Airborne observations and simulations of three-dimensional radiative interactions between Arctic boundary layer clouds and ice floes

• Published in: Atmospheric chemistry and physics Vol. 15; no. 14; pp. 8147 - 8163
• Main Authors: Schäfer, M, Bierwirth, E, Ehrlich, A, Jäkel, E, Wendisch, M
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 23-07-2015; Copernicus Publications
• Subjects: Airborne sensing; Aircraft; Albedo; Algorithms; Altitude; Altitude effects; Arctic clouds; Boundary layers; Clouds; Clouds (Meteorology); Distance; Ice; Ice edge; Ice floes; Ice properties; Investigations; Marine; Meteorological satellites; Optical properties; Optical thickness; Pack ice; Radiative transfer; Reflectance; Remote sensing; Retrieval; Sea ice; Sea ice properties; Simulation; Snow; Three dimensional; Uncertainty; Vertical distribution; Wavelength; Arctic region; PN, Arctic
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-15-8147-2015

Based on airborne spectral imaging observations, three-dimensional (3-D) radiative effects between Arctic boundary layer clouds and highly variable Arctic surfaces were identified and quantified. A method is presented to discriminate between sea ice and open water under cloudy conditions based on airborne nadir reflectivity gamma lambda measurements in the visible spectral range. In cloudy cases the transition of gamma lambda from open water to sea ice is not instantaneous but horizontally smoothed. In general, clouds reduce gamma lambda above bright surfaces in the vicinity of open water, while gamma lambda above open sea is enhanced. With the help of observations and 3-D radiative transfer simulations, this effect was quantified to range between 0 and 2200 m distance to the sea ice edge (for a dark-ocean albedo of alpha water = 0.042 and a sea-ice albedo of alpha ice = 0.91 at 645 nm wavelength). The affected distance Delta L was found to depend on both cloud and sea ice properties. For a low-level cloud at 0-200 m altitude, as observed during the Arctic field campaign VERtical Distribution of Ice in Arctic clouds (VERDI) in 2012, an increase in the cloud optical thickness tau from 1 to 10 leads to a decrease in Delta L from 600 to 250 m. An increase in the cloud base altitude or cloud geometrical thickness results in an increase in Delta L; for tau = 1/10 Delta L = 2200 m/1250 m in case of a cloud at 500-1000 m altitude. To quantify the effect for different shapes and sizes of ice floes, radiative transfer simulations were performed with various albedo fields (infinitely long straight ice edge, circular ice floes, squares, realistic ice floe field). The simulations show that Delta L increases with increasing radius of the ice floe and reaches maximum values for ice floes with radii larger than 6 km (500-1000 m cloud altitude), which matches the results found for an infinitely long, straight ice edge. Furthermore, the influence of these 3-D radiative effects on the retrieved cloud optical properties was investigated. The enhanced brightness of a dark pixel next to an ice edge results in uncertainties of up to 90 and 30 % in retrievals of tau and effective radius reff, respectively. With the help of Delta L, an estimate of the distance to the ice edge is given, where the retrieval uncertainties due to 3-D radiative effects are negligible.