Frédéric Szczap

THE I3RC: Bringing Together the Most Advanced Radiative Transfer Tools for Cloudy Atmospheres

The interaction of clouds with solar and terrestrial radiation is one of the most important topics of climate research. In recent years it has been recognized that only a full three-dimensional (3D) treatment of this interaction can provide answers to many climate and remote sensing problems, leading to the worldwide development of numerous 3D radiative transfer (RT) codes. The international Intercomparison of 3D Radiation Codes (I3RC), described in this paper, sprung from the natural need to compare the performance of these 3D RT codes used in a variety of current scientific work in the atmospheric sciences. I3RC supports intercomparison and development of both exact and approximate 3D methods in its effort to 1) understand and document the errors/limits of 3D algorithms and their sources; 2) provide “baseline” cases for future code development for 3D radiation; 3) promote sharing and production of 3D radiative tools; 4) derive guidelines for 3D radiative tool selection; and 5) improve atmospheric science education in 3D RT. Results from the two completed phases of I3RC have been presented in two workshops and are expected to guide improvements in both remote sensing and radiative energy budget calculations in cloudy atmospheres.

Effective radiative properties of bounded cascade nonabsorbing clouds: Definition of the equivalent homogeneous cloud approximation

In the present study we investigated the radiative properties of inhomogeneous nonabsorbing clouds under the Equivalent plane-parallel Homogeneous Cloud Approximation (EHCA), by using the one-dimensional (1-D) bounded cascade inhomogeneous clouds. The effective optical depth was defined under the EHCA by requiring the identity of the radiant flux components of the radiation budget between the inhomogeneous clouds and their equivalent homogeneous counterparts. Such requirement provides a rational framework to define the effective optical depth of the inhomogeneous nonabsorbing clouds. We analyzed the dependency of the effective optical depth on the horizontal scale of averaging and solar incidence angle and specified the conditions under which an inhomogeneous cloud segment could be treated as a plane-parallel homogeneous cloud. A parameterization of the effective optical depth was proposed as a function of the mean optical depth and a relative cloud inhomogeneity parameter. Finally, we compared the EHCA with the effective thickness approximation, both based on the definition of the effective optical depth, and discussed the difference between their respective effective optical depths.

Effective radiative properties of bounded cascade absorbing clouds: Definition of an effective single-scattering albedo

We applied the equivalent homogeneous cloud approximation (EHCA) to the bounded cascade inhomogeneous absorbing clouds and defined their effective radiative properties. It is found that we have to introduce an effective single-scattering albedo in addition to an effective optical depth to treat the inhomogeneous absorbing clouds under the plane-parallel homogeneous cloud assumption. For an inhomogeneous absorbing cloud, a pair of the effective parameters can be estimated from each one of three possible pairs taken from the area-averaged reflectance, transmittance and absorptance. We found that the behavior of these effective properties was quite similar to those observed for the inhomogeneous non absorbing clouds except that two effective parameters were to be examined instead of only one effective parameter for the nonabsorbing clouds. Empirical relations for both the effective optical depth and the single-scattering albedo were given as a function of the local mean optical depth and relative local cloud inhomogeneity. We showed that the effective single-scattering albedo could not be properly introduced under the effective thickness approximation (ETA), which indicates an important conceptual difference between the EHCA and the ETA. Finally, we discussed possible consequences of the effective single-scattering albedo, defined in this study, with respect to the anomalous absorption phenomenon.

Inhomogeneity effects of 1D and 2D bounded cascade model clouds on their effective radiative properties

Abstract Inhomogeneous clouds generated with the bounded cascade process were used in a number of recent studies on the interaction between the radiative transfer process and inhomogeneous clouds. In this study, we investigate how the EHCA could be applied to 1D and 2D bounded cascade inhomogeneous absorbing clouds generated with different pairs of fractal parameters H and f . Firstly, we found that the empirical formulas we previously established were still applicable to other 1D and 2D inhomogeneous clouds with different single scattering albedo and with various types of fluctuations of the optical depth, except the case of very intermittent inhomogeneous clouds generated with H = 0 whatever f and H = 0.25 and f ≥ 0.3 (i.e. ϱ τ > 1.5 ∼ 2, where ϱ τ is a relative cloud inhomogeneity parameter defined as the standard deviation of the optical depth normalized by the mean optical depth). Secondly, we also found that the bias in the bidirectional reflectance factor between 1D and 2D inhomogeneous clouds radiance and their EHCA counterpart remained rather small (≤ 5%), again except the case of very intermittent inhomogeneous clouds with ϱ τ > 1.5 ∼ 2, for which this bias reached 20% to 60%.

EUREC 4 A: A Field Campaign to Elucidate the Couplings Between Clouds, Convection and Circulation

Trade-wind cumuli constitute the cloud type with the highest frequency of occurrence on Earth, and it has been shown that their sensitivity to changing environmental conditions will critically influence the magnitude and pace of future global warming. Research over the last decade has pointed out the importance of the interplay between clouds, convection and circulation in controling this sensitivity. Numerical models represent this interplay in diverse ways, which translates into different responses of trade-cumuli to climate perturbations. Climate models predict that the area covered by shallow cumuli at cloud base is very sensitive to changes in environmental conditions, while process models suggest the opposite. To understand and resolve this contradiction, we propose to organize a field campaign aimed at quantifying the physical properties of trade-cumuli (e.g., cloud fraction and water content) as a function of the large-scale environment. Beyond a better understanding of clouds-circulation coupling processes, the campaign will provide a reference data set that may be used as a benchmark for advancing the modelling and the satellite remote sensing of clouds and circulation. It will also be an opportunity for complementary investigations such as evaluating model convective parameterizations or studying the role of ocean mesoscale eddies in air–sea interactions and convective organization.

Evaluation of cloud heterogeneity effects on total and polarized visible radiances as measured by POLDER/PARASOL and consequences for retrieved cloud properties

Several sensors are dedicated to cloud observations. Among them, POLDER/PARASOL measures total and polarized visible radiances in up to 16 directions. As for other sensors, the POLDER cloud retrieval algorithm is based on the assumption that clouds are plan-parallel, homogeneous and infinite. To assess the cloud heterogeneities impacts on POLDER radiances and thus on retrieved cloud parameters, we developed a tri-dimensional radiative transfer model called 3DMCPOL allowing the computation of total and polarized radiances of 3D cloud fields. The input cloud properties are simulated with a model called 3Dcloud, which is based on a simplified dynamic/thermodynamic scheme to get cloud characteristic shape coupled with a Fourier stochastic approach to enforce cloud scale invariance. The POLDER algorithm is next applied to the simulated radiances to assess the 3D errors on the retrieved cloud parameters.

Cirrus Heterogeneity Effects on Cloud Optical Properties Retrieved with an Optimal Estimation Method from MODIS VIS to TIR Channels.

This study presents preliminary results on the effect of cirrus heterogeneities on top-of-atmosphere (TOA) simulated radiances or reflectances for MODIS channels centered at 0.86, 2.21, 8.56, 11.01 and 12.03 µm, and on cloud optical properties retrieved with a research-level optimal estimation method (OEM). Synthetic cirrus cloud fields are generated using a 3D cloud generator (3DCLOUD) and radiances/reflectances are simulated using a 3D radiative transfer code (3DMCPOL). We find significant differences between the heterogeneity effects on either visible and near-infrared (VNIR) or thermal infrared (TIR) radiances. However, when both wavelength ranges are combined, heterogeneity effects are dominated by the VNIR horizontal radiative transport effect. As a result, small optical thicknesses are overestimated and large ones are underestimated. Retrieved effective diameter are found to be slightly affected, contrarily to retrievals using TIR channels only.This study presents preliminary results on the effect of cirrus heterogeneities on top-of-atmosphere (TOA) simulated radiances or reflectances for MODIS channels centered at 0.86, 2.21, 8.56, 11.01 and 12.03 µm, and on cloud optical properties retrieved with a research-level optimal estimation method (OEM). Synthetic cirrus cloud fields are generated using a 3D cloud generator (3DCLOUD) and radiances/reflectances are simulated using a 3D radiative transfer code (3DMCPOL). We find significant differences between the heterogeneity effects on either visible and near-infrared (VNIR) or thermal infrared (TIR) radiances. However, when both wavelength ranges are combined, heterogeneity effects are dominated by the VNIR horizontal radiative transport effect. As a result, small optical thicknesses are overestimated and large ones are underestimated. Retrieved effective diameter are found to be slightly affected, contrarily to retrievals using TIR channels only.

A 3D polarized Monte Carlo LIDAR system simulator for studying effects of cirrus inhomogeneities on CALIOP/CALIPSO measurements

To estimate cirrus inhomogeneity effects on the apparent backscatter and on the apparent depolarization ratio measured by CALIOP/CALIPSO, a 3D polarized Monte Carlo LIDAR simulator was developed. Comparisons were done with the fast Hogans LIDAR simulator. Early results show that clouds inhomogeneous effects seem to be negligible on the apparent backscatter but not on the apparent depolarization ratio.

Assessment of cloud heterogeneities effects on brightness temperatures simulated with a 3D Monte Carlo code in the thermal infrared

This study presents preliminary results to investigate the impact of cirrus cloud heterogeneities on the thermal infrared radiative transfer in the Earth atmosphere. We use the code 3DCLOUD to generate cloud scenes and the 3D radiative transfer code 3DMCPOL extended to the thermal infrared to simulate measurements of the Infrared Imaging Radiometer (IIR) onboard the satellite CALIPSO. This paper shows that, the differences between 3D and ID brightness temperatures fields at a spatial resolution of 1 km are significant (up to 7K) and strongly dependent of the macrophysical and microphysical cloud properties.

Scale dependence of cirrus heterogeneity effects. Part II: MODIS VNIR and SWIR channels

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-ofthe-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 2.13μm solar reflectances over a cirrus 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 offnadir 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, nearinfrared and shortwave infrared channels compared with the thermal infrared channels discussed in Part I of this study. The contrast of 3-D radiative effects between solar and thermal infrared channels may be a significant issue for retrieval techniques that simultaneously use radiative measurements across a wide range of solar reflectance and infrared wavelengths. Published by Copernicus Publications on behalf of the European Geosciences Union. 12106 T. Fauchez et al.: Cirrus heterogeneity effects for MODIS NIR and SWIR channels