Optical thickness and effective radius of Arctic boundary-layer clouds retrieved from airborne nadir and imaging spectrometry

Bierwirth, Eike; Ehrlich, André; Wendisch, Manfred; Gayet, Jean-François; Gourbeyre, Christophe; Dupuy, Régis; Herber, Andreas; Neuber, Roland; Lampert, Astrid

doi:10.5194/amt-6-1189-2013

Cited by 33 publications

(22 citation statements)

References 32 publications

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“…The albedo contrast in such areas is the highest we can observe on Earth. For visible wavelengths, the albedo of open water is low (0.042 at 645 nm; Bowker et al, 1985), while that of ice-/snow-covered ocean is high (0.91 at 645 nm; Bowker et al, 1985). These differences significantly decrease in the near-infrared wavelength range (α water = 0.01 and α snow = 0.04 at λ = 1.6 µm wavelength; Bowker et al, 1985), but still slightly alter the radiative transfer.…”

Section: Schäfer Et Al: 3-d Influence Of Ice Edges On Radiative Tmentioning

confidence: 99%

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

et al. 2015

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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 γ λ measurements in the visible spectral range. In cloudy cases the transition of γ λ from open water to sea ice is not instantaneous but horizontally smoothed. In general, clouds reduce γ λ above bright surfaces in the vicinity of open water, while γ λ 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 α water = 0.042 and a sea-ice albedo of α ice = 0.91 at 645 nm wavelength). The affected distance 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 L from 600 to 250 m. An increase in the cloud base altitude or cloud geometrical thickness results in an increase in L; for τ = 1/10 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 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 r eff , respectively. With the help of L, an estimate of the distance to the ice edge is given, where the retrieval uncertainties due to 3-D radiative effects are negligible.

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Section: Schäfer Et Al: 3-d Influence Of Ice Edges On Radiative Tmentioning

confidence: 99%

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

et al. 2015

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“…Further, more extensive simulations with detailed sensitivity studies concerning the various governing parameters are necessary here together with similar comparisons to field data, which should also cover different thermodynamic situations (e.g., moisture, temperature above the To analyze this phenomenon in a higher resolution a holographic instrument capable of detecting particles in a three-dimensional sample volume of a few cm 3 in a single snapshot (Spuler and Fugal, 2011;Schlenczek et al 2014) allows investigation of still smaller mixing scales for both size modes. Also new imaging remote sensing techniques (e.g., as presented by Bierwirth et al, 2013) could help to identify the horizontal variability of the cloud top regions on different scales and thus to quantify the interplay between eddy driven mixing and cloud microphysical processes.…”

Section: Theoretical Considerations and Modelingmentioning

confidence: 99%

Arctic low-level boundary layer clouds: in situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the top layer of liquid phase clouds

et al. 2015

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Abstract. Aircraft borne optical in situ size distribution measurements were performed within Arctic boundary layer clouds with a special emphasis on the cloud top layer during the VERtical Distribution of Ice in Arctic clouds (VERDI) campaign in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside the cloud the droplet mean diameters grew from 10 to 20 µm. In the upper transition zone (i.e., adjacent to the cloud-free air aloft) changes from monomodal to bimodal droplet size distributions (Mode 1 with 20 µm and Mode 2 with 10 µm diameter) were observed. It is shown that droplets of both modes coexist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating data point populations from different air volumes. The formation of the second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol (which was detected directly above the stratus cloud) might form a second mode of small cloud droplets. However, theoretical considerations and model calculations (adopting direct numerical simulation, DNS) revealed that, instead, turbulent mixing and evaporation of larger droplets are the most likely reasons for the formation of the second droplet size mode in the uppermost region of the clouds.

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“…The Arctic Study of Tropospheric Aerosols, clouds and Radiation experiments (ASTAR; Herber et al, 2004;Jourdan et al, 2010;Ehrlich et al, 2009;Gayet et al, 2009;Lampert et al, 2009) iii. The Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols, and Transport (POLARCAT-France; Delanoë et al, 2013;Law et al, 2008;Quennehen et al, 2011) Bierwirth et al, 2013) was performed in the Svalbard region in May 2010 with the AWI Polar-5 aircraft.…”

Section: Airborne Campaignsmentioning

confidence: 99%

Vertical distribution of microphysical properties of Arctic springtime low-level mixed-phase clouds over the Greenland and Norwegian seas

et al. 2017

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Abstract. This study aims to characterize the microphysical and optical properties of ice crystals and supercooled liquid droplets within low-level Arctic mixed-phase clouds (MPCs). We compiled and analyzed cloud in situ measurements from four airborne spring campaigns (representing 18 flights and 71 vertical profiles in MPCs) over the Greenland and Norwegian seas mainly in the vicinity of the Svalbard archipelago. Cloud phase discrimination and representative vertical profiles of the number, size, mass and shape of ice crystals and liquid droplets are established. The results show that the liquid phase dominates the upper part of the MPCs. High concentrations (120 cm −3 on average) of small droplets (mean values of 15 µm), with an averaged liquid water content (LWC) of 0.2 g m −3 are measured at cloud top. The ice phase dominates the microphysical properties in the lower part of the cloud and beneath it in the precipitation region (mean values of 100 µm, 3 L −1 and 0.025 g m −3 for diameter, particle concentration and ice water content (IWC), respectively). The analysis of the ice crystal morphology shows that the majority of ice particles are irregularly shaped or rimed particles; the prevailing regular habits found are stellars and plates. We hypothesize that riming and diffusional growth processes, including the WegenerBergeron-Findeisen (WBF) mechanism, are the main growth mechanisms involved in the observed MPCs. The impact of larger-scale meteorological conditions on the vertical profiles of MPC properties was also investigated. Large values of LWC and high concentration of smaller droplets are possibly linked to polluted situations and air mass origins from the south, which can lead to very low values of ice crystal size and IWC. On the contrary, clean situations with low temperatures exhibit larger values of ice crystal size and IWC. Several parameterizations relevant for remote sensing or modeling studies are also determined, such as IWC (and LWC) -extinction relationship, ice and liquid integrated water paths, ice concentration and liquid water fraction according to temperature.

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Optical thickness and effective radius of Arctic boundary-layer clouds retrieved from airborne nadir and imaging spectrometry

Cited by 33 publications

References 32 publications

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

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

Arctic low-level boundary layer clouds: in situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the top layer of liquid phase clouds

Vertical distribution of microphysical properties of Arctic springtime low-level mixed-phase clouds over the Greenland and Norwegian seas

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