Scale dependence of cirrus horizontal heterogeneity effects on TOA measurements – Part I: MODIS brightness temperatures in the thermal infrared

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Ken Carslaw, Thomas Koop, Rolf Sander & William Thomas Sturges

  • Atmos. Chem. Phys., 17, 8489-8508, 2017
    © Author(s) 2017. This work is distributed under
    the Creative Commons Attribution 3.0 License.

    13 Jul 2017 - Research article

    Thomas Fauchez1,2, Steven Platnick2, Kerry Meyer3,2, Céline Cornet4, Frédéric Szczap5, and Tamás Várnai6,2 1Universities Space Research Association (USRA), Columbia, MD, USA
    2NASA Goddard Space Flight Center, Greenbelt, MD, USA
    3Goddard Earth Sciences Technology and Research, Universities Space Research Association, Columbia, MD, USA
    4Laboratoire d’Optique Atmosphèrique, UMR 8518, Université Lille 1, Villeneuve d’Ascq, France
    5Laboratoire de Météorologie Physique, UMR 6016, Université Blaise Pascal, Clermont Ferrand, France
    6University of Maryland Baltimore County: Joint Center for Earth Systems Technology and the Department of Physics, Baltimore, MD, USA

    Received: 20 Jan 2017 – Discussion started: 07 Feb 2017
    Revised: 16 May 2017 – Accepted: 24 May 2017 – Published: 13 Jul 2017

    Abstract. This paper presents a study on the impact of cirrus cloud heterogeneities on MODIS simulated thermal infrared (TIR) brightness temperatures (BTs) at the top of the atmosphere (TOA) as a function of spatial resolution from 50 m to 10 km. A realistic 3-D cirrus field is generated by the 3DCLOUD model (average optical thickness of 1.4, cloud-top and base altitudes at 10 and 12 km, respectively, consisting of aggregate column crystals of Deff = 20 µm), and 3-D thermal infrared radiative transfer (RT) is simulated with the 3DMCPOL code.

    According to previous studies, differences between 3-D BT computed from a heterogenous pixel and 1-D RT computed from a homogeneous pixel are considered dependent at nadir on two effects: (i) the optical thickness horizontal heterogeneity leading to the plane-parallel homogeneous bias (PPHB) and the (ii) horizontal radiative transport (HRT) leading to the independent pixel approximation error (IPAE).

    A single but realistic cirrus case is simulated and, as expected, the PPHB mainly impacts the low-spatial-resolution results (above ∼ 250 m) with averaged values of up to 5–7 K, while the IPAE mainly impacts the high-spatial-resolution results (below ∼ 250 m) with average values of up to 1–2 K.

    A sensitivity study has been performed in order to extend these results to various cirrus optical thicknesses and heterogeneities by sampling the cirrus in several ranges of parameters. For four optical thickness classes and four optical heterogeneity classes, we have found that, for nadir observations, the spatial resolution at which the combination of PPHB and HRT effects is the smallest, falls between 100 and 250 m. These spatial resolutions thus appear to be the best choice to retrieve cirrus optical properties with the smallest cloud heterogeneity-related total bias in the thermal infrared. For off-nadir observations, the average total effect is increased and the minimum is shifted to coarser spatial resolutions.
    Citation: Fauchez, T., Platnick, S., Meyer, K., Cornet, C., Szczap, F., and Várnai, T.: Scale dependence of cirrus horizontal heterogeneity effects on TOA measurements – Part I: MODIS brightness temperatures in the thermal infrared, Atmos. Chem. Phys., 17, 8489-8508, https://doi.org/10.5194/acp-17-8489-2017, 2017

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