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Recent studies reveal that a higher fraction of coarse mineral dust particles than that estimated by climate model simulations has been observed in the atmosphere, leading to a more significant pos...

Thunderstorms occur all over the world, and produce flashes (optical and radio waves). From space, only the light scattered by the cloud is visible. Understanding the radiative transfer of light produced by the lightning discharges in the clouds is therefore fundamental. Observations made by low orbit satellites for twenty years gave the first global map of electrical activity of thunderstorms. Many on-board instruments can now detect lightning. For the first time, the current generation of geostationary meteorological satellites is equipped with lightning imagers. These satellites strongly contribute to the real-time alert of severe weather associated with thunderstorms. Simultaneously, the ASIM mission on board the International Space Station, can measure lightning at different wavelengths, from near-UV to near-IR (imagery and photometry) and provide complementary measurements to those of the geostationary satellites.The present study aims to better quantify the radiative transfer of the light emitted by lightning discharges through the cloud. We characterize optical lightning waveforms and images detected by satellites with three-dimensional simulation of photons transport through clouds. A forward three dimensional radiative code based on a Monte-Carlo approach (Cornet et al., 2010) is used in order to accurately simulate the scattering/absorption processes by cloud particles and molecules. The light emitted by the lightning source is simulated as a large number of photons with different temporal and spatial distribution. The simulations have been done for different wavelengths from the near-UV to the near infra-red close to those observed by the ASIM mission. Simulation results are compared to previous results from Light et al. (2001) and Luque et al. (2020) in the case of simple homogeneous water clouds. Furthermore, a sensitivity study is presented concerning the effect of the position, vertical extension and temporal character of the emitting source as well as the cloud microphysics on the signal observed at the top of the atmosphere.ReferencesCornet, C, L. C-Labonnote, F. Szczap (2010), Three-dimensional polarized monte carlo atmospheric radiative transfer model (3dmcpol): 3d effects on polarized visible reflectances of a cirrus cloud: Journal of Quantitative Spectroscopy and Radiative Transfer, 111(1), 174-186Light, T, D. Suszcynsky, M. Kirkland, A. Jacobson (2001), Simulations of lightning optical waveforms as seen clouds by satellites: Journal of Geophysical Research: Atmospheres, 106(D15), 17103-17114.A. Luque, F. J. Gordillo-Vázquez, D. Li, A. Malagón-Romero, F. J. Pérez-Invernón, A. Schmalzried, S. Soler, O. Chanrion, M. Heumesser, T. Neubert, et al. (2020), Modeling lightning observations from space-based platforms (cloudscat. Jl 1.0): Geoscientific Model Development, 13(11), 5549–5566

Besides their strong contribution to weather forecast improvement through data assimilation in clear-sky conditions, thermal infrared sounders on board polar orbiting platforms are now playing a key role in monitoring changes in atmospheric composition. However, it is known that clear sky observations are only a small part of the entire set of measurements, the remaining part is only slightly used as they are contaminated by either aerosols and/or clouds. Moreover, ice or liquid cloud retrieval of column and profile properties from passive and active measurements respectively help us in reaching a better understanding of climate processes. If the information provided by the latter has allowed a significant advance in our knowledge of the vertical distribution of condensed water, it suffers from spatial coverage compared to passive measurements. It is therefore fundamental to better characterize cloud properties from passive measurements by using, for example, high spectral resolution instruments such as IASI and the future IASI-NG.An information content analysis based on Shannon's formalism has been used to determine the level and the spectral repartition of the information about the ice cloud properties in the IASI and IASI-NG spectra. Based on this analysis, we have developped and tested an algorithm which allows to retrieve from an optimal estimation approach the cloud integrated ice water content together with the cloud layer altitude. We have taken into account the Signal-to-Noise ratio of each specific instrument and the uncertainties due to the non-retrieved atmospheric and surface parameters. The forward model is the fast radiative transfer model RTTOV which has been developped for satellite data assimilation in Numerical Weather Prediction (NWP) models. The ice cloud microphysical model is based on the ensemble model of Baran and Labonnote (2007), where the bulk ice optical properties have been parametrized as a function of the ice water content (expressed in g/m³) and in cloud temperature.The present study aims to quantify the potential and limits of thermal infrared sounders such as IASI or IASI-NG to retrieve ice cloud properties by using a representative dataset from the global operational short range forecast of the european center of medium-range weather forecast.

Following the release of the version 4 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data products from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, a new version (version 4; V4) of the CALIPSO Imaging Infrared Radiometer (IIR) Level 2 data products has been developed. The IIR Level 2 data products include cloud effective emissivities and cloud microphysical properties such as effective diameter and ice or liquid water path estimates. Dedicated retrievals for water clouds were added in V4, taking advantage of the high sensitivity of the IIR retrieval technique to small particle sizes. This paper (Part I) describes the improvements in the V4 algorithms compared to those used in the version 3 (V3) release, while results will be presented in a companion (Part II) paper. The IIR Level 2 algorithm has been modified in the V4 data release to improve the accuracy of the retrievals in clouds of very small (close to 0) and very large (close to 1) effective emissivities. To reduce biases at very small emissivities that were made evident in V3, the radiative transfer model used to compute clear-sky brightness temperatures over oceans has been updated and tuned for the simulations using Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) data to match IIR observations in clear-sky conditions. Furthermore, the clear-sky mask has been refined compared to V3 by taking advantage of additional information now available in the V4 CALIOP 5 km layer products used as an input to the IIR algorithm. After sea surface emissivity adjustments, observed and computed brightness temperatures differ by less than ±0.2 K at night for the three IIR channels centered at 08.65, 10.6, and 12.05 µm, and inter-channel biases are reduced from several tens of Kelvin in V3 to less than 0.1 K in V4. We have also improved retrievals in ice clouds having large emissivity by refining the determination of the radiative temperature needed for emissivity computation. The initial V3 estimate, namely the cloud centroid temperature derived from CALIOP, is corrected using a parameterized function of temperature difference between cloud base and top altitudes, cloud absorption optical depth, and CALIOP multiple scattering correction factor. As shown in Part II, this improvement reduces the low biases at large optical depths that were seen in V3 and increases the number of retrievals. As in V3, the IIR microphysical retrievals use the concept of microphysical indices applied to the pairs of IIR channels at 12.05 and 10.6 µm and at 12.05 and 08.65 µm. The V4 algorithm uses ice look-up tables (LUTs) built using two ice habit models from the recent “TAMUice2016” database, namely the single-hexagonal-column model and the eight-element column aggregate model, from which bulk properties are synthesized using a gamma size distribution. Four sets of effective diameters derived from a second approach are also reported in V4. Here, the LUTs are analytical functions relating microphysical index applied to IIR channels 12.05 and 10.6 µm and effective diameter as derived from in situ measurements at tropical and midlatitudes during the Tropical Composition, Cloud, and Climate Coupling (TC4) and Small Particles in Cirrus Science and Operations Plan (SPARTICUS) field experiments.

Following the release of the version 4 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data products from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, a new version 4 (V4) of the CALIPSO Imaging Infrared Radiometer (IIR) Level 2 data products has been developed. The IIR Level 2 data products include cloud effective emissivities and cloud microphysical properties such as effective diameter (De) and water path estimates for ice and liquid clouds. This paper (Part II) shows retrievals over ocean and describes the improvements made with respect to version 3 (V3) as a result of the significant changes implemented in the V4 algorithms, which are presented in a companion paper (Part I). The analysis of the three-channel IIR observations (08.65, 10.6, and 12.05 µm) is informed by the scene classification provided in the V4 CALIOP 5 km cloud layer and aerosol layer products. Thanks to the reduction of inter-channel effective emissivity biases in semi-transparent (ST) clouds when the oceanic background radiance is derived from model computations, the number of unbiased emissivity retrievals is increased by a factor of 3 in V4. In V3, these biases caused inconsistencies between the effective diameters retrieved from the 12/10 (βeff12/10 = τa,12/τa,10) and 12/08 (βeff12/08 = τa,12/τa,08) pairs of channels at emissivities smaller than 0.5. In V4, microphysical retrievals in ST ice clouds are possible in more than 80 % of the pixels down to effective emissivities of 0.05 (or visible optical depth ∼0.1). For the month of January 2008, which was chosen to illustrate the results, median ice De and ice water path (IWP) are, respectively, 38 µm and 3 g m−2 in ST clouds, with random uncertainty estimates of 50 %. The relationship between the V4 IIR 12/10 and 12/08 microphysical indices is in better agreement with the “severely roughened single column” ice habit model than with the “severely roughened eight-element aggregate” model for 80 % of the pixels in the coldest clouds (<210 K) and 60 % in the warmest clouds (>230 K). Retrievals in opaque ice clouds are improved in V4, especially at night and for 12/10 pair of channels, due to corrections of the V3 radiative temperature estimates derived from CALIOP geometric altitudes. Median ice De and IWP are 58 µm and 97 g m−2 at night in opaque clouds, with again random uncertainty estimates of 50 %. Comparisons of ice retrievals with Moderate Resolution Imaging Spectroradiometer (MODIS)/Aqua in the tropics show a better agreement of IIR De with MODIS visible–3.7 µm than with MODIS visible–2.1 µm in the coldest ST clouds and the opposite for opaque clouds. In prevailingly supercooled liquid water clouds with centroid altitudes above 4 km, retrieved median De and liquid water path are 13 µm and 3.4 g m−2 in ST clouds, with estimated random uncertainties of 45 % and 35 %, respectively. In opaque liquid clouds, these values are 18 µm and 31 g m−2 at night, with estimated uncertainties of 50 %. IIR De in opaque liquid clouds is smaller than MODIS visible–2.1 µm and visible–3.7 µm by 8 and 3 µm, respectively.

The present study aims to quantify the potential of hyperspectral thermal infrared sounders such as the Infrared Atmospheric Sounding Interferometer (IASI) and the future IASI next generation (IASI-NG) for retrieving the ice cloud layer altitude and thickness together with the ice water path. We employed the radiative transfer model Radiative Transfer for TOVS (RTTOV) to simulate cloudy radiances using parameterized ice cloud optical properties. The radiances have been computed from an ice cloud profile database coming from global operational short-range forecasts at the European Center for Medium-range Weather Forecasts (ECMWF) which encloses the normal conditions, typical variability, and extremes of the atmospheric properties over one year (Eresmaa and McNally (2014)). We performed an information content analysis based on Shannon&rsquo, s formalism to determine the amount and spectral distribution of the information about ice cloud properties. Based on this analysis, a retrieval algorithm has been developed and tested on the profile database. We considered the signal-to-noise ratio of each specific instrument and the non-retrieved atmospheric and surface parameter errors. This study brings evidence that the observing system provides information on the ice water path (IWP) as well as on the layer altitude and thickness with a convergence rate up to 95% and expected errors that decrease with cloud opacity until the signal saturation is reached (satisfying retrievals are achieved for clouds whose IWP is between about 1 and 300 g/m2).

Following the release of the Version 4 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data products from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, a new version 4 (V4) of the CALIPSO Imaging Infrared Radiometer (IIR) Level 2 data products has been developed. The IIR Level 2 data products include cloud effective emissivities and cloud microphysical properties such as effective diameter and ice or liquid water path estimates. Dedicated retrievals for water clouds were added in V4, taking advantage of the high sensitivity of the IIR retrieval technique to small particle sizes. This paper (Part I) describes the improvements in the V4 algorithms compared to those used in the version 3 (V3) release, while results will be presented in a companion (Part II) paper. To reduce biases at very small emissivities that were made evident in V3, the radiative transfer model used to compute clear sky brightness temperatures over oceans has been updated and tuned for the simulations using MERRA-2 data to match IIR observations in clear sky conditions. Furthermore, the clear-sky mask has been refined compared to V3 by taking advantage of additional information now available in the V4 CALIOP 5-km layer products used as an input to the IIR algorithm. After sea surface emissivity adjustments, observed and computed brightness temperatures differ by less than ± 0.2 K at night for the three IIR channels centered at 08.65, 10.6, and 12.05 µm, and inter-channel biases are reduced from several tens of Kelvin in V3 to less than 0.1 K in V4. We have also aimed at improving retrievals in ice clouds having large optical depths by refining the determination of the radiative temperature needed for emissivity computation. The initial V3 estimate, namely the cloud centroid temperature derived from CALIOP, is corrected using a parameterized function of temperature difference between cloud base and top altitudes, cloud absorption optical depth, and the CALIOP multiple scattering correction factor. As shown in Part II, this improvement reduces the low biases at large optical depths that were seen in V3, and increases the number of retrievals in dense ice clouds. As in V3, the IIR microphysical retrievals use the concept of microphysical indices applied to the pairs of IIR channels at 12.05 μm and 10.6 μm and at 12.05 μm and 08.65 μm. The V4 algorithm uses ice look-up tables (LUTs) built using two ice crystal models from the recent TAMUice 2016 database, namely the single hexagonal column model and the 8-element column aggregate model, from which bulk properties are synthesized using a gamma size distribution. Four sets of effective diameters derived from a second approach are also reported in V4. Here, the LUTs are analytical functions relating microphysical index applied to IIR channels 12.05 µm and 10.6 µm and effective diameter as derived from in situ measurements at tropical and mid-latitudes during the TC4 and SPARTICUS field experiments.

Following the release of the Version 4 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data products from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, a new version 4 (V4) of the CALIPSO Imaging Infrared Radiometer (IIR) Level 2 data products has been developed. The IIR Level 2 data products include cloud effective emissivities and cloud microphysical properties such as effective diameter (De) and ice or liquid water path estimates. This paper (Part II) shows retrievals over ocean and describes the improvements made with respect to version (V3) as a result of the significant changes implemented in the V4 algorithms, which are presented in a companion paper (Part I). The analysis of the three-channel IIR observations (08.65 μm, 10.6 μm, and 12.05 μm) is informed by the scene classification provided in the V4 CALIOP 5-km cloud layer and aerosol layer products. Thanks to the reduction of inter-channel effective emissivity biases in semi-transparent (ST) clouds when the oceanic background radiance is derived from model computations, the number of unbiased emissivity retrievals is increased by a factor 3 in V4. In V3, these biases caused inconsistencies between the effective diameters retrieved from the 12/10 and 12/08 pairs of channels at emissivities smaller than 0.5. In V4, microphysical retrievals in ST ice clouds are possible in more than 80 % of the pixels down to effective emissivities of 0.05 (or visible optical depth ~ 0.1). For the month of January 2008 chosen to illustrate the results, median ice De and ice water path (IWP) are, respectively, 38 µm and 3 g⋅m−2 in ST clouds, with random uncertainty estimates of 50 %. The relationship between the V4 IIR 12/10 and 12/08 microphysical indices is in better agreement with the severely roughened single column ice crystal model than with the severely roughened 8-element aggregate model for 80 % of the pixels in the coldest clouds ( 230 K). Retrievals in opaque ice clouds are improved in V4, especially at night and for 12/10 pair of channels, owing to corrections of the V3 radiative temperature estimates derived from CALIOP geometric altitudes. Median ice De and IWP are 58 µm and 97 g⋅m−2 at night in opaque clouds, with again random uncertainty estimates of 50 %. Comparisons of ice retrievals with Aqua/Moderate Resolution Imaging Spectroradiometer (MODIS) in the tropics show a better agreement of IIR De with MODIS visible/3.7 µm than with MODIS visible/2.1 µm in the coldest ST clouds and the opposite for opaque clouds. In prevailingly supercooled liquid water clouds with centroid altitudes above 4 km, retrieved median De and liquid water path are 13 µm and 3.4 g.m−2 in ST clouds, with estimated random uncertainties of 45 % and 35 % respectively. In opaque liquid clouds, these values are 18 µm and 31 g.m−2 at night, with estimated uncertainties of 50 %. IIR De in opaque liquid clouds is smaller than MODIS visible/2.1 and visible/3.7 by 8 µm and 3 µm, respectively.

Radiative cooling and heating impact the liquid water balance of fog and therefore play an important role in determining their persistence or dissipation. We demonstrate that a quantitative analysis of the radiation-driven condensation and evaporation is possible in real time using ground-based remote sensing observations (cloud radar, ceilometer, microwave radiometer). Seven continental fog events in midlatitude winter are studied, and the radiative processes are further explored through sensitivity studies. The longwave (LW) radiative cooling of the fog is able to produce 40–70 g m−2 h−1 of liquid water by condensation when the fog liquid water path exceeds 30 g m−2 and there are no clouds above the fog, which corresponds to renewing the fog water in 0.5–2 h. The variability is related to fog temperature and atmospheric humidity, with warmer fog below a drier atmosphere producing more liquid water. The appearance of a cloud layer above the fog strongly reduces the LW cooling relative to a situation with no cloud above; the effect is strongest for a low cloud, when the reduction can reach 100 %. Consequently, the appearance of clouds above will perturb the liquid water balance in the fog and may therefore induce fog dissipation. Shortwave (SW) radiative heating by absorption by fog droplets is smaller than the LW cooling, but it can contribute significantly, inducing 10–15 g m−2 h−1 of evaporation in thick fog at (winter) midday. The absorption of SW radiation by unactivated aerosols inside the fog is likely less than 30 % of the SW absorption by the water droplets, in most cases. However, the aerosols may contribute more significantly if the air mass contains a high concentration of absorbing aerosols. The absorbed radiation at the surface can reach 40–120 W m−2 during the daytime depending on the fog thickness. As in situ measurements indicate that 20–40 % of this energy is transferred to the fog as sensible heat, this surface absorption can contribute significantly to heating and evaporation of the fog, up to 30 g m−2 h−1 for thin fog, even without correcting for the typical underestimation of turbulent heat fluxes by the eddy covariance method. Since the radiative processes depend mainly on the profiles of temperature, humidity and clouds, the results of this paper are not site specific and can be generalised to fog under different dynamic conditions and formation mechanisms, and the methodology should be applicable to warmer and moister climates as well. The retrieval of approximate emissivity of clouds above fog from cloud radar should be further developed.

We investigate the impact of cirrus cloud heterogeneity on the direct emission by cloud or surface and on the scattering by ice particles in the thermal infrared (TIR). Realistic 3-D cirri are modeled with the 3DCLOUD code, and top-of-atmosphere radiances are simulated by the 3-D Monte Carlo radiative transfer (RT) algorithm 3DMCPOL for two (8.65 micrometers and 12.05 micrometers) channels of the Imaging Infrared Radiometer on CALIPSO. At nadir, comparisons of 1-D and 3-D RT show that 3-D radiances are larger than their 1-D counterparts for direct emission but smaller for scattered radiation. For our cirrus cases, 99% of the 3-D total radiance is computed by the third scattering order, which corresponds to 90% of the total computational effort, but larger optical thicknesses need more scattering orders. To radically accelerate the 3-D RT computations (using only few percent of 3-D RT time with a Monte Carlo code), even in the presence of large optical depths, we develop a hybrid model based on exact 3-D direct emission, the first scattering order from 1-D in each homogenized column, and an empirical adjustment linearly dependent on the optical thickness to account for higher scattering orders. Good agreement is found between the hybrid model and the exact 3-D radiances for two very different cirrus models without changing the empirical parameters. We anticipate that a future deterministic implementation of the hybrid model will be fast enough to process multiangle thermal imagery in a practical tomographic reconstruction of 3-D cirrus fields.

The aim of this work is to describe a Spectral Mapping Method (SMM) to split spectral intervals into smaller sets of wavenumbers, called intervals of comonotonicity, over which gas spectra in distinct states are rigorously linked through a strictly increasing function. Over small intervals of comonotonicity, the proposed method becomes, in theory, exact. The step-by-step process of construction of intervals of comonotonicity is described and explained. The present work focuses on the two-cell problem. Despite its full generality for the treatment of the blurring effect in k-distribution approaches, the strength of the method is illustrated: 1/ with a high number of subintervals on an IR signature configuration, widely recognized as among the most challenging in band model theory; 2/ with only two subintervals, in a highly non-uniform two-cell configuration. Comparisons with reference Line-By-Line calculations and a mapping technique founded on a scaled map illustrate its relevance for radiative heat transfer and spectroscopic applications.

Accurate calculations of shortwave reflectances in clear-sky aerosol-laden atmospheres are necessary for various applications in atmospheric sciences. However, computational cost becomes increasingly important for some applications such as data assimilation of top-of-atmosphere reflectances in models of atmospheric composition. This study aims to provide a benchmark that can help in assessing these two requirements in combination. We describe a protocol and input data for 44 080 cases involving various solar and viewing geometries, four different surfaces (one oceanic bidirectional reflectance function and three albedo values for a Lambertian surface), eight aerosol optical depths, five wavelengths, and four aerosol types. We first consider two models relying on the discrete ordinate method: VLIDORT (in vector and scalar configurations) and DISORT (scalar configuration only). We use VLIDORT in its vector configuration as a reference model and quantify the loss of accuracy due to (i) neglecting the effect of polarization in DISORT and VLIDORT (scalar) models and (ii) decreasing the number of streams in DISORT. We further test two other models: the 6SV2 model, relying on the successive orders of scattering method, and Forward-Lobe Two-Stream Radiance Model (FLOTSAM), a new model under development by two of the authors. Typical mean fractional errors of 2.8 % and 2.4 % for 6SV2 and FLOTSAM are found, respectively. Computational cost depends on the input parameters but also on the code implementation and application as some models solve the radiative transfer equations for a range of geometries while others do not. All necessary input and output data are provided as a Supplement as a potential resource for interested developers and users of radiative transfer models.

Le Cnes, en collaboration avec le CNRS/Insu et Météo-France, organisait en janvier 2020 à Toulouse la troisième édition de l'atelier Trattoria (Transfert radiatif dans les atmosphères terrestres pour les observations spatiales). Cet atelier est principalement consacré aux codes de transfert radiatif dans l'atmosphère terrestre pour les applications de télédétection spatiale, opérant sur l'ensemble de la gamme des longueurs d'onde de l'ultraviolet aux micro-ondes. Ces codes numériques sont fondamentaux pour la préparation des instruments de télédétection, ainsi que pour le traitement et l'exploitation des données satellitaires. Cet atelier était ouvert à tous les chercheurs, ingénieurs, post-doctorants et doctorants du domaine. Les résultats et recommandations de l'atelier doivent servir de guide au Cnes et aux divers participants et utilisateurs français et européens de codes de transfert radiatif. The CNES, in collaboration with the CNRS/INSU and Météo-France, organized in January 2020 in Toulouse the third edition of the Trattoria workshop (Transfert radiatif dans les atmosphères terrestres pour les observations spatiales). This workshop is mainly devoted to radiation transfer codes in the Earth's atmosphere for space remote sensing applications, operating over the entire wavelength range from ultraviolet to microwaves. These numerical codes are fundamental for the preparation of remote sensing instruments, as well as for the processing and exploitation of satellite data. This workshop was open to all researchers, engineers, post-doctoral and doctoral students in the field. The results and recommendations of the workshop should serve as a "guide" for CNES and the various French and European participants and users of radiative transfer codes.

We use computational 1D radiative transfer modeling and the formalism of optimal estimation to quantify the cloud information content in the oxygen A- and B-band channels of the Earth Polychromatic Imaging Camera (EPIC) on the Deep Space Climate ObserVatoRy (DSCOVR) platform. EPIC/DSCOVR images the sunlit hemisphere of our planet from ≈ 1,500,000 km away with ≈ 8 km pixels at the center of the disc. EPIC pixel-scale spectral data is used to estimate in-band/continuum radiance ratios for O2’s A- and B-bands, from which one can derive, in principle, both cloud top height and cloud geometrical thickness. We use the general framework of optimal estimation theory to show that in practice, once measurement error is factored in, only cloud top height can be reliably inferred. With that limitation in mind, we discuss the ramifications for retrieval algorithm development.

A fruitful approach for light-droplet interaction in atmospheric optics, the quantum optical analogy combined with partial wave analysis, is used to characterize (location, width) very sharp tunneling optical resonances (TORs). For this purpose, a fast and flexible technique of computation, the transfer matrix method, has been developed; it is described herein. This paper proposes explicit calculations of TORs, considering isolated droplets and a droplet population, and so goes further than earlier studies. Applied in the context of POLDER observations, from the visible to the near infrared, it is shown that TORs enhance cross sections with respect to Mie's theory as currently computed in atmospheric optics. Precisely, for a typical warm cloud droplet population, this enhancement can reach almost 30% in the case of absorption. Scattering and extinction cross sections are also higher than those of Mie's theory, but to a lesser extent. As a conclusion, it is suggested that tunneling should be taken into account more explicitly when addressing light scattering by droplets. No scattering inversion technique is investigated in this paper, but for the time being, the results presented can be used as look-up tables (at least orders of magnitude) for practical computations in atmospheric remote sensing. Finally, the approach presented herein is proposed as a perspective to be used in different situations involving other spherical (or almost spherical) scatterers that otherwise should be addressed approximately.

Atmospheric aging promotes internal mixing of black carbon (BC), leading to an enhancement of light absorption and radiative forcing. The relationship between BC mixing state and consequent absorption enhancement was never estimated for BC found in the Arctic region. In the present work, we aim to quantify the absorption enhancement and its impact on radiative forcing as a function of microphysical properties and mixing state of BC observed in situ at the Zeppelin Arctic station (78∘ N) in the spring of 2012 during the CLIMSLIP (Climate impacts of short-lived pollutants in the polar region) project. Single-particle soot photometer (SP2) measurements showed a mean mass concentration of refractory black carbon (rBC) of 39 ng m−3, while the rBC mass size distribution was of lognormal shape, peaking at an rBC mass-equivalent diameter (DrBC) of around 240 nm. On average, the number fraction of particles containing a BC core with DrBC>80 nm was less than 5 % in the size range (overall optical particle diameter) from 150 to 500 nm. The BC cores were internally mixed with other particulate matter. The median coating thickness of BC cores with 220 nm < DrBC< 260 nm was 52 nm, resulting in a core–shell diameter ratio of 1.4, assuming a coated sphere morphology. Combining the aerosol absorption coefficient observed with an Aethalometer and the rBC mass concentration from the SP2, a mass absorption cross section (MAC) of 9.8 m2 g−1 was inferred at a wavelength of 550 nm. Consistent with direct observation, a similar MAC value (8.4 m2 g−1 at 550 nm) was obtained indirectly by using Mie theory and assuming a coated-sphere morphology with the BC mixing state constrained from the SP2 measurements. According to these calculations, the lensing effect is estimated to cause a 54 % enhancement of the MAC compared to that of bare BC particles with equal BC core size distribution. Finally, the ARTDECO radiative transfer model was used to estimate the sensitivity of the radiative balance to changes in light absorption by BC as a result of a varying degree of internal mixing at constant total BC mass. The clear-sky noontime aerosol radiative forcing over a surface with an assumed wavelength-dependent albedo of 0.76–0.89 decreased, when ignoring the absorption enhancement, by −0.12 W m−2 compared to the base case scenario, which was constrained with mean observed aerosol properties for the Zeppelin site in Arctic spring. The exact magnitude of this forcing difference scales with environmental conditions such as the aerosol optical depth, solar zenith angle and surface albedo. Nevertheless, our investigation suggests that the absorption enhancement due to internal mixing of BC, which is a systematic effect, should be considered for quantifying the aerosol radiative forcing in the Arctic region.

Atmospheric analysis data from the European Center for Medium-Range Weather Forecasts (ECMWF) have been acquired and are used to characterize the meteorological situation in Maroua, Cameroon ( 10.614 ∘ N, 14.361 ∘ E) at 12:00 UTC. These are then used to simulate downward global solar radiation (DGSR) with the moderate-resolution transmittance (MODTRAN) radiative transfer code (RTC). In comparison with meteorological data measured during the year 2014 in Maroua, ECMWF atmospheric quantities at ground level, in general, showed good correlation coefficients and slight differences. It is shown that ECMWF atmospheric profiles can thus be used to complete the scarce atmospheric data and to study the atmosphere state and dynamics, such as the African monsoon phenomenon detected in this region, which regulates the rainy season. In addition, they are more suitable to simulate clear-sky DGSR compared to MODTRAN standard atmospheric profiles. The causes and effects of the substantial bias and weak correlation coefficient observed with ECMWF wind data and the constant underestimation of simulated DGSR in comparison with ground-based measurements are investigated. The paper emphasizes the need for a better characterization of the Maroua atmosphere state and dynamics as well as the simulation of more accurate and reliable DGSR under any atmospheric conditions.

The Eyjafjallajökull eruption, which occurred during May 2010, is used as a case study to evaluate the consistency of the detection and characterization of volcanic ash plumes from different thermal infrared instruments. In this study, the well-known split window technique is used to retrieve the optical thickness and the effective particle size, and to estimate the mass concentration of volcanic particles from brightness temperatures measured in the infrared atmospheric window (8–12 μm). Retrievals are obtained for several mineral compositions whose optical properties are computed using Mie theory accounting for spectral variations of the refractive index. The impacts of errors in atmospheric parameters on the a posteriori uncertainties have been analysed. This analysis confirmed that major sources of errors are the layer altitude, the particle composition and, most of all, the size distribution for which uncertainties in retrievals can reach 50% in mass loading estimates. This retrieval algorithm is then applied to measurements acquired near-simultaneously from MODIS, SEVIRI and IASI space-borne instruments, using two channels around 11 μm and 12 μm. The retrievals are in close agreement when taking into account the different spatial and spectral configurations, and deviations between retrievals remain less than the uncertainties due to errors in atmospheric parameters. This analysis demonstrates the robustness of the retrieval method and the consistency of observations from these instruments for volcanic ash plume monitoring.

International audience

According to climate models’ simulations, cloud altitude change is the dominant contributor of the positive ensemble mean longwave cloud feedback. Nevertheless, the cloud altitude longwave feedback mechanism and its amplitude struggle yet to be verified in observations. An accurate, stable in time, and potentially long-term observation of a cloud property summarizing the cloud vertical distribution and driving the longwave cloud radiative effect is needed to hope to achieve a better understanding of the cloud altitude longwave feedback mechanism. This study proposes the direct lidar measurement of the atmosphere opacity altitude is a good candidate to derive the needed observed cloud property. This altitude is the level at which a space-borne lidar beam is fully attenuated when probing an optically opaque cloud. By combining this altitude with the direct lidar measurement of the cloud top altitude, we derive the radiative temperature of opaque clouds that linearly drives, as we show, the outgoing longwave radiation. This linear relationship provides a simple formulation of the cloud radiative effect in the longwave domain for opaque clouds and so, helps to understand the cloud altitude longwave feedback mechanism. We find that in presence of an opaque cloud, a cloud temperature change of 1 K modifies its cloud radiative effect by 2 W m−2. We show that this linear relationship holds true at single atmospheric column scale with radiative transfer simulations, at instantaneous radiometer footprint scale of the Clouds and the Earth's Radiant Energy System (CERES), and at monthly mean 2° x 2° gridded scale. Opaque clouds cover 35 % of the ice-free ocean and contribute to 73 % of the global mean cloud radiative effect. Thin clouds cover 36 % and contribute to 27 %.

This paper presents a study of the impact of cirrus cloud heterogeneities on the thermal infrared brightness temperatures at the top of the atmosphere (TOA). Realistic 3-D cirri are generated by a cloud generator based on simplified thermodynamic and dynamic equations and on the control of invariant scale properties. The 3-D thermal infrared radiative transfer is simulated with a Monte Carlo model for three typical spectral bands in the infrared atmospheric window. Comparisons of TOA brightness temperatures resulting from 1-D and 3-D radiative transfer show significant differences for optically thick cirrus (τ > 0.3 at 532 nm) and are mainly due to the plane-parallel approximation (PPA). At the spatial resolution of 1 km × 1 km, two principal parameters control the heterogeneity effects on brightness temperatures: i) the optical thickness standard deviation inside the observation pixel, ii) the brightness temperature contrast between the top of the cirrus~and the clear-sky atmosphere. Furthermore, we show that the difference between 1-D and 3-D brightness temperatures increases with the zenith view angle from two to ten times between 0° and 60° due to the tilted independent pixel approximation (TIPA).

The Greenhouse gases Observing SATellite (GOSAT) mission, and in particular the Thermal And Near infrared Sensor for carbon Observations–Fourier Transform Spectrometer (TANSO-FTS) instrument, has the advantage of being able to measure simultaneously the same field of view in different spectral ranges with a high spectral resolution. These features allow studying the benefits of using multispectral measurements to improve the CO2 and CH4 retrievals. In order to quantify the impact of the spectral synergy on the retrieval accuracy, we performed an information content (IC) analysis from simulated spectra corresponding to the three infrared bands of TANSO-FTS. The advantages and limitations of using thermal and shortwave infrared simultaneously are discussed according to surface type and state vector composition. The IC is then used to determine the most informative spectral channels for the simultaneous retrieval of CO2 and CH4. The results show that a channel selection spanning the three infrared bands can improve the computation time and retrieval accuracy. Therefore, a selection of less than 700 channels from the thermal infrared (TIR) and shortwave infrared (SWIR) bands allows retrieving CO2 and CH4 simultaneously with a similar accuracy to using all channels together to retrieve each gas separately.

This article is the second in a series of studies investigating the benefits of multispectral measurements to improve the atmospheric parameter retrievals. In the first paper, we presented an information content (IC) analysis from the thermal infrared (TIR) and shortwave infrared (SWIR) bands of Thermal And Near infrared Sensor for carbon Observations–Fourier Transform Spectrometer (TANSO-FTS) instrument dedicated to greenhouse gas retrieval in clear sky conditions. This second paper presents the potential of the spectral synergy from TIR to visible for aerosol characterization, and their impact on the retrieved CO2 and CH4 column concentrations. The IC is then used to determine the most informative spectral channels for the simultaneous retrieval of greenhouse gas total columns and aerosol parameters. The results show that a channel selection spanning the four bands can improve the computation time and retrieval accuracy. Therefore, the spectral synergy allows obtaining up to almost seven different aerosol parameters, which is comparable to the most informative dedicated instruments. Moreover, a channel selection from the TIR to visible bands allows retrieving CO2 and CH4 total columns simultaneously in the presence of one aerosol layer with a similar accuracy to using all channels together to retrieve each gas separately in clear sky conditions.

International audience; Projections of aerosol emissions for 2030 have been recently generated and implemented in a comprehensive chemistry-transport model to analyse the future evolution of the aerosol radiative forcing over Europe. In this study, numerical developments based on an off-line coupling between the regional chemistry-transport model CHIMERE (extended by an aerosol optical module) and the radiative transfer code GAME have been implemented in order to simulate the interaction of physico-chemically resolved aerosols with radiation at regional scale. This novel approach is used to examine the shortwave aerosol direct radiative forcing response to two airpollution reduction scenarios for 2030 over Europe. Our study suggests that measures introduced to improve futureair quality could have large implication on the aerosol climate forcing over Europe. Results of simulations indicate that abatement of aerosols in the near future could lead to a decrease of the aerosol cooling effect at the surface and at the top of the atmosphere over the main anthropogenic emission regions. Especially over the Moscow region, different strategies of reduction for scattering sulphate and absorbing black carbon aerosols between the two scenarios could result, however, in either a reduction or an enhancement in atmospheric radiative forcing.

Our goal is to establish a climatology of cirrus cloud properties over Lille, France (50.60°N, 3.14 °E) using a ground-based lidar. A statistical analysis of mid-latitude cirrus clouds from lidar data in Lille over the period 2008-2013 is presented and discussed. The macrophysical properties (cloud base altitude, cloud top altitude, geometrical thickness, mid-cloud temperature) and optical properties (cloud optical thickness and lidar ratio) are evaluated and compared between the ground-based and the spaceborne lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) measurements for the period 2008-2013. We found similar results of macrophysical properties derived from both lidars. In addition a difference in the optical properties results is due to the multiple scattering and the heterogeneity of the observed scenes.

In this paper we combine SO2 and ash plume dispersion modelling with satellite and surface remote sensing observations to study the regional influence of a relatively weak volcanic eruption from Mount Etna on the optical and micro-physical properties of Mediterranean aerosols. We analyse the Mount Etna eruption episode of 25–27 October 2013. The evolution of the plume along the trajectory is investigated by means of the FLEXible PARTicle Lagrangian dispersion (FLEXPART) model. The satellite data set includes true colour images, retrieved values of volcanic SO2 and ash, estimates of SO2 and ash emission rates derived from MODIS (MODerate resolution Imaging Spectroradiometer) observations and estimates of cloud top pressure from SEVIRI (Spinning Enhanced Visible and InfraRed Imager). Surface remote sensing measurements of aerosol and SO2 made at the ENEA Station for Climate Observations (35.52° N, 12.63° E; 50 m a.s.l.) on the island of Lampedusa are used in the analysis. The combination of these different data sets suggests that SO2 and ash, despite the initial injection at about 7.0 km altitude, reached altitudes around 10–12 km and influenced the column average aerosol particle size distribution at a distance of more than 350 km downwind. This study indicates that even a relatively weak volcanic eruption may produce an observable effect on the aerosol properties at the regional scale. The impact of secondary sulfate particles on the aerosol size distribution at Lampedusa is discussed and estimates of the clear-sky direct aerosol radiative forcing are derived. Daily shortwave radiative forcing efficiencies, i.e. radiative forcing per unit AOD (aerosol optical depth), are calculated with the LibRadtran model. They are estimated between −39 and −48 W m−2 AOD−1 at the top of the atmosphere and between −66 and −49 W m−2 AOD−1 at the surface, with the variability in the estimates mainly depending on the aerosol single scattering albedo. These results suggest that sulfate particles played a large role in the transported plume composition and radiative forcing, while the contribution by ash particles was small in the volcanic plume arriving at Lampedusa during this event.

The evaluation of aerosol radiative effect on broadband hemispherical solar flux is often performed using simplified spectral and directional scattering characteristics of atmospheric aerosol and underlying surface reflectance. In this study we present a rigorous yet fast computational tool that accurately accounts for detailed variability of both spectral and angular scattering properties of aerosol and surface reflectance in calculation of direct aerosol radiative effect. The tool is developed as part of the GRASP (Generalized Retrieval of Aerosol and Surface Properties) project. We use the tool to evaluate instantaneous and daily average radiative efficiencies (radiative effect per unit aerosol optical thickness) of several key atmospheric aerosol models over different surface types. We then examine the differences due to neglect of surface reflectance anisotropy, nonsphericity of aerosol particle shape and accounting only for aerosol angular scattering asymmetry instead of using full phase function. For example, it is shown that neglecting aerosol particle nonsphericity causes mainly overestimation of the aerosol cooling effect and that magnitude of this overestimate changes significantly as a function of solar zenith angle (SZA) if the asymmetry parameter is used instead of detailed phase function. It was also found that the nonspherical–spherical differences in the calculated aerosol radiative effect are not modified significantly if detailed BRDF (bidirectional reflectance distribution function) is used instead of Lambertian approximation of surface reflectance. Additionally, calculations show that usage of only angular scattering asymmetry, even for the case of spherical aerosols, modifies the dependence of instantaneous aerosol radiative effect on SZA. This effect can be canceled for daily average values, but only if sun reaches the zenith; otherwise a systematic bias remains. Since the daily average radiative effect is obtained by integration over a range of SZAs, the errors vary with latitude and season. In summary, the present analysis showed that use of simplified assumptions causes systematic biases, rather than random uncertainties, in calculation of both instantaneous and daily average aerosol radiative effect. Finally, we illustrate application of the rigorous aerosol radiative effect calculations performed as part of GRASP aerosol retrieval from real POLDER/PARASOL satellite observations.

The development of ODS materials for the cladding for Sodium Fast Reactors is a key issue to achieve the objectives required for the GEN IV reactors. CEA, AREVA and EDF have launched in 2007 an important program to determine the optimal fabrication parameters, and to measure and understand the microstructure and properties before, under and after irradiation of such cladding materials. The aim of this paper is to present the French program and the major results obtained recently at CEA on Fe–9/14/18Cr1WTiY 2 O 3 ferritic/martensitic ODS materials. The first step of the program was to consolidate Fe–9/14/18Cr ODS materials as plates and bars to study the microstructure and the mechanical properties of the new alloys. The second step consists in producing tubes at a geometry representative of the cladding of new Sodium Fast Reactors. The optimization of the fabrication route at the laboratory scale is conducted and different tubes were produced. Their microstructure depends on the martensitic (Fe–9Cr) or ferritic (Fe–14Cr) structure. To join the plug to the tube, the reference process is the welding resistance. A specific approach is developed to model the process and support the development of the welds performed within the “SOPRANO” facility. The development at CEA of Fe–9/14/18Cr new ODS materials for the cladding for GENIV Sodium Fast Reactors is in progress. The first microstructural and mechanical characterizations are very encouraging and the full assessment and qualification of this new alloys and products will pass through the irradiation of specimens, tubes, fuel pins and subassemblies up to high doses.

In this paper we combine SO2/ash plume dispersion modelling, satellite and surface remote sensing observations to study the regional influence of a relatively weak volcanic eruption from Mount Etna on the optical and micro-physical properties of Mediterranean aerosols. We analyse the Mount Etna eruption episode of 25–27 October 2013. The evolution of the plume along the trajectory is investigated by means of the FLEXPART (FLEXible PARTicle dispersion model) Lagrangian dispersion model. The satellite dataset includes true colour images, retrieved values of volcanic SO2 and ash, and estimates of SO2 and ash emission rates derived from MODIS (MODerate resolution Imaging Spectroradiometer) observations, and estimates of cloud top pressure from SEVIRI (Spinning Enhanced Visible and InfraRed Imager). Surface remote sensing measurements of aerosol and SO2 made at the ENEA Station for Climate Observations (35.52° N, 12.63° E, 50 m a.s.l.) on the island of Lampedusa are used in the analysis. The combination of these different datasets suggests that SO2 and ash, despite the initial injection occurred at about 7.0 km altitude, reached altitudes around 10–12 km and influenced the aerosol size distribution at a distance more than 350 km downwind. This study indicates that even a relatively weak volcanic eruption may produce an observable effect on the aerosol properties at the regional scale. The impact of secondary sulphate particles on the aerosol size distribution at Lampedusa is discussed, and estimates of the clear sky direct aerosol radiative forcing are derived. Daily shortwave radiative forcing efficiencies are calculated with the LibRadtran model. They are estimated between −39 and −48 W m−2 AOD−1 at the top of the atmosphere, and between −66 and −49 W m−2 AOD−1, at the surface, with the variability in the estimates mainly depending on the aerosol single scattering albedo. These results suggest that sulphate particles played a large role, while the contribution by ash particles was small in the volcanic plume arriving at Lampedusa during this event.

This paper presents the sensitivity to various atmospheric parameters of two height assignment methods that aim to retrieve the cloud-top height of semitransparent clouds. The use of simulated Meteosat-8 radiances has the advantage that the pressure retrieved by a given method can be compared to the initial pressure set to the cloud in the model, which is exactly known. The methods retrieve the pressure of a perfectly opaque cloud to within a few hectopascals. However, considering more realistic ice clouds, methods are sensitive to all of the tested atmospheric parameters and, especially, to the cloud microphysics, which can bias the results of the CO2-slicing method by several tens of hectopascals. The cloud-top pressure retrieval is especially difficult for thinner clouds with optical thicknesses smaller than 2, for which the errors can reach several tens of hectopascals. The methods have also been tested after introducing realistic perturbations in the temperature and humidity profiles and on the clear-sky surface radiances. The corresponding averages of errors on the retrieved pressures are also very large, especially for thin clouds. In multilayer cloud situations the height assignment methods do not work properly, placing the cloud-top height somewhere between the two cloud layers for most cirrus cloud layers with optical thicknesses between 0.1 and 10.

Current infrared narrowband instruments, such as Moderate Resolution Imaging Spectrometer (MODIS) or Spinning Enhanced Visible and InfraRed Imager (SEVIRI), can be used to locate volcanic SO 2 plumes. A three-channel technique has been proposed by Prata et. al. [Prata, A.J., Rose, W.I., Self, S., O'Brien, D.M., 2003. Global, long-term sulphur dioxide measurements from TOVS data: a new tool for studying explosive volcanism and climate. AGU Geophysical Monograph 139: Volcanism and the Earth's Atmosphere (pp. 75–92). Ed. by A. Robock and C. Oppenhaimer.], which considers the difference in brightness temperature (BT) in the SO 2 absorption band around 7–8 µm between an observed value and an estimated value interpolated from two other channels. The technique relies on the assumption that the interpolated BT value is a good approximation of the BT that would have been observed in the absence of SO 2 . In this study we use a sophisticated radiative transfer model in order to test this assumption and assess the sensitivity of this three-channel technique to the vertical profiles of temperature and water vapour, the surface temperature, the vertical distribution of the SO 2 , the viewing geometry, the presence of ash, and the presence of low- and high-level clouds. The capabilities and limitations of such a technique are discussed. An example is also shown as a proof of concept using data from the SEVIRI instrument onboard the MSG satellite.

Sensitivity of the solutions of forward and inverse problems of the atmospheric-surface parameters retrieval to spectral shifts of channels is considered. The solution of the forward problem is obtained by radiative transfer simulation. The retrieval (solution of the inverse problem) is obtained by linear (optimal interpolation) and non-linear (variational) techniques. It is shown that atmospheric temperature profiles exhibit high sensitivity to the spectral shift, while the humidity profiles are moderately sensitive while the sea surface temperatures retrievals are insensitive. Two approaches are proposed to reduce the effect of channel spectral shift, one is based on channel selection and the other approach is related to proper calibration of the cost function. We performed the numerical simulations using the parameters of AIRS spectrometer to illustrate the sensitivity of forward and inverse problems. The results of the simulation show that the inversion error can be significantly reduced by the proposed techniques.

Materials of the core internals of Pressurized Water Reactors (austenitic stainless steels) are submitted to neutron irradiation. Some baffle/former bolt cracking were detected by non-destructive examinations and removed for metallurgical examination. The most likely cause to cracking was deduced to be Irradiation Assisted Stress Corrosion Cracking.To understand the ageing mechanisms associated to irradiation and propose life predictions of the component, irradiation damage is investigated experimentally by Transmission Electron Microscopy for two representative austenitic stainless steels, SA 304L and CW 316. The microstructure of these austenitic stainless steels constitutive of internals structures of PWR is characterized by TEM and compared to the microstructure obtained after irradiation for high doses up to 40 dpa at 320°C in experimental OSIRIS (mixed flux spectrum) and BOR-60 (fast breeders) reactors and at 300°C in experimental SM-2 (mixed flux spectrum) reactor. Only minor differences were detected.

This paper provides a detailed evaluation of cloud absorption optical depths retrieved at 12.05 μm and comparisons to extinction optical depths retrieved at 0.532 μm from perfectly co-located observations of single-layered semi-transparent cirrus over ocean made by the Imaging Infrared Radiometer (IIR) and the Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP) flying on-board the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite. The blackbody radiance taken in the IIR Version 3 algorithm is evaluated, and IIR retrievals are corrected accordingly. IIR infrared absorption optical depths are then compared to CALIOP visible extinction optical depths when the latter can be directly derived from the measured apparent 2-way transmittance through the cloud. Numerical simulations and IIR retrievals of ice crystal sizes suggest that the ratios of CALIOP extinction and IIR absorption optical depths should remain roughly constant with respect to temperature. Instead, these ratios are found to increase quasi-linearly by about 40% as the temperature at the layer centroid altitude decreases from 240 to 200 K. This behavior is explained by variations of the multiple scattering factor ηT to be applied to correct the measured transmittance, which is taken equal to 0.6 in the CALIOP Version 3 algorithm, and which is found here to vary with temperature (and hence cloud particle size) from ηT = 0.8 at 200 K to ηT = 0.5 at 240 K for clouds with optical depth larger than 0.3. The revised parameterization of ηT introduces a concomitant temperature dependence in the simultaneously derived CALIOP lidar ratios that is consistent with observed changes in CALIOP depolarization ratios and particle habits derived from IIR measurements.

This paper presents a study, based on simulations, of the impact of cirrus cloud heterogeneities on the retrieval of cloud parameters (optical thickness and effective diameter) for the Imaging Infrared Radiometer (IIR) on board CALIPSO. Cirrus clouds are generated by the stochastic model 3DCLOUD for two different cloud fields and for several averaged cloud parameters. One cloud field is obtained from a cirrus observed on 25 May 2007 during the airborne campaign CIRCLE-2 and the other is a cirrus uncinus. The radiative transfer is simulated with the 3DMCPOL code. To assess the errors due to cloud heterogeneities, two related retrieval algorithms are used: (i) the split-window technique to retrieve the ice crystal effective diameter and (ii) an algorithm similar to the IIR operational algorithm to retrieve the effective emissivity and the effective optical thickness. Differences between input parameters and retrieved parameters are compared as a function of different cloud properties such as the mean optical thickness, the heterogeneity parameter and the effective diameter. The optical thickness heterogeneity for each 1 km × 1 km observation pixel is represented by the optical thickness standard deviation computed using 100 m × 100 m subpixels. We show that optical thickness heterogeneity may have a strong impact on the retrieved parameters, mainly due to the plane-parallel approximation (PPA assumption). In particular, for cirrus clouds with ice crystal diameter of approximately 10 μm, the averaged error on the retrieved effective diameter and optical thickness is about 2.5 μm (~ 25%) and −0.20 (~ 12%), respectively. Then, these biases decrease with increasing effective size due to a decrease of the cloud absorption and, thus, the PPA bias. Cloud horizontal heterogeneity effects are greater than other possible sources of retrieval errors such as those due to cloud vertical heterogeneity impact, surface temperature or atmospheric temperature profile uncertainty and IIR retrieval uncertainty. Cloud horizontal heterogeneity effects are larger than the IIR retrieval uncertainty if the standard deviation of the optical thickness, inside the observation pixel, is greater than 1.

Cirrus cloud absorption optical depths retrieved at 12.05 μm are compared to extinction optical depths retrieved at 0.532 μm from perfectly co-located observations of single-layered semi-transparent cirrus over ocean made by the Imaging Infrared Radiometer (IIR) and the Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP) flying on board the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite. IIR infrared absorption optical depths are compared to CALIOP visible extinction optical depths when the latter can be directly derived from the measured apparent two-way transmittance through the cloud. An evaluation of the CALIOP multiple scattering factor is inferred from these comparisons after assessing and correcting biases in IIR and CALIOP optical depths reported in version 3 data products. In particular, the blackbody radiance taken in the IIR version 3 algorithm is evaluated, and IIR retrievals are corrected accordingly. Numerical simulations and IIR retrievals of ice crystal sizes suggest that the ratios of CALIOP extinction and IIR absorption optical depths should remain roughly constant with respect to temperature. Instead, these ratios are found to increase quasi-linearly by about 40 % as the temperature at the layer centroid altitude decreases from 240 to 200 K. It is discussed that this behavior can be explained by variations of the multiple scattering factor ηT applied to correct the measured apparent two-way transmittance for contribution of forward-scattering. While the CALIOP version 3 retrievals hold ηT fixed at 0.6, this study shows that ηT varies with temperature (and hence cloud particle size) from ηT = 0.8 at 200 K to ηT = 0.5 at 240 K for single-layered semi-transparent cirrus clouds with optical depth larger than 0.3. The revised parameterization of ηT introduces a concomitant temperature dependence in the simultaneously derived CALIOP lidar ratios that is consistent with observed changes in CALIOP depolarization ratios and particle habits derived from IIR measurements.

International audience; This study is intended to illustrate the potential advantage of combining lidar measurements and the split-window technique based on the infrared spectral information contained at the 8.65-, 11.15-, and 12.05-μm bands for inferring the microphysical properties of cirrus clouds. The lidar returns are employed to detect cirrus clouds. The optical properties of nonspherical ice crystals computed from the state-of-the-art scattering computational methods are used for the present forward radiative transfer simulation that fully accounts for both gaseous absorption and multiple scattering processes in the atmosphere. A combination of the radiances at the three infrared (IR) bands with lidar backscatter returns cannot uniquely specify the effective size of ice crystals because of its dependence on the particle aspect ratios. To avoid the shortcoming associated with a potential multivalued retrieval, lidar depolarization observation is used to constrain the specification of the particle aspect ratio in the retrieval implementation based on a precalculated lookup library. The present methodology for inferring the microphysical properties of cirrus clouds is implemented for nine cases by using the measurements from a 532-nm lidar located at the Palaiseau, France, ground-based site and the infrared spectral bands from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra platform. It is shown that the three IR wavelengths are quite complementary in constraining the retrieval of the particle size, leading to a significant advance in comparison with two-channel techniques, whereas the aspect ratio constraint due to lidar depolarization reduces the uncertainty of retrieved particle size by more than 20% for 70% of the cases and more than 65% for 40% of the cloud cases.

A method for the in-flight spectral calibration of the oxygen A-band channel of MERIS is presented. This calibration is based on measurements of the atmospheric transmission in the oxygen A-band, using the relation between oxygen transmission and surface pressure. The accuracy of the method has been estimated by using radiative transfer calculations for realistic atmospheric conditions, assuming spectral shifts on the MERIS response. When the calibration is applied over bright surfaces, the expected accuracy of the spectral shift determination is about - 0.01 nm.

The inversion of aerosol characteristics over DDV (Dark Dense Vegetation) targets is an intermediate and important step whenever atmospheric correction over land for the Envisat/MERIS instrument is performed. We present here the results of an error budget for every step of this inversion. We show that the crucial step is DDV selection based on a threshold on the ARVI (Atmospherically Resistant Vegetation Index). The cornerstone of this inversion is aerosol climatology, as aerosol scattering properties depend on the aerosol refractive index. Finally, we conclude that the major source of uncertainty is the imaginary part of the aerosol refractive index. This is critical as the atmospheric correction over land algorithm handles it with difficulty.