Scale dependence of cirrus heterogeneity effects. Part ii: modis nir and swir channels

Fauchez, Thomas; Platnick, Steven; Varnai, Tamas; Meyer, Kerry; Cornet, Celine; Szczap, Frederic

Document type :

Article dans une revue scientifique: Article original

DOI :

10.5194/acp-18-12105-2018

Permalink :

http://hdl.handle.net/20.500.12210/105072

Title :

Scale dependence of cirrus heterogeneity effects. Part ii: modis nir and swir channels

Author(s) :

Fauchez, Thomas [Auteur]
NASA Goddard Space Flight Center [GSFC]
Universities Space Research Association [Washington] [USRA]
Platnick, Steven [Auteur]
NASA Goddard Space Flight Center [GSFC]
Varnai, Tamas [Auteur]
University of Maryland [Baltimore County] [UMBC]
NASA Goddard Space Flight Center [GSFC]
Meyer, Kerry [Auteur]
NASA Goddard Space Flight Center [GSFC]
Cornet, Celine [Auteur]

Laboratoire d'Optique Atmosphérique (LOA) - UMR 8518
Szczap, Frederic [Auteur]
Laboratoire de Météorologie Physique [LaMP]

Journal title :

Atmospheric Chemistry and Physics

Abbreviated title :

Atmos. Chem. Phys.

Volume number :

Publication date :

2018-08-22

ISSN :

1680-7316

HAL domain(s) :

Physique [physics]

English abstract : [en]

In a context of global climate change, the understanding of the radiative role of clouds is crucial. On average, ice clouds such as cirrus have a significant positive radiative effect, but under some conditions the effect ...
Show more >In a context of global climate change, the understanding of the radiative role of clouds is crucial. On average, ice clouds such as cirrus have a significant positive radiative effect, but under some conditions the effect may be negative. However, many uncertainties remain regarding the role of ice clouds on Earth's radiative budget and in a changing climate. Global satellite observations are particularly well suited to monitoring clouds, retrieving their characteristics and inferring their radiative impact. To retrieve ice cloud properties (optical thickness and ice crystal effective size), current operational algorithms assume that each pixel of the observed scene is plane-parallel and homogeneous, and that there is no radiative connection between neighboring pixels. Yet these retrieval assumptions are far from accurate, as real radiative transfer is 3-D. This leads to the plane-parallel and homogeneous bias (PPHB) plus the independent pixel approximation bias (IPAB), which impacts both the estimation of top-of-the-atmosphere (TOA) radiation and the retrievals. An important factor that determines the impact of these assumptions is the sensor spatial resolution. High-spatial-resolution pixels can better represent cloud variability (low PPHB), but the radiative path through the cloud can involve many pixels (high IPAB). In contrast, low-spatial-resolution pixels poorly represent the cloud variability (high PPHB), but the radiation is better contained within the pixel field of view (low IPAB). In addition, the solar and viewing geometry (as well as cloud optical properties) can modulate the magnitude of the PPHB and IPAB. In this, Part II of our study, we simulate TOA 0.86 and 2.13 µm solar reflectances over a cir-rus uncinus scene produced by the 3DCLOUD model. Then, 3-D radiative transfer simulations are performed with the 3DMCPOL code at spatial resolutions ranging from 50 m to 10 km, for 12 viewing geometries and nine solar geometries. It is found that, for simulated nadir observations taken at resolution higher than 2.5 km, horizontal radiation transport (HRT) dominates biases between 3-D and 1-D reflectance calculations, but these biases are mitigated by the side illumination and shadowing effects for off-zenith solar geometries. At resolutions coarser than 2.5 km, PPHB dominates. For off-nadir observations at resolutions higher than 2.5 km, the effect that we call THEAB (tilted and homogeneous extinction approximation bias) due to the oblique line of sight passing through many cloud columns contributes to a large increase of the reflectances, but 3-D radiative effects such as shadowing and side illumination for oblique Sun are also important. At resolutions coarser than 2.5 km, the PPHB is again the dominant effect. The magnitude and resolution dependence of PPHB and IPAB is very different for visible, near-infrared and shortwave infrared channels compared with the thermal infrared channels discussed in Part I of this study.Show less >

Language :

Anglais

Audience :

Internationale

Popular science :

Non

Administrative institution(s) :

CNRS
Université de Lille

Collections :

Laboratoire d'Optique Atmosphérique (LOA) - UMR 8518

Submission date :

2024-01-30T11:45:46Z
2024-02-27T09:22:50Z

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Except where otherwise noted, this item's license is described as Attribution 3.0 United States

Version	Link	Modification date
2	20.500.12210/105072*	2024-02-27T09:17:01Z
1	20.500.12210/105072.1	2024-01-30T11:45:46Z

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Scale dependence of cirrus heterogeneity ...

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