Estimation of absorbed PAR across Scandinavia from satellite measurements: Part I: Incident PAR

The primary productivity of a plant community can be modeled as the product of the amount of photosynthetically active radiation (PAR) absorbed by the canopy and a light use efficiency factor, where the amount of absorbed PAR (APAR) is the product of the fractional absorption and the amount of incident PAR. By implementing a method, PARcalc, using atmospheric data from the Moderate Resolution Imaging Spectroradiometer (MODIS), incident PAR is estimated in this study. In addition, since many PAR datasets are generated by converting shortwave radiation into PAR, the ratio of PAR to shortwave radiation was also investigated. PARcalc models the photosynthetic photon flux density (PPFD) as a product of atmospheric transmittance, the cosine of the Sun zenith angle, and the solar constant. The atmospheric transmittance includes the attenuation of radiation by Rayleigh and aerosol scattering, and absorption by water and ozone. A cloud transmittance factor which is primarily a function of the cloud optical thickness is added in order to cope with cloudy conditions. The model was implemented at two sites in Sweden, Asa and Norunda, where in situ measurements of PPFD were made during the spring and summer of 2004. Modeled time-series were evaluated against the measurements, and daily sums of PPFD were calculated by fitting of a sine function in combination with linear interpolation of the instantaneous estimates from sunrise to sunset. This gave correlation coefficients at Norunda and Asa of 0.80 and 0.77, respectively, when comparing modeled and measured daily insolation. The average relative errors were 24% and 25%. Corresponding figures for five day averages were 0.91 and 0.86; and 9.3% and 11.9%. Instantaneous estimates of PPFD were modeled with correlation coefficients of 0 88-0 93 and average relative errors from 17.0%. These numbers were acquired when using measured values for determining cloudiness; the corresponding figures when the method is fully implemented using satellite data are 0.84 to 0.71 and 24.9%, respectively. The ratio of PAR to shortwave radiation was measured at Norunda 1 Jan to 31 Oct 2004 and was found to vary between 0.27 and 0.48 on a daily basis with an average of 0.43 for the whole period.     

Uncertainty estimates for the FAPAR operational products derived from MERIS — Impact of top-of-atmosphere radiance uncertainties and validation with field data

This paper discusses the accuracy of the operational Medium Resolution Imaging Spectrometer (MERIS) Level 2 land product which corresponds to the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). The FAPAR value is estimated from daily MERIS spectral measurements acquired at the top-of-atmosphere, using a physically based approach. The products are operationally available at the reduced spatial resolution, i.e. 1.2 km, and can be computed at the full spatial resolution, i.e. at 300 m, from the top-of-atmosphere MERIS data by using the same algorithm. The quality assessment of the MERIS FAPAR products capitalizes on the availability of five years of data acquired globally. The actual validation exercise is performed in two steps including, first, an analysis of the accuracy of the FAPAR algorithm itself with respect to the spectral measurements uncertainties and, second, with a direct comparison of the FAPAR time series against ground-based estimations as well as similar FAPAR products derived from other optical sensor data. The results indicate that the impact of top-of-atmosphere radiance uncertainties on the operational MERIS FAPAR products accuracy is expected to be at about 5-10% and the agreement with the ground-based estimates over different canopy types is achieved within ± 0.1.     

Fire radiative energy for quantitative study of biomass burning: derivation from the BIRD experimental satellite and comparison to MODIS fire products

A major focus in global change research is to quantify the amount of gaseous and particulate pollutants emitted from terrestrial vegetation fires. Determination of the emitted radiant energy released during biomass combustion episodes (the so-called fire radiative energy or FRE) has been suggested as a new tool for determining variations in biomass combustion rates and the rate of production of atmospheric pollutants. We review the physical principals behind the remote determination of FRE and present an alternative method for its derivation via analysis of ‘fire pixel’ radiances in the middle infrared spectral region. We compare our method to the existing FRE retrieval approach used in the EOS Moderate Resolution Imaging Spectro-radiometer (MODIS) fire products, and to retrievals of FRE based on derived fire temperature and area made via the so-called Bi-spectral method. We test each FRE retrieval method using both simulated data and imagery from a new experimental space mission, the Bi-spectral InfraRed Detection (BIRD) small satellite, which has sensors specifically designed for the study of active fires. We analyse near simultaneous MODIS and BIRD data of the fires that burned around Sydney, Australia in January 2002. Despite the markedly different pixel size and spectral coverage of these sensors, where the spatial extent of the fire pixel groups detected by MODIS and BIRD are similar, the derived values of FRE for these fires agree to within ±15 %. However, in certain fires, the lower spatial resolution of MODIS appears to prevent many of the less intensely radiating fire pixels being detected as such, meaning MODIS underestimates FRE for these fires by up to 46% in comparison to BIRD. Though the FRE release of each of these low intensity fire pixels is relatively low, their comparatively large number makes their overall FRE significant. Thus, total FRE release of the Sydney fires on 5 January 2002 is estimated to be 6.5×10⁹ J s⁻¹ via BIRD but 4.0×10⁹ J s⁻¹ via MODIS. The ability of BIRD to resolve individual fire fronts further allows the first accurate calculation of ‘radiative’ fireline intensity from spaceborne measurements, providing values of 15-75 kJ s⁻¹ m⁻¹ for fire fronts that are up to 9 km in length. Finally, we analyse the effectiveness of the satellite-based FRE retrieval methods in estimating the FRE from the active flaming and smouldering components only (FRE_Active, believed to be proportional to the rate of biomass combustion), despite the sensor receiving additional radiance from the ‘cooling ground’. The MIR radiance method appears particularly strong in this regard, allowing FRE_Active to be estimated to within ±30% in the range 100-100,000 J s⁻¹ m⁻². These results provide further confidence in the ability of spaceborne missions to derive physically meaningful values of FRE that could be used to support biomass burning emissions inventories. Future comparisons between FRE derived via MODIS and those from higher spatial resolution BIRD or airborne imagery may allow the MODIS-derived FRE values to be ‘calibrated’ for any systematic underestimation. We therefore expect FRE to become an important tool for enhancing global studies of terrestrial vegetation fires with infrared remote sensing, particularly as the majority of large fires are now imaged four times per day via the MODIS instruments on the Terra and Aqua spacecraft.     

An assessment of photosynthetic light use efficiency from space: Modeling the atmospheric and directional impacts on PRI reflectance

Estimation of photosynthetic light use efficiency (ε) from satellite observations is an important component of climate change research. The photochemical reflectance index, a narrow waveband index based on the reflectance at 531 and 570 nm, allows sampling of the photosynthetic activity of leaves; upscaling of these measurements to landscape and global scales, however, remains challenging. Only a few studies have used spaceborne observations of PRI so far, and research has largely focused on the MODIS sensor. Its daily global coverage and the capacity to detect a narrow reflectance band at 531 nm make it the best available choice for sensing ε from space. Previous results however, have identified a number of key issues with MODIS-based observations of PRI. First, the differences between the footprint of eddy covariance (EC) measurements and the MODIS footprint, which is determined by the sensor's observation geometry make a direct comparison between both data sources challenging and second, the PRI reflectance bands are affected by atmospheric scattering effects confounding the existing physiological signal. In this study we introduce a new approach for upscaling EC based ε measurements to MODIS. First, EC-measured ε values were “translated” into a tower-level optical PRI signal using AMSPEC, an automated multi-angular, tower-based spectroradiometer instrument. AMSPEC enabled us to adjust tower-measured PRI values to the individual viewing geometry of each MODIS overpass. Second, MODIS data were atmospherically corrected using a Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm, which uses a time series approach and an image-based rather than pixel-based processing for simultaneous retrievals of atmospheric aerosol and surface bidirectional reflectance (BRDF). Using this approach, we found a strong relationship between tower-based and spaceborne reflectance measurements (r² = 0.74, p < 0.01) throughout the vegetation period of 2006. Swath (non-gridded) observations yielded stronger correlations than gridded data (r² = 0.58, p < 0.01) both of which included forward and backscatter observations. Spaceborne PRI values were strongly related to canopy shadow fractions and varied with different levels of ε. We conclude that MAIAC-corrected MODIS observations were able to track the site-level physiological changes from space throughout the observation period.     

Input–output feedback linearizing control of linear induction motor taking into consideration the end-effects. Part II: Simulation and experimental results

This is the second part of a paper, divided in two parts, dealing with the application of the input–output feedback linearization (FL) control technique to linear induction motors (LIMs).The first part has treated the theoretical formulation of the input–output feedback linearization control technique as to be applied to linear induction motors. This second part describes the set of tests, both in numerical simulations and experiments, performed to assess the validity of the control technique. In particular, it addresses the issues of the sensitivity of the FL control versus the LIM electrical parameters’ variations and the improvements achievable by considering the LIM dynamic end effects in the control formulation.The proposed FL technique has been further compared, under the same closed-loop bandwidths of the flux and speed systems, with the industrial standard in terms of high performance control technique: field oriented control (FOC).     

Thermochemical analysis for the reduction behavior of FeO in EAF slag via Aluminothermic Smelting Reduction (ASR) process: Part II. Effect of aluminum dross and lime fluxing on Fe and Mn recovery

We investigated Fe recovery from EAF slag by means of aluminothermic smelting reduction (ASR) at 1773 K with Al dross as the reductant, especially the effect of the added amount of the fluxing agent CaO on the Fe recovery. The maximum reaction temperature calculated using FactSage™ 7.0 decreased with increasing CaO addition, but the experimentally measured maximum temperatures increased with increasing CaO addition. We calculated the amounts of various phases before and after Al dross addition under different conditions of added CaO. FeO and Al₂O₃ contents in molten slag sharply varied within the first 5 min of the reaction, stabilizing soon thereafter. The aluminothermic reduction of FeO appeared to proceed rapidly and in good stoichiometric balance, based upon the mass balance between the consumption of FeO and MnO (ΔFeO and ΔMnO) and the production of Al₂O₃ (∆Al₂O₃). Iron recovery from EAF slag was maximized at about 90% when 40 g of CaO was added to 100 g slag. Furthermore, Mn could also be reduced from the EAF slags by the metallic Al in the Al dross reductant. The solid compounds of spinel (MgO∙Al₂O₃) and MgO were precipitated from the slag during the FeO reduction reaction, as confirmed by means of XRD analysis and thermochemical computations. To maximize Fe recovery from EAF slag, it is crucial to control the slag composition, namely to ensure high fluidity by suppressing the formation of solid compounds.