Observing phase transitions in mixed-phase clouds using polarimetric and spectral imagery during (AC)3 and CIRRUS-HL

在 (AC)3 和 CIRRUS-HL 期间使用偏振和光谱图像观察混合相云中的相变

• 批准号: 442667104
• 负责人: Professor Dr. Bernhard Mayer
• 金额: --
• 依托单位: Meteorologisches Institut
• 依托单位国家: 德国
• 项目类别: Infrastructure Priority Programmes
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/442667104?language=en
• 关键词: Observing; phase; transitions; mixed; clouds

Aim of the project is to develop a method to detect phase and microphysical properties for mixed-phase clouds from passive remote sensing. Particularly, we intend to determine liquid and ice fraction of clouds at a horizontal resolution of better than 100 m in order to study small-scale processes which are highly relevant for cloud lifetime. The two research questions of this proposal are: At which accuracy is the observation of the cloud thermodynamic phase partitioning possible, utilizing the combination of multi-angle polarimetric and spectral imagery? What are the typical spatio-temporal characteristics of low-level mixed phased cloud amount and their phase change during Arctic air-mass exchanges, in particular cold-air outbreaks and warm-air intrusions?Cloud phase at cloud top strongly affects cloud cover and longevity of cloud decks in the Arctic. The ice formation via the Bergeron-Findeisen process leads to larger precipitating ice particles compared to supercooled liquid water droplets. On one hand the increased particle size and the larger emissivity of ice compared to liquid water over large parts of the spectral thermal window region increase the emissivity of the cloud. At the same time ice sedimentation depletes cloud water and reduces cloudiness and lifetime of the cloud decks. The representation of these processes is a core deficit in current estimates of arctic climate change. Especially the time-scale of the phase transition is not well represented in models and not well constrained by observations.One reason for the modeling and observational deficiencies is the small-scale variability of the involved quantities both in the vertical and horizontal directions: The phase transition fromliquid to ice takes place in rather thin cloud layers of typically several 100 m geometrical thickness, and up- and downdrafts affecting the phase transition occur at a horizontal scale of a few 10 to a few 100 m.To this end we suggest the advancement of passive cloud phase detection methods towards phase mixture quantification methods, combining two complimentary remote sensing approaches: (1) spectral radiance observations as used in various approaches such as the MODIS optical thickness, phase, and particle size retrievals; (2) imaging of the angular polarized radiance distribution which allows exploiting features specific for liquid water (e.g. cloudbow, backscatter glory) and ice (e.g. specular reflection). Spatial and temporal scales of the phase transition from liquid to ice in the cloud decks will be characterized. Only recently these advancements have become possible by the extension of our hyperspectral sensor specMACS by a wide-field, polarization-sensitive 2D camera (specMACS-P) which first demonstrated its capabilities during the test flights for EUREC4A. The new sensor provides horizontal resolution better than 100 m.

该项目的目的是开发一种方法，从被动遥感中探测混合相云的相态和微物理特性。特别是，我们打算确定液体和冰的云分数在水平分辨率优于100米，以研究小尺度的过程，这是高度相关的云的寿命。这一建议的两个研究问题是：在什么精度是云热力学相划分的观测可能的，利用多角度偏振和光谱图像的组合？在北极气团交换，特别是冷空气爆发和暖空气入侵期间，低层混合相态云量及其相态变化的典型时空特征是什么？云顶的云相强烈影响北极地区的云量和云顶的寿命。通过Bergeron-Findeisen过程的冰形成导致与过冷液态水滴相比更大的沉淀冰粒。一方面，与液态水相比，在光谱热窗口区域的大部分上，冰的颗粒尺寸的增加和更大的发射率增加了云的发射率。与此同时，冰的沉积会耗尽云水，减少云量和云板的寿命。这些过程的代表性是目前北极气候变化估计的核心不足。特别是阶段的时间尺度 过渡在模型中没有很好地表现出来，也没有很好地受到观测的约束。模型和观测不足的一个原因是所涉及的量在垂直和水平方向上的小尺度变化：从液体到冰的相变发生在几何厚度通常为数百米的相当薄的云层中，影响相变的上升和下降气流发生在几个10到几个100米的水平尺度上。为此，我们建议将被动云相位检测方法向相位混合物量化方法发展，结合两种互补的遥感方法：（1）光谱辐射观测，用于各种方法，如MODIS光学厚度，相位和颗粒大小反演;（2）角偏振辐射分布的成像，其允许利用液态水特有的特征（例如云虹、后向散射光）和冰（例如镜面反射）。空间和时间尺度的相变从液体到冰的云甲板的特点。直到最近，我们的高光谱传感器specMACS通过宽视场偏振敏感2D相机（specMACS-P）的扩展才实现了这些进步，该相机在EUREC 4A的试飞中首次展示了其功能。新传感器提供优于100米的水平分辨率。