Abstract
Abstract. 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 γλ measurements in the visible spectral range. In cloudy cases the transition of γλ from open water to sea ice is not instantaneous but horizontally smoothed. In general, clouds reduce γλ above bright surfaces in the vicinity of open water, while γλ 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 αwater = 0.042 and a sea-ice albedo of αice = 0.91 at 645 nm wavelength). The affected distance Δ 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 τ from 1 to 10 leads to a decrease in Δ L from 600 to 250 m. An increase in the cloud base altitude or cloud geometrical thickness results in an increase in Δ L; for τ = 1/10 Δ 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 Δ 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 τ and effective radius reff, respectively. With the help of Δ L, an estimate of the distance to the ice edge is given, where the retrieval uncertainties due to 3-D radiative effects are negligible.
Highlights
As shown by observations and simulations, the Arctic climate changes faster and stronger than the global climate (e. g., Sanderson et al, 2011; Overland et al, 2011)
Due to the high contrast in the surface albedo of sea ice and open water, the data showed a distinct difference between γλ above water and sea ice surfaces
Threshold γλ values derived from both measurements and radiative transfer simulations are in good agreement and were found to be robust for the separation of the surfaces
Summary
As shown by observations and simulations, the Arctic climate changes faster and stronger than the global climate (e. g., Sanderson et al, 2011; Overland et al, 2011). The focus lies on those 3-D radiative effects that are related to the horizontal photon transport between cloud and surface due to isotropic reflection of the incident radiation on the bright sea ice. The goal is to quantify the magnitude and horizontal extent of those 3-D effects as well as their influence on cloud retrievals from the visible wavelength range with a high spatial resolution. For a solar zenith angle ( ) of 45◦, Zinner et al (2010) found that the remote sensing of stratocumulus was not biased by 3-D effects, while that of scattered cumulus was sensitive to horizontal heterogeneities This leads to the assumption that retrievals of cloud microphysical and optical properties can be treated by 1-D simulations if the distance to ice-open water boundaries is sufficiently large.
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