Abstract
Abstract. Arctic boundary-layer clouds were investigated with remote sensing and in situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. The clouds formed in a cold air outbreak over the open Greenland Sea. Beside the predominant mixed-phase clouds pure liquid water and ice clouds were observed. Utilizing measurements of solar radiation reflected by the clouds three methods to retrieve the thermodynamic phase of the cloud are introduced and compared. Two ice indices IS and IP were obtained by analyzing the spectral pattern of the cloud top reflectance in the near infrared (1500–1800 nm wavelength) spectral range which is characterized by ice and water absorption. While IS analyzes the spectral slope of the reflectance in this wavelength range, IS utilizes a principle component analysis (PCA) of the spectral reflectance. A third ice index IA is based on the different side scattering of spherical liquid water particles and nonspherical ice crystals which was recorded in simultaneous measurements of spectral cloud albedo and reflectance. Radiative transfer simulations show that IS, IP and IA range between 5 to 80, 0 to 8 and 1 to 1.25 respectively with lowest values indicating pure liquid water clouds and highest values pure ice clouds. The spectral slope ice index IS and the PCA ice index IP are found to be strongly sensitive to the effective diameter of the ice crystals present in the cloud. Therefore, the identification of mixed-phase clouds requires a priori knowledge of the ice crystal dimension. The reflectance-albedo ice index IA is mainly dominated by the uppermost cloud layer (τ<1.5). Therefore, typical boundary-layer mixed-phase clouds with a liquid cloud top layer will be identified as pure liquid water clouds. All three methods were applied to measurements above a cloud field observed during ASTAR 2007. The comparison with independent in situ microphysical measurements shows the ability of the three approaches to identify the ice phase in Arctic boundary-layer clouds.
Highlights
The impact of clouds on the radiation budget of Arctic regions constitutes a crucial uncertainty in predicting Arctic climate change as reported in the Arctic Climate Impact Assessment (Corell, 2004)
Ehrlich et al.: Cloud phase identification of Arctic boundary-layer clouds found that the low surface albedo of the ice-free ocean reduces the upwelling radiation above the clouds and the cloud albedo by up to 30% compared to clouds over highly reflecting sea ice
In this study we present similar methods of cloud phase identification using airborne measurements of spectral solar cloud reflectance combined with radiative transfer simulations
Summary
The impact of clouds on the radiation budget of Arctic regions constitutes a crucial uncertainty in predicting Arctic climate change as reported in the Arctic Climate Impact Assessment (Corell, 2004). Before retrieving cloud properties a preselection algorithm distinguishes between ice, mixed-phase and liquid water clouds (Key and Intrieri, 2000; King et al, 2004; Kokhanovsky et al, 2006). This phase discrimination is often based on two methods using the brightness temperatures of thermal infrared (IR; 5−50 μm) channels and the cloud reflectance at channels for near infrared wavelength range (NIR, 700−2500 nm).
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