ALBERT

Description: Turnover of soil bacterial diversity driven by wide-scale environmental heterogeneity Nature Communications 4, 1434 (2013). doi:10.1038/ncomms2431 Authors: L. Ranjard, S. Dequiedt, N. Chemidlin Prévost-Bouré, J. Thioulouse, N.P.A. Saby, M. Lelievre, P. A. Maron, F.E.R Morin, A. Bispo, C. Jolivet, D. Arrouays & P. Lemanceau

Description: The study assesses the contribution of aerosols to the extinction of visible radiation in the mist-fog-mist cycle. Measurements of the microphysical and optical properties of hydrated aerosols with diameters larger than 400 nm, composing the accumulation mode, which are the most efficient to interact with visible radiation, were carried out near Paris, during November 2011, in ambient conditions. Eleven mist-fog-mist cycles were observed, with cumulated fog duration of 95 h, and cumulated mist-fog-mist duration of 240 h. In mist, aerosols grew up by taking up water at relative humidities larger than 93%, causing a visibility decrease below 5 km. While visibility decreased down to few km, the mean size of the hydrated aerosols increased, and their number concentration (Nha) increased from approximately 160 to approximately 600 cm−3. When fog formed, droplets became the strongest contributors to visible radiation extinction, and liquid water content (LWC) increased beyond 7 mg m−3. Hydrated aerosols of the accumulation mode co-existed with droplets, as interstitial non-activated aerosols. Their size continued to increase, and a significant proportion of aerosols achieved diameters larger than 2.5 μm. The mean transition diameter between the accumulation mode and the small droplet mode was 4.0 ± 1.1 μm. Moreover Nha increased on average by 60% after fog formation. Consequently the mean aerosol contribution to extinction in fog was 20 ± 15% for diameter smaller than 2.5 μm and 6 ± 7% beyond. The standard deviation is large because of the large variability of Nha in fog, which could be smaller than in mist or three times larger. The particle extinction coefficient in fog can be computed as the sum of a droplet component and an aerosol component, which can be approximated by 3.5 Nha (Nha in cm−3 and particle extinction coefficient in Mm−1). We observed an influence of the main formation process on Nha, but not on the contribution to fog extinction by aerosols. Indeed in fogs formed by stratus lowering (STL), the mean Nha was 360 ± 140 cm−3, close to the value observed in mist, while in fogs formed by nocturnal radiative cooling under cloud-free sky (RAD), the mean Nha was 600 ± 350 cm−3. But because visibility (extinction) in fog was also lower (larger) in RAD than in STL fogs, the contribution by aerosols to extinction depended little on the fog formation process. Similarly, the proportion of hydrated aerosols over all aerosols (dry and hydrated) did not depend on the fog formation process. Measurements show that visibility in RAD fogs was smaller than in STL fogs because: (1) LWC was larger in RAD than in STL fogs, (2) droplets were smaller, (3) as already said, hydrated aerosols composing the accumulation mode were more numerous.

Description: The Spinning Enhanced Visible and InfraRed Imager (SEVIRI) aboard Meteosat Second Generation (MSG) launched in 2003 by EUMETSAT is dedicated to the Nowcasting applications and Numerical Weather Prediction and to provide information for climate monitoring and research. We use the data in visible and near infrared channels to derive the Aerosol Optical Thickness (AOT) over land. The algorithm is based on the assumption that the Top Of the Atmosphere (TOA) reflectance increases with the aerosol load. This is a reasonable assumption except in case of absorbing aerosols above bright surfaces. We assume that the minimum in a 14-day time series of the TOA reflectance is, once corrected from gaseous scattering and absorption, representative of the surface reflectance. The AOT and the aerosol model (a set of 5 models are used), are retrieved by matching the simulated TOA reflectance with the TOA reflectances measured by SEVIRI in its visible and Near Infra-Red (NIR) spectral bands. The high temporal resolution of the data acquisition by SEVIRI allows to retrieve the AOT every 15 min with a spatial resolution of 3km at sub-satellite point, over the whole SEVIRI disk which covers Europe, Africa and part of South America. The resulting AOT, a Level 2 product at the same temporal and spatial resolution than SEVIRI, is presented and evaluated in this paper. The AOT has been validated using ground based measurements from AERONET, a sun-photometer network, focusing over Europe for 3 months in 2006. The SEVIRI estimates correlate well with the AERONET measurements, r = 0.64, with a slight underestimate, bias = −0.017. The sources of errors are mainly the cloud contamination and the bad estimation of the surface reflectance. The temporal evolutions exhibited by both dataset show very good agreement which allows to conclude that the AOT Level 2 product from SEVIRI can be used to quantify the aerosol content and to monitor its daily evolution with a high temporal frequency. The comparison with daily maps of MODIS AOT level 3 product shows qualitative good agreements in the retrieved geographic patterns of AOT. Given the high spatial and temporal resolutions obtained with this approach, our results have clear potential for applications ranging from air quality monitoring to climate studies.

Description: The Spinning Enhanced Visible and InfraRed Imager (SEVIRI) aboard Meteosat Second Generation (MSG) launched in 2003 by EUMETSAT is dedicated to the Nowcasting applications and Numerical Weather Prediction and to the provision of observations for climate monitoring and research. We use the data in visible and near infrared (NIR) channels to derive the aerosol optical thickness (AOT) over land. The algorithm is based on the assumption that the top of the atmosphere (TOA) reflectance increases with the aerosol load. This is a reasonable assumption except in case of absorbing aerosols above bright surfaces. We assume that the minimum in a 14-days time series of the TOA reflectance is, once corrected from gaseous scattering and absorption, representative of the surface reflectance. The AOT and the aerosol model (a set of 5 models is used), are retrieved by matching the simulated TOA reflectance with the TOA reflectances measured by SEVIRI in its visible and NIR spectral bands. The high temporal resolution of the data acquisition by SEVIRI allows to retrieve the AOT every 15 min with a spatial resolution of 3 km at sub-satellite point, over the entire SEVIRI disk covering Europe, Africa and part of South America. The resulting AOT, a level 2 product at the native temporal and spatial SEVIRI resolutions, is presented and evaluated in this paper. The AOT has been validated using ground based measurements from AErosol RObotic NETwork (AERONET), a sun-photometer network, focusing over Europe for 3 months in 2006. The SEVIRI estimates correlate well with the AERONET measurements, r = 0.64, with a slight overestimate, bias = −0.017. The sources of errors are mainly the cloud contamination and the bad estimation of the surface reflectance. The temporal evolutions exhibited by both datasets show very good agreement which allows to conclude that the AOT Level 2 product from SEVIRI can be used to quantify the aerosol content and to monitor its daily evolution with a high temporal frequency. The comparison with daily maps of Moderate Resolution Imaging Spectroradiometer (MODIS) AOT level 3 product shows qualitative good agreement in the retrieved geographic patterns of AOT. Given the high spatial and temporal resolutions obtained with this approach, our results have clear potential for applications ranging from air quality monitoring to climate studies. This paper presents a first evaluation and validation of the derived AOT over Europe in order to document the overall quality of a product that will be made publicly available to the users of the aforementioned research communities.

Description: A robust method to estimate the cloud microphysical properties from visible (0.67 μm) and near infrared (1.6 μm) measurements of reflected sunlight is presented. The method does not determine cloud particle phase and size separately. Instead it assigns a cloud particle type to every pixel that is most representative for the radiation measurements. The corresponding radiative transfer model calculations will yield the most accurate values for optical thickness. Furthermore, an estimate of the particle size is obtained, which is used in estimates of liquid water path. Radiative transfer calculations have been performed for eleven cloud particle models assuming a single, plane-parallel and homogeneous layer. Standard gamma distributions with varying effective radii have been chosen for liquid water droplet whereas imperfect hexagonal ice crystal with different aspect ratio and size were selected for ice particles. It is shown that the ratio of the visible reflectivity to the near infrared reflectivity as a function of the visible reflectivity allows a consistent classification of cloud particles with respect to size and phase over a large area. The method is tested with measurements from the Along Track Scanning Radiometer instrument (ATSR-2) on board ERS-2 for a marine stratocumulus cloud and a cirrus cloud over the North Sea. For both cases, the variation of the measured ratio as a function of the measured visible reflectivity is well simulated by liquid water droplet distribution with an effective radius between 4 and 10 micrometers for the stratocumulus and by imperfect hexagonal ice crystal with a size of 60 μm for cirrus. The method was used in the CLIWANET-project and will be the basis to the algorithm for AVHRR and SEVIRI radiances for EUMETSAT's Sattelite Application facility on climate monitoring.

Description: The study assesses the contribution of aerosols to the extinction of visible radiation in the mist–fog–mist cycle. Relative humidity is large in the mist–fog–mist cycle, and aerosols most efficient in interacting with visible radiation are hydrated and compose the accumulation mode. Measurements of the microphysical and optical properties of these hydrated aerosols with diameters larger than 0.4 μm were carried out near Paris, during November 2011, under ambient conditions. Eleven mist–fog–mist cycles were observed, with a cumulated fog duration of 96 h, and a cumulated mist–fog–mist cycle duration of 240 h. In mist, aerosols grew by taking up water at relative humidities larger than 93%, causing a visibility decrease below 5 km. While visibility decreased down from 5 to a few kilometres, the mean size of the hydrated aerosols increased, and their number concentration (Nha) increased from approximately 160 to approximately 600 cm−3. When fog formed, droplets became the strongest contributors to visible radiation extinction, and liquid water content (LWC) increased beyond 7 mg m−3. Hydrated aerosols of the accumulation mode co-existed with droplets, as interstitial non-activated aerosols. Their size continued to increase, and some aerosols achieved diameters larger than 2.5 μm. The mean transition diameter between the aerosol accumulation mode and the small droplet mode was 4.0 ± 1.1 μm. Nha also increased on average by 60 % after fog formation. Consequently, the mean contribution to extinction in fog was 20 ± 15% from hydrated aerosols smaller than 2.5 μm and 6 ± 7% from larger aerosols. The standard deviation was large because of the large variability of Nha in fog, which could be smaller than in mist or 3 times larger. The particle extinction coefficient in fog can be computed as the sum of a droplet component and an aerosol component, which can be approximated by 3.5 Nha (Nha in cm−3 and particle extinction coefficient in Mm−1. We observed an influence of the main formation process on Nha, but not on the contribution to fog extinction by aerosols. Indeed, in fogs formed by stratus lowering (STL), the mean Nha was 360 ± 140 cm−3, close to the value observed in mist, while in fogs formed by nocturnal radiative cooling (RAD) under cloud-free sky, the mean Nha was 600 ± 350 cm−3. But because visibility (extinction) in fog was also lower (larger) in RAD than in STL fogs, the contribution by aerosols to extinction depended little on the fog formation process. Similarly, the proportion of hydrated aerosols over all aerosols (dry and hydrated) did not depend on the fog formation process. Measurements showed that visibility in RAD fogs was smaller than in STL fogs due to three factors: (1) LWC was larger in RAD than in STL fogs, (2) droplets were smaller, (3) hydrated aerosols composing the accumulation mode were more numerous.