François-Marie Bréon

Application of the CALIOP layer product to evaluate the vertical distribution of aerosols estimated by global models: AeroCom phase i results

[1] The CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) layer product is used for a multimodel evaluation of the vertical distribution of aerosols. Annual and seasonal aerosol extinction profiles are analyzed over 13 sub-continental regions representative of industrial, dust, and biomass burning pollution, from CALIOP 2007–2009 observations and from AeroCom (Aerosol Comparisons between Observations and Models) 2000 simulations. An extinction mean height diagnostic (Za) is defined to quantitatively assess the models’ performance. It is calculated over the 0–6 km and 0–10 km altitude ranges by weighting the altitude of each 100 m altitude layer by its aerosol extinction coefficient. The mean extinction profiles derived from CALIOP layer products provide consistent regional and seasonal specificities and a low inter-annual variability. While the outputs from most models are significantly correlated with the observed Za climatologies, some do better than others, and 2 of the 12 models perform particularly well in all seasons. Over industrial and maritime regions, most models show higher Za than observed by CALIOP, whereas over the African and Chinese dust source regions, Za is underestimated during Northern Hemisphere Spring and Summer. The positive model bias in Za is mainly due to an overestimate of the extinction above 6 km. Potential CALIOP and model limitations, and methodological factors that might contribute to the differences are discussed.

A satellite- and model-based assessment of the 2003 Russian fires: Impact on the Arctic region

n this paper, we address the issues of the representation of boreal fires in a global chemistry and transport model (GEOS-Chem) as well as their contribution to the Arctic aerosol optical thickness and black carbon (BC) deposition, with a focus on the 2003 Russian fires. We use satellite observations from the MOPITT, POLDER and MODIS sensors to evaluate the model performances in simulating the fire pollution export over the North Pacific. Our results show that aerosol and carbon monoxide (CO) outflow is best reproduced in our model when fire emissions are (1) increased to 72 Tg for CO, 0.5 Tg C for BC, and 5.3 Tg C for organic carbon over the entire fire season; (2) prescribed on a daily basis; and (3) injected up to 4.5 km in July and August. The use of daily, rather than monthly, biomass burning emission inventories improves significantly the representation of the aerosol outflow, but has little impact on CO. The injection of fire emissions above the boundary layer influences both the CO and aerosol columns but only during the late fire season. The model improvements with respect to the standard configuration induce an increase of a factor up to 2 on the aerosol optical thickness and the mass of BC deposited in the Northern Hemisphere. According to our improved simulation, the 2003 Russian fires contributed to 16–33% of the aerosol optical thickness and to 40–56% of the mass of BC deposited, north of 75°N in spring and summer. They contribute to the aerosol optical thickness by more than 30% during the days of Arctic haze events in spring and summer.

Aerosol vertical distribution in dust outflow over the Atlantic: Comparisons between GEOS‐Chem and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)

Vertically-resolved attenuated backscatter from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission and Aerosol Optical Thickness (AOT)from Moderate-resolution Imaging Spectroradiometer (MODIS) are used to characterize the Saharan dust outflow over the Atlantic and to evaluate results from a global chemical and transport model (GEOS-Chem). We first document two events of dust plume transport from the Sahara to the American coast representative of summer and winter conditions. The GEOSChem model reproduces fairly well the aerosol patterns over the Atlantic although observed AOT are overestimated close to the source regions and underestimated in the remote regions, suggesting a too weak transport toward the western Atlantic and/or a too strong deposition over the Caribbean sea. Observed and model-derived attenuated backscatter profiles agree qualitatively well, suggesting that the altitudes of the aerosol layers over the Atlantic is well reproduced. We find that dust plumes extend up to 6 km in summer and up to 4 km in winter over the Atlantic close to the source regions and that they gradually descent throughout their travel over the Atlantic. We then use our simulation to characterize the deposition flux of dust in this region. Half of the dust deposited on the surface of the open ocean in 2006 in this area occurs during summer, 20% during each of winter and spring, and 10% in the fall. During the July 2006 one-week dust episode that we investigated, 5 Tg of dust were deposited (4% of the annual total in the region).

Variability of tropical upper tropospheric humidity 1979–1998

We update the time series of upper tropospheric humidity observations, begun in 1979 through May 1998, which includes the major El Nino event of 1997–1998. The intercalibration of different satellites is updated and compared to a physically based intercalibration. The results show excellent agreement and indicate that the main source of intersatellite bias is the known filter response function of the different instruments. Interannual variability of spatial fields is dominated by the major El Nino events in the 1979–1998 time period. Tropical average anomaly time series of upper tropospheric humidity, however, are dominated by a pronounced seasonal preference in extremes. Large deviations from the anomaly time series are twice as likely to occur in boreal winter and spring as in boreal summer and fall. During boreal winter and spring, the tropical basic state circulation permits the opening of a westerly wave duct such that midlatitude Rossby waves can propagate into the subtropics and tropics. We hypothesize that variation in the Rossby wave activity modulates the vertical water vapor flux and is responsible for large variations in the tropical upper tropospheric humidity time series.

Calibration of the Meteosat water vapor channel using collocated NOAA/HIRS 12 measurements

The Meteosat geostationary satellites carry a filtered radiometer channel centered at 6.2 μm for the measurement of upper tropospheric humidity. The operational calibration is derived from radiative transfer calculations applied to radiosonde measurements; large fluctuations in the calibration have been noticed. Here, we apply another method based on collocations with NOAA/High-Resolution Infrared Radiometer Sounder (HIRS) channel 12. Radiative transfer calculations show that Meteosat measurements can be accurately predicted from HIRS channel 12 brightness temperatures. In addition, the HIRS instrument has onboard calibration of the complete optics, which makes it suitable as a reference for the calibration of other spaceborne radiometers. Application of the method to 3 years of Meteosat-5 data shows calibration fluctuations significantly smaller than those provided by the operational method. This result indicates that the short-term stability of the instrument calibration is much better than 5%, the variability given by the operational calibration. The proposed calibration coefficient is smaller by about 10–15%, which results in equivalent brightness temperatures biased by about 3–4 K and 35–50% relative error on the upper tropospheric humidity. Hypotheses of the causes for such large differences between the two calibration procedure results are proposed.

Evaluation of the aerosol vertical distribution in global aerosol models through comparison against CALIOP measurements: AeroCom phase II results

The ability of 11 models in simulating the aerosol vertical distribution from regional to global scales, as part of the second phase of the AeroCom model intercomparison initiative (AeroCom II), is assessed and compared to results of the first phase. The evaluation is performed using a global monthly gridded data set of aerosol extinction profiles built for this purpose from the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) Layer Product 3.01. Results over 12 subcontinental regions show that five models improved, whereas three degraded in reproducing the interregional variability in Z α0-6 km, the mean extinction height diagnostic, as computed from the CALIOP aerosol profiles over the 0-6 km altitude range for each studied region and season. While the models performance remains highly variable, the simulation of the timing of the Z α0-6 km peak season has also improved for all but two models from AeroCom Phase I to Phase II. The biases in Z α0-6 km are smaller in all regions except Central Atlantic, East Asia, and North and South Africa. Most of the models now underestimate Z α0-6 km over land, notably in the dust and biomass burning regions in Asia and Africa. At global scale, the AeroCom II models better reproduce the Z α0-6 km latitudinal variability over ocean than over land. Hypotheses for the performance and evolution of the individual models and for the intermodel diversity are discussed. We also provide an analysis of the CALIOP limitations and uncertainties contributing to the differences between the simulations and observations.

Evidence of Atmospheric Contamination on the Measurement of the Spectral Response of the GMS-5 Water Vapor Channel

Abstract The GMS-5 geostationary satellite carries a channel centered at 6.7 μm for the measurement of upper-tropospheric humidity. This channel’s spectral response shows structures that are similar to those shown by the atmospheric transmission. This note shows that these structures probably result from water vapor absorption between the calibration source and the instrument while making the response measurement. A corrected filter is proposed after normalization by the inferred atmospheric transmission. The brightness temperatures computed by a radiative transfer model using the spurious response exhibit a warm bias of about 1 K.

Using Copernicus Atmosphere Monitoring Service Products to Constrain the Aerosol Type in the Atmospheric Correction Processor MAJA

The quantitative use of space-based optical imagery requires atmospheric correction to separate the contributions from the surface and the atmosphere. The MACCS (Multi-sensor Atmospheric Correction and Cloud Screening)-ATCOR (Atmospheric and Topographic Correction) Joint Algorithm, called MAJA, is a numerical tool designed to perform cloud detection and atmospheric correction. For the correction of aerosols effects, MAJA makes an estimate of the aerosol optical thickness (AOT) based on multi-temporal and multi-spectral criteria, but there is insufficient information to infer the aerosol type. The current operational version of MAJA uses an aerosol type which is constant with time, and this assumption impacts the quality of the atmospheric correction. In this study, we assess the potential of using an aerosol type derived from the Copernicus Atmosphere Monitoring Service (CAMS) operational analysis. The performances, with and without the CAMS information, are evaluated. Firstly, in terms of the aerosol optical thickness retrievals, a comparison against sunphotometer measurements over several sites indicates an improvement over arid sites, with a root-mean-square error (RMSE) reduced by 28% (from 0.095 to 0.068), although there is a slight degradation over vegetated sites (RMSE increased by 13%, from 0.054 to 0.061). Secondly, a direct validation of the retrieved surface reflectances at the La Crau station (France) indicates a reduction of the relative bias by 2.5% on average over the spectral bands. Thirdly, based on the assumption that surface reflectances vary slowly with time, a noise criterion was set up, exhibiting no improvement over the spectral bands and the validation sites when using CAMS data, partly explained by a slight increase in the surface reflectances themselves. Finally, the new method presented in this study provides a better way of using the MAJA processor in an operational environment because the aerosol type used for the correction is automatically inferred from CAMS data, and is no longer a parameter to be defined in advance.

Results of POLDER in-flight absolute calibration

POLDER is a CNES instrument on-board ADEOS polar orbiting satellite, which was successfully launched in August 1996. In November 1996, POLDER entered its nominal acquisition phase and functioned perfectly until ADEOS early end of service in June 1997. POLDER is a multispectral imaging radiometer/polarimeter designed to collect global and repetitive observations of the solar radiation reflected by the Earth/atmosphere system, with a wide field of view (2400 km) and a moderate geometric resolution (6 km). The instrument concept is based on telecentric optics, on a rotating wheel carrying 15 spectral filters and polarizers, and on a bidimensional CCD detector array. In addition to the classical measurement and mapping characteristics of a narrow-band imaging radiometer, POLDER has a unique ability to measure polarized reflectances using three polarizers (for three of its eight spectral bands, 443 to 910 nm), and to observe target reflectances from 13 different viewing directions during a single satellite pass. One of POLDER original features is that its in-flight radiometrical calibration does not rely on any on-board device. Many calibration methods using well-characterized calibration targets have been developed to achieve a very high calibration accuracy. This paper presents the various methods involved in the absolute in-flight calibration plan and the results obtained during the calibration phase of the instrument: absolute calibration over molecular scattering, inter-band calibration over sunglint and clouds, inter-calibration with OCTS, water vapor channels calibration over sunglint using meteorological analysis. A brief description of the algorithm and of the performances of each method is given.