A Novel 3-D Digital Holographic Instrument for Imaging Ice Crystals in Cirrus

卷云持续覆盖近40%的地球表面，现在已知对全球辐射收支有重大影响。例如，热带风暴产生的卷云砧板被顺风吹到数百公里外，限制了海面的变暖。这反过来可能会影响随后的热带风暴的产生，甚至可能在控制南方涛动(即厄尔尼诺事件)方面发挥作用。使用水晶收集器和复制器进行的有限飞机观测表明，卷云主要由冰晶组成，冰晶的形状复杂，大小从大约10微米到1毫米不等。然而，这些研究中使用的收集器和复制器仅限于进行粗略的空间观测，并不能很好地测量晶体浓度。目前的飞机光学成像探头不能很好地测量150微米左右的晶体浓度。这是传统光学系统固有的局限性，因为景深(DOF)以及样品体积随着晶体半径的平方而减小。在这里，PI提出了一种新的光学仪器，它使用了一种新型的数字全息成像系统(DHIS)，与传统成像系统相比，它的自由度增加了100倍以上。在第一阶段，PI对DHIS进行数值模拟和实验室测试，该DHIS将提供卷云中冰晶的三维信息。这台机载仪器将在第二阶段建造和测试，可以提供直径从5微米到2.5毫米的冰晶的数字化三维全息图，速度高达100赫兹。

Cirrus clouds continuously cover nearly 40% of the earth's surface and are now known to have a significant influence on the global radiation budget. For example, tropical storms generate cirrus anvils that are blown hundreds of kilometers downwind and limit warming of the sea surface. This in turn may influence the subsequent generation of tropical storms and may even play a role in controlling the Sourthern Oscillation (i.e., the El Nino event). Limited aircraft observations using crystal collectors and replicators suggest that cirrus are composed mostly of ice crystals with complex shapes that range in size from about 10 um to 1 mm. However, the collectors and replicators used in these studies are limited to making coarse spatial observations and do not provide good measurements of crystal concentration. The current aircraft optical imaging probes are not capable of making good measurements of crystal concentrations for sizes about 150 um. This is an inherent limitation of conventional optical systems because the depth of field (DOF), and consequently the sample volume, decreases with the square of the crystal radius. Here, the PI proposes a new optical instrument using a novel digital holographic imaging system (DHIS) which increases DOF compared to conventional imaging systems by more than a factor of 100. In Phase I, the PI performs numerical simulations and laboratory tests of a DHIS which will provide three-dimensional information on ice crystals in cirrus. The airborne instrument, to be built and tested in Phase II, could provide digitized 3-D holograms at rates up to 100 Hz of ice crystals with diameters from 5 um to 2.5 mm.