Thermal radiative fluxes through inhomogeneous cloud fields: a sensitivity study using a new stochastic cloud generator

Benassi, Albert; Szczap, Frédéric; Davis, Anthony B.; Masbou, M.; Cornet, Céline; Bleuyard, P.

doi:10.1016/j.atmosres.2004.03.018

Cited by 21 publications

(19 citation statements)

References 56 publications

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“…For scales smaller than the inertial domain, viscosity phenomena smooth and homogenize the fluid movement and the spectral energy is no longer correlated with the wave number (Benassi et al, 2004). The limit is not clearly defined because of limitations due to instrument resolution.…”

Section: Scale Invariant Propertiesmentioning

confidence: 99%

“…Microphysical quantities such as liquid water content (LWC) or ice water content (IWC), optical quantities such as extinction coefficient, or radiative field quantities such as radiances, reflectances and brightness temperatures are not randomly distributed from small to large scales but often follow a power law in Fourier space (Benassi et al, 2004;Cahalan and Snider, 1989;Davis et al, 1994Davis et al, , 1996Davis et al, 1997;Fauchez et al, 2014, etc.). Indeed, Kolmogorov theory (Kolmogorov, 1941) shows that in the inertial domain, where the turbulence is isotropic, and at the equilibrium with large scales, spectral energy as a function of the wave number k is described by a power law spectrum E(k) with an exponent β ∼ −5/3 named spectral slope.…”

Section: Scale Invariant Propertiesmentioning

confidence: 99%

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

et al. 2017

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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, cloudtop and base altitudes at 10 and 12 km, respectively, consisting of aggregate column crystals of D eff = 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-spatialresolution results (above ∼ 250 m) with averaged values of up to 5-7 K, while the IPAE mainly impacts the high-spatialresolution 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.

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Section: Scale Invariant Propertiesmentioning

confidence: 99%

Section: Scale Invariant Propertiesmentioning

confidence: 99%

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

et al. 2017

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“…The scale invariant properties observed in real clouds can be controlled. The power spectra of the logarithm of their optical properties (optical depth, liquid water content or liquid water path for low clouds and ice water content for high clouds) typically exhibits a spectral slope of around −5/3 (Davis et al, 1994(Davis et al, , 1996(Davis et al, , 1997(Davis et al, , 1999Cahalan et al, 1994;Benassi et al, 2004;Hogan and Kew, 2005;Hill et al, 2012;Fauchez et al, 2014) from small scale (a few metres) to the "integral scale" or the outer scale (few tenths of a kilometre to one-hundred kilometres), where the spectrum becomes flat (i.e. decorrelation occurs).…”

Section: F Szczap Et Al: a Flexible Three-dimensional Cloud Generatmentioning

confidence: 99%

“…Examples of these types of cloud models are: the bounded cascade model Marshak et al, 1998), the iterative amplitude adapted Fourier transform (IAAFT) algorithm (Venema et al, 2006), the SITCOM model (Di Guiseppe and Thompkins, 2003), the tree-driven mass accumulation process (tdMAP) model (Benassi et al, 2004), the model developed by Evans and Wiscombe (2004) for low liquid clouds (stratocumulus and cumulus) or by Alexandrov et al (2010) and the Cloudgen model (Hogan and Kew, 2005) for high ice clouds (cirrus). These stochastic models are based on fractal or Fourier framework.…”

Section: F Szczap Et Al: a Flexible Three-dimensional Cloud Generatmentioning

confidence: 99%

A flexible three-dimensional stratocumulus, cumulus and cirrus cloud generator (3DCLOUD) based on drastically simplified atmospheric equations and the Fourier transform framework

et al. 2014

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Abstract. The 3DCLOUD algorithm for generating stochastic three-dimensional (3-D) cloud fields is described in this paper. The generated outputs are 3-D optical depth (τ ) for stratocumulus and cumulus fields and 3-D ice water content (IWC) for cirrus clouds. This model is designed to generate cloud fields that share some statistical properties observed in real clouds such as the inhomogeneity parameter ρ (standard deviation normalized by the mean of the studied quantity), the Fourier spectral slope β close to −5/3 between the smallest scale of the simulation to the outer L out (where the spectrum becomes flat). Firstly, 3DCLOUD assimilates meteorological profiles (humidity, pressure, temperature and wind velocity). The cloud coverage C, defined by the user, can also be assimilated, but only for stratocumulus and cumulus regime. 3DCLOUD solves drastically simplified basic atmospheric equations, in order to simulate 3-D cloud structures of liquid or ice water content. Secondly, the Fourier filtering method is used to constrain the intensity of ρ, β, L out and the mean of τ or IWC of these 3-D cloud structures. The 3DCLOUD model was developed to run on a personal computer under Matlab environment with the Matlab statistics toolbox. It is used to study 3-D interactions between cloudy atmosphere and radiation.

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“…Results from cuboidal cloud geometries were extended in Masunaga and Nakajima (2001) by relaxing the blackbody assumption and addressing how optical thickness influences the effective cloud fraction. Benassi et al (2004) further extended these studies by using realistic cloud structures, broken or not, that were generated with a data-driven stochastic cloud model to compute the MD thermal radiative transfer. Scattering effects in the longwave have traditionally been thought to play only a minor role compared to absorption, and in a study of the relative contributions of cloud geometry and scattering Takara and Ellingson (1996) concluded that MD effects of cloud geometry indeed dominate over scattering.…”

Section: ϫ2mentioning

confidence: 99%

Multidimensional Longwave Forcing of Boundary Layer Cloud Systems

Mechem

Kogan

Ovtchinnikov

et al. 2008

Journal of the Atmospheric Sciences

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The importance of multidimensional (MD) longwave radiative effects on cloud dynamics is evaluated in an eddy-resolving model (ERM)-the two-dimensional analog to large-eddy simulation (LES)-framework employing multidimensional radiative transfer [Spherical Harmonics Discrete Ordinate Method (SHDOM)]. Simulations are performed for a case of unbroken, marine boundary layer stratocumulus and a broken field of trade cumulus. "Snapshot" calculations of MD and independent pixel approximation (IPA; 1D) radiative transfer applied to simulated cloud fields show that the total radiative forcing changes only slightly, although the MD effects significantly modify the spatial structure of the radiative forcing. Simulations of each cloud type employing MD and IPA radiative transfer, however, differ little. For the solid cloud case, relative to using IPA, the MD simulation exhibits a slight reduction in entrainment rate and boundary layer total kinetic energy (TKE) relative to the IPA simulation. This reduction is consistent with both the slight decrease in net radiative forcing and a negative correlation between local vertical velocity and radiative forcing, which implies a damping of boundary layer eddies. Snapshot calculations of the broken cloud case suggest a slight increase in radiative cooling, although few systematic differences are noted in the interactive simulations. This result is attributed to the fact that radiative cooling is a relatively minor contribution to the total energetics. For the cloud systems in this study, the use of IPA longwave radiative transfer is sufficiently accurate to capture the dynamical behavior of boundary layer clouds. Further investigations are required to generalize this conclusion for other cloud types and longer time integrations.

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Thermal radiative fluxes through inhomogeneous cloud fields: a sensitivity study using a new stochastic cloud generator

Cited by 21 publications

References 56 publications

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

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

A flexible three-dimensional stratocumulus, cumulus and cirrus cloud generator (3DCLOUD) based on drastically simplified atmospheric equations and the Fourier transform framework

Multidimensional Longwave Forcing of Boundary Layer Cloud Systems

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