Microphysical properties of single and mixed-phase Arctic clouds derived from ground-based AERI observations.

摘要

A novel new approach to retrieve microphysical properties from mixed-phase clouds is presented. This algorithm retrieves cloud optical depth, ice fraction, and the effective size of the water and ice particles from ground-based, high-resolution infrared radiance observations. The theoretical basis is that the absorption coefficient of ice is greater than that of liquid water from 10–13 μm, whereas liquid water is more absorbing than ice from 16–25 μm. However, due to the strong absorption of water vapor in the rotational absorption band, the 16–25 μm spectral region becomes opaque for significant water vapor burdens (i.e., for precipitable water vapor over approximately 1 cm). The Arctic is characterized by its dry and cold atmosphere, as well as a preponderance of mixed-phase clouds, and thus this approach is applicable to Arctic clouds. Since this approach uses infrared observations, cloud properties are retrieved at night and during the long polar wintertime period as well as during daytime periods.; The interactions among the clouds, atmosphere, and surface in the Arctic are extremely complex, and these interactions are less understood than at mid-latitudes. This lack of understanding is due to the small number of observations of cloud properties in the Arctic, which is primarily due to the difficulty in detecting and retrieving cloud properties from space. The retrieval algorithm developed here offers the necessary data set to study the interactions of the clouds with the surface and atmosphere and to validate existing and new satellite remote sensing techniques, especially during the polar winter. As an example, frequency distributions of the cloud properties retrieved during a 7 month Arctic experiment demonstrate many interesting features of Arctic clouds. These results demonstrate that approximately 50% of the clouds are mixed-phase, a lack of temperature dependence in the ice fraction for temperatures above 240 K, seasonal trends in the optical depth with the clouds being thinner in winter and becoming more optically thick in the late spring, and a seasonal trend in the effective size of the water droplets in liquid-only and mixed-phase clouds that is most likely related to aerosol concentration.