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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 28 Oct 2019

Submitted as: research article | 28 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Estimation of cloud optical thickness, single scattering albedo and effective droplet radius using a shortwave radiative closure study in Payerne

Christine Aebi1,2,a, Julian Gröbner1, Stelios Kazadzis1, Laurent Vuilleumier3, Antonis Gkikas4, and Niklaus Kämpfer2 Christine Aebi et al.
  • 1Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos, Switzerland
  • 2Oeschger Center for Climate Change Research and Institute of Applied Physics, University of Bern, Bern, Switzerland
  • 3Federal Office of Meteorology and Climatology MeteoSwiss, Payerne, Switzerland
  • 4Department of Physics, University of Ioannina, Ioannina, Greece
  • anow at: Royal Meteorological Institute of Belgium, Brussels, Belgium

Abstract. We have used a method based on ground-based solar radiation measurements and radiative transfer models (RTM) in order to estimate the following cloud optical properties: cloud optical thickness (COT), cloud single scattering albedo (SSAc) and effective droplet radius (reff). The method is based on the minimisation of the difference between modelled and measured downward shortwave radiation (DSR). The optical properties are estimated for more than 3,000 stratus-altostratus (St-As) and 206 cirrus-cirrostratus (Ci-Cs) measurements during 2013–2017, at the Baseline Surface Radiation Network (BSRN) station in Payerne, Switzerland. The RTM libRadtran is used to simulate the total DSR, as well as its direct and diffuse components. The model inputs of additional atmospheric parameters are either ground- or satellite-based measurements. The cloud cases are identified by the use of an all-sky cloud camera. For the low- to mid-level cloud class St-As, 95 % of the estimated COT values from DSR measurements (COTDSR) are between 11.9 and 91.5 with a geometric mean and standard deviation of 33.81 and 1.67, respectively. The comparison of these COTDSR values with COTBarnard values retrieved from an independent empirical equation, results in a mean difference of −1.20 ± 2.73 and is thus within the method uncertainty. However, there is a larger mean difference of around 18 between COTDSR and COT values derived from MODIS level-2 (L2), Collection 6.1 (C6.1) data (COTMODIS). The estimated reff (from liquid water path (LWP) and COTDSR) for St-As are between 2.1 and 20.4 μm. For the high-level cloud class Ci-Cs, COTDSR is derived considering the direct radiation and 95 % of the values are between 0.32 and 1.40. For Ci-Cs, 95 % of the SSAc values are estimated to be between 0.84 and 0.99 using diffuse radiation measurements. The COT values for Ci-Cs are also estimated from data from precision filter radiometers (PFR) at various wavelengths. The herein presented method could be applied and validated at other stations with direct and diffuse radiation measurements.

Christine Aebi et al.
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Christine Aebi et al.
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Short summary
Clouds are one of the largest sources of uncertainties in climate models. The current study estimates the cloud optical thickness, the effective droplet radius and the single scattering albedo of stratus-altostratus and cirrus-cirrostratus clouds in Payerne, Switzerland, by combining ground- and satellite-based measurements and radiative transfer models. The estimated values are thereafter compared with data retrieved from other methods.
Clouds are one of the largest sources of uncertainties in climate models. The current study...