Jean-Claude Roger

Atmospheric correction of visible to middle‐infrared EOS‐MODIS data over land surfaces: Background, operational algorithm and validation

The NASA moderate resolution imaging spectroradiometer (MODIS) instrument will provide a global and improved source of information for the study of land surfaces with a spatial resolution of up to 250 m. Prior to the derivation of various biophysical parameters based on surface reflectances, the top of the atmosphere signals need to be radiometrically calibrated and corrected for atmospheric effects. The present paper describes in detail the state of the art techniques that will be used for atmospheric correction of MODIS bands 1 through 7, centered at 648, 858, 470, 555, 1240, 1640, and 2130 nm, respectively. Previous operational correction schemes have assumed a standard atmosphere with zero or constant aerosol loading and a uniform, Lambertian surface. The MODIS operational atmospheric correction algorithm, reported here, uses aerosol and water vapor information derived from the MODIS data, corrects for adjacency effects and takes into account the directional properties of the observed surface. This paper also describes the operational implementation of these techniques and its optimization. The techniques are applied to remote sensing data from the Landsat Thematic Mapper (TM), the NOAA advanced very high resolution radiometer (AVHRR), and the MODIS airborne simulator (MAS) and validated against ground-based measurements from the Aerosol Robotic Network (AERONET).

Atmospheric correction over land for MERIS

A three-stage atmospheric correction is proposed for the Medium Resolution Imaging Spectrometer (MERIS) from a validated formulation of the signal. We correct first for the gaseous transmittance. Assuming the ozone correction is well defined, we illustrate the need to include a correction for water vapour continuum which covers most of the MERIS bands. The water vapour transmittance can be computed from the water vapour content obtained from a twoband ratio at 900nm and 890nm. We demonstrate that a direct association between the transmittance in a given band and the two band ratio is more accurate due to the removal of the coupling between absorption and scattering. Secondly, the Rayleigh correction depends on the barometric pressure determined here from a two band ratio method with the oxygen A band. Good accuracy is obtained for the pressure when accounting for the coupling between scattering and gas absorption, which mostly depends on the surface reflectance. The Rayleigh reflectance is computed from a...

Angular Illumination and Truncation of Three Different Integrating Nephelometers: Implications for Empirical, Size-Based Corrections

Integrating nephelometers are widely used for monitoring and research applications related to air pollution and climate. Several commercial versions of the instrument are available and are in wide use in the community. This article reports on results from a calibration and intercomparison workshop where several units of the three most widely used nephelometer models were tested with respect to their CO2 calibration accuracy and stability and non-idealities of their angular illumination function. Correction factors that result from the non-ideal illumination due to truncation of the sensing volumes in the near-forward and near-backward angular ranges and for non-Lambertian illumination from the light sources are presented, in particular for two models that have not previously been tested in this respect. The correction factors ranged from 0.95 to 1.15 depending on the model of nephelometer and aerosol size distribution. Recommendations for operational data analysis in context of these and previous performance tests are presented.

Aerosol direct radiative forcing over Djougou (northern Benin) during the African Monsoon Multidisciplinary Analysis dry season experiment (Special Observation Period-0)

The purpose of this work is to investigate the direct radiative forcing of aerosols over the supersite of Djougou (northern Benin) during the African Monsoon Multidisciplinary Analyses dry season experiment. We focus our simulations on the top of atmosphere, bottom of atmosphere, and atmosphere radiative forcings. During the dry season period, Sun photometer measurements indicate a rather turbid atmosphere with a mean aerosol optical depth for the overall period of 0.78 ± 0.24 (at 440 nm). The aerosol absorption coefficient estimated at the surface ranged between 2.3 and 37.3 Mm−1 (mean value 15.2 ± 10.6 Mm−1 at 520 nm) and the scattering coefficient between 44.5 and 232.3 Mm−1 (mean 145 ± 59 Mm−1 at 520 nm). This leads to a single scattering albedo of between 0.81 and 0.98 (at 520 nm) with a mean (and standard deviation) value of 0.91 ± 0.05, indicating moderately absorbing aerosols. In parallel, micropulse lidar measurements indicate the presence of two distinct aerosol layers, with a first one located between the surface and 1 km and a second one located above 1.5–4.0 km. On the basis of surface and aircraft observations, sunphotometer measurements, lidar profiles, and Moderate Resolution Imagaing Spectroradiometer sensor an estimation of the daily clear sky direct radiative forcing has been estimated for the 17–24 January 2006 period. Simulations indicate that aerosols reduce significantly the solar energy reaching the surface (mean ΔFBOA = −61.5 W/m2) by reflection to space (mean ΔFTOA = −18.4 W/m2) but predominantly by absorption of the solar radiation into the atmosphere (mean ΔFATM = +43.1 W/m2). The mean heating rate at the surface and within the elevated biomass burning layer is considerably enhanced by 1.50 and 1.90 K day−1, respectively.

A Method to Retrieve the Reflectivity Signature at 3.75 μm from AVHRR Data

Abstract Global monitoring of land surface properties has primarily relied on the AVHRR red and near-infrared channels through use of the NDVI. The AVHRR Channel 3, centered at 3.75 μm, has been shown to be sensitive to vegetation on a local scale. A method to separate the reflected and emitted components in this channel has been developed. The 3.75-μm reflectivity is computed by subtracting the thermal contribution from the total signal and dividing the remaining signal component by atmospheric transmission and solar irradiance. The thermal contribution is estimated by using thermal infrared Channels 4 and 5 as well as NDVI to estimate infrared surface emissivities. The atmospheric transmission is computed with MODTRAN2 and uses integrated water vapor derived from the Split Window Technique. The formula derived are validated over ocean using sun glint observations and land using the FIFE-87 data set. Despite the uncertainties inherent to the procedure we adopted, quantitative use of the derived reflectance at 3.75 μm appears possible.

Radiative forcing of haze during a forest fire in Spain

Intense fires occurred in northwestern Spain on 6 September 2000, filling a valley with smoke haze. Aerosol size distribution measurements were performed during 1 day with a thermal inversion, so the aging process of the smoke aerosol could be closely monitored. In 3.5 h, the fine aerosol increased up to 0.06 μm in the geometric median diameter of the fine mode. This aging process enhanced the scattering ability of aerosols. On the basis of several hypotheses on the data obtained, shortwave radiative forcing at surface level, at top level, and in the atmosphere was estimated: instantaneous surface forcing reached up to between −80.4 and −67.4 W/m2, top of the atmosphere (TOA) instantaneous forcing reached up to between −23.4 and +4.9 W/m2, and instantaneous atmosphere forcing reached up to between +44.2 and +85.3 W/m2. The study reveals not only the absorption of solar radiation in the atmosphere by smoke aerosols but also an aerosol-induced case study, where TOA cooling forcing shifts to warming for specific aerosol single scattering albedo. The daily mean heating rate of the smoke haze was estimated at 5.9 ± 0.6 K/d.

Aerosol complexity in megacities: From size‐resolved chemical composition to optical properties of the Beijing atmospheric particles

Megacities need adapted tools for the accurate modeling of aerosol impacts. For this purpose a new experimental data processing has been worked out for Beijing aerosols as case study. Size-resolved aerosol particles were extensively sampled during winter and summer 2003 and subsequently fully chemically characterized. The product is an aerosol model presenting a new particle pattern (mode number, size and chemistry) without any prerequisite constrain either on the mode number or on each mode chemical composition. Six modes were found and five of them consistently appear as internally mixed particles organized around a black carbon or a dust core coated by organic and/or inorganic material. Data were checked by robust comparisons with other experimental data (particle number, sunphotometer-derived derived data). We found the presence of two accumulation modes in different internal mixing and optical calculations show that the Beijing aerosol single scattering albedo (ωo # 0.90) is significantly higher than expected. Such an approach would allow realistic modeling of atmospheric particle impacts under complex situations.

Atmospheric Correction Inter-Comparison Exercise

The Atmospheric Correction Inter-comparison eXercise (ACIX) is an international initiative with the aim to analyse the Surface Reflectance (SR) products of various state-of-the-art atmospheric correction (AC) processors. The Aerosol Optical Thickness (AOT) and Water Vapour (WV) are also examined in ACIX as additional outputs of an AC processing. In this paper, the general ACIX framework is discussed; special mention is made of the motivation to initiate this challenge, the inter-comparison protocol and the principal results. ACIX is free and open and every developer was welcome to participate. Eventually, 12 participants applied their approaches to various Landsat-8 and Sentinel-2 image datasets acquired over sites around the world. The current results diverge depending on the sensors, products and sites, indicating their strengths and weaknesses. Indeed, this first implementation of processor inter-comparison was proven to be a good lesson for the developers to learn the advantages and limitations of their approaches. Various algorithm improvements are expected, if not already implemented, and the enhanced performances are yet to be investigated in future ACIX experiments.

A 30+ Year AVHRR Land Surface Reflectance Climate Data Record and Its Application to Wheat Yield Monitoring

The Advanced Very High Resolution Radiometer (AVHRR) sensor provides a unique global remote sensing dataset that ranges from the 1980’s to the present. Over the years, several efforts have been made on the calibration of the different instruments to establish a consistent land surface reflectance time-series and to augment the AVHRR data record with data from other sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS). In this paper, we present a summary of all the corrections applied to the AVHRR Surface Reflectance and NDVI Version 4 Product, developed in the framework of the National Oceanic and Atmospheric Administration (NOAA) Climate Data Record (CDR) program. These corrections result from assessment of the geo-location, improvement of the cloud masking and calibration monitoring. Additionally, we evaluate the performance of the surface reflectance over the AERONET sites by a cross-comparison with MODIS, which is an already validated product, and evaluation of a downstream Leaf Area Index (LAI) product. We demonstrate the utility of this long time-series by estimating the winter wheat yield over the USA. The methods developed by [1] and [2] are applied to both the MODIS and AVHRR data. Comparison of the results from both sensors during the MODIS-era shows the consistency of the dataset with similar errors of 10%. When applying the methods to AVHRR historical data from the 1980’s, the results have errors equivalent to those derived from MODIS.

Revisiting Short-Wave-Infrared (SWIR) Bands for Atmospheric Correction in Coastal Waters

The shortwave infrared (SWIR) bands on the existing Earth Observing missions like MODIS have been designed to meet land and atmospheric science requirements. The future geostationary and polar-orbiting ocean color missions, however, require highly sensitive SWIR bands (> 1550nm) to allow for a precise removal of aerosol contributions. This will allow for reasonable retrievals of the remote sensing reflectance (Rrs) using standard NASA atmospheric corrections over turbid coastal waters. Design, fabrication, and maintaining high-performance SWIR bands at very low signal levels bear significant costs on dedicated ocean color missions. This study aims at providing a full analysis of the utility of alternative SWIR bands within the 1600nm atmospheric window if the bands within the 2200nm window were to be excluded due to engineering/cost constraints. Following a series of sensitivity analyses for various spectral band configurations as a function of water vapor amount, we chose spectral bands centered at 1565 and 1675nm as suitable alternative bands within the 1600nm window for a future geostationary imager. The sensitivity of this band combination to different aerosol conditions, calibration uncertainties, and extreme water turbidity were studied and compared with that of all band combinations available on existing polar-orbiting missions. The combination of the alternative channels was shown to be as sensitive to test aerosol models as existing near-infrared (NIR) band combinations (e.g., 748 and 869nm) over clear open ocean waters. It was further demonstrated that while in extremely turbid waters the 1565/1675 band pair yields Rrs retrievals as good as those derived from all other existing SWIR band pairs (> 1550nm), their total calibration uncertainties must be < 1% to meet current science requirements for ocean color retrievals (i.e., Δ Rrs (443) < 5%). We further show that the aerosol removal using the NIR and SWIR bands (available on the existing polar-orbiting missions) can be very sensitive to calibration uncertainties. This requires the need for monitoring the calibration of these bands to ensure consistent multi-mission ocean color products in coastal/inland waters.