its shape as well as its size; an aspherical particle experiences different aerodynamic forces that could cause it to reside longer in the supersaturation region and thus could enhance its growth. The aspherical nature of PMCs can affect the characterization of PMC particle size distributions and the interpretation of PMC microphysics. Lastly, the presence of PMCs may affect sodium concentrations and the greater surface area-to-volume ratio of an aspherical particle may enhance the interaction of PMC particles with the mesospheric sodium species. Owing to the submicron size of PMC particles and the visible wavelengths used to probe them, Mie-Rayleigh scattering theory has predominantly been applied to study PMCs. However, these studies have assumed that the PMC particles are spherical -- a fundamental assumption with little experimental support. An aspherical particle causes incident linearly polarized light to depolarize when scattered. Lidar observations have shown nonzero PMC depolarization ratios in lidar measurements from Norway. Recently the Sondrestrom, Greenland, lidar system has also measured nonzero depolarization ratios at visible wavelengths. The results indicated that PMC particles can at times be aspherical. The goal of this research is to evaluate the aspherical nature of PMCs at Sondrestrom. Another new and emerging research area is the study of the interaction of PMCs with the mesospheric metal species. Sondrestrom sodium and Rayleigh lidar measurements have indicated a reduction in sodium density at the lower boundary of the sodium layer when PMCs are present. These new observations suggest a connection between PMCs and sodium that involves either heterogeneous chemistry in the presence of ice or the physical uptake of sodium onto existing ice particles. The investigators will study seasonal changes of sodium at high latitudes, including the effect of ice on the sodium distribution, by using a seven-year database of Sondrestrom lidar measurements and by using a sodium chemistry model. The research plan includes an analysis of PMC depolarization measurements made by the Sondrestrom lidar in the summers of 2003, 2004, and 2005 to address the question about the aspherical nature of the PMCs. The investigators will develop a model based on aspherical particle scattering to address the question of PMC shape and size. In addition, six years of coincident PMC and sodium density measurements will be compiled and the relationship of PMC occurrence to sodium density depletion will be detailed. In addition, new observations from 1064 nm PMC backscatter measurements in the summers of 2005 and 2006 will be used. The combination of 532 nm and 1064 nm backscatter measurements, with depolarization measurements at 532 nm, will be used to better constrain the PMC model and improve the shape and size parameter estimates. The seasonal variability in the sodium layer observed with the lidar will be used to test sodium chemistry models. The PMC depolarization measurements since 2003 and the modeling effort will provide a reasonable database that will be used to characterize basic PMC properties. PMCs are believed to be an indicator of global change. Thus, it is important to understand the physical and chemical mechanisms by which they are formed.