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

点击翻译按钮获取中文摘要

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.