The Experts below are selected from a list of 1839 Experts worldwide ranked by ideXlab platform
D. E. Heard - One of the best experts on this subject based on the ideXlab platform.
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Concentrations of OH and HO2 radicals during NAMBLEX: measurements and steady state analysis
Atmospheric Chemistry and Physics, 2006Co-Authors: S. C. Smith, J. D. Lee, W. J. Bloss, G. P. Johnson, T. Ingham, D. E. HeardAbstract:OH and HO2 concentrations were measured simultaneously at the Mace Head Atmospheric Research Station in the summer of 2002 during the NAMBLEX (North Atlantic Marine Boundary Layer EXperiment) field campaign. OH was measured by laser-induced fluorescence employing the FAGE (Fluorescence Assay by Gas Expansion) technique, with a mean daytime detection limit of 2.7×105 molecule cm?3 (5 min acquisition period; signal-to-noise ratio = 1). HO2 was detected as OH following its chemical conversion through addition of NO, with a mean detection limit of 4.4×106 molecule cm?3. The diurnal variation of OH was measured on 24 days, and that of HO2 on 17 days. The Local Solar Noon OH concentrations ranged between (3?8)×106 molecule cm?3, with a 24 h mean concentration of 9.1×105 molecule cm?3. The Local Solar Noon HO2 concentrations were (0.9?2.1)×108 molecule cm?3 (3.5?8.2 pptv), with a 24 h mean concentration of 4.2×107 molecule cm?3 (1.6 pptv). HO2 radicals in the range (2?3)×107 molecule cm?3 were observed at night. During NAMBLEX, a comprehensive suite of supporting measurements enabled a detailed study of the behaviour of HOx radicals under primarily clean marine conditions. Steady state expressions are used to calculate OH and HO2 concentrations and to evaluate the effect of different free-radical sources and sinks. The diurnally averaged calculated to measured OH ratio was 1.04±0.36, but the ratio displays a distinct diurnal variation, being less than 1 during the early morning and late afterNoon/evening, and greater than 1 in the middle of the day. For HO2 there was an overprediction, with the agreement between calculated and measured concentrations improved by including reaction with measured IO and BrO radicals and uptake to aerosols. Increasing the concentration of IO radicals included in the calculations to above that measured by a DOAS instrument with an absorption path located mainly over the ocean, reflecting the domination of the inter-tidal region as an iodine source at Mace Head, led to further improvement. The results are compared with previous measurements at Mace Head, and elsewhere in the remote marine boundary layer.
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Concentrations of OH and HO<sub>2</sub> radicals during NAMBLEX: measurements and steady state analysis
Atmospheric Chemistry and Physics Discussions, 2005Co-Authors: S. C. Smith, J. D. Lee, W. J. Bloss, G. P. Johnson, D. E. HeardAbstract:Abstract. OH and HO2 concentrations were measured simultaneously at the Mace Head Atmospheric Research Station in the summer of 2002 during the NAMBLEX (North Atlantic Marine Boundary Layer EXperiment) field campaign. OH was measured by laser-induced fluorescence employing the FAGE (Fluorescence Assay by Gas Expansion) technique, with a mean daytime detection limit of 2.7×105 molecule cm−3 (5 min acquisition period; signal-to-noise ratio = 1). HO2 was detected as OH following its chemical conversion through addition of NO, with a mean detection limit of 4.4×106 molecule cm−3. The diurnal variation of OH was measured on 24 days, and that of HO2 on 17 days. The Local Solar Noon OH concentrations ranged between (3–8)×106 molecule cm−3, with a 24 h mean concentration of 9.1×105 molecule cm−3. The Local Solar Noon HO2 concentrations were (0.9–2.1)×108 molecule cm−3 (3.5–8.2 pptv), with a 24 h mean concentration of 4.2×107 molecule cm−3. HO2 radicals in the range (2–3)×107 molecule cm−3 were observed at night. During NAMBLEX, a comprehensive suite of supporting measurements enabled a detailed study of the behaviour of HOx radicals under primarily clean marine conditions. Case study periods highlight the typical radical levels observed under different conditions. Steady state expressions are used to calculate OH and HO2 concentrations and to evaluate the effect of different free-radical sources and sinks. The diurnally averaged calculated to measured OH ratio was 1.04±0.36, but the ratio displays a distinct diurnal variation, being less than 1 during the early morning and late afterNoon/evening, and greater than 1 in the middle of the day. For HO2 there was an overprediction, with the agreement between calculated and measured concentrations improved by including reaction with measured IO and BrO radicals and uptake to aerosols. Increasing the concentration of IO radicals included in the calculations to above that measured by a DOAS instrument with an absorption path located mainly over the ocean, reflecting the domination of the inter-tidal region as an iodine source at Mace Head, led to further improvement. The results are compared with previous measurements at Mace Head, and elsewhere in the remote marine boundary layer.
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Concentrations of OH and HO2 radicals during NAMBLEX: measurements and steady state analysis
Atmospheric Chemistry and Physics Discussions, 2005Co-Authors: S. C. Smith, J. D. Lee, W. J. Bloss, G. P. Johnson, D. E. HeardAbstract:OH and HO2 concentrations were measured simultaneously at the Mace Head Atmospheric Research Station in the summer of 2002 during the NAMBLEX (North Atlantic Marine Boundary Layer EXperiment) field campaign. OH was measured by laser-induced fluorescence employing the FAGE (Fluorescence Assay by Gas Expansion) technique, with a mean daytime detection limit of 2.7×105 molecule cm?3 (5 min acquisition period; signal-to-noise ratio = 1). HO2 was detected as OH following its chemical conversion through addition of NO, with a mean detection limit of 4.4×106 molecule cm?3. The diurnal variation of OH was measured on 24 days, and that of HO2 on 17 days. The Local Solar Noon OH concentrations ranged between (3?8)×106 molecule cm?3, with a 24 h mean concentration of 9.1×105 molecule cm?3. The Local Solar Noon HO2 concentrations were (0.9?2.1)×108 molecule cm?3 (3.5?8.2 pptv), with a 24 h mean concentration of 4.2×107 molecule cm?3. HO2 radicals in the range (2?3)×107 molecule cm?3 were observed at night. During NAMBLEX, a comprehensive suite of supporting measurements enabled a detailed study of the behaviour of HOx radicals under primarily clean marine conditions. Case study periods highlight the typical radical levels observed under different conditions. Steady state expressions are used to calculate OH and HO2 concentrations and to evaluate the effect of different free-radical sources and sinks. The diurnally averaged calculated to measured OH ratio was 1.04±0.36, but the ratio displays a distinct diurnal variation, being less than 1 during the early morning and late afterNoon/evening, and greater than 1 in the middle of the day. For HO2 there was an overprediction, with the agreement between calculated and measured concentrations improved by including reaction with measured IO and BrO radicals and uptake to aerosols. Increasing the concentration of IO radicals included in the calculations to above that measured by a DOAS instrument with an absorption path located mainly over the ocean, reflecting the domination of the inter-tidal region as an iodine source at Mace Head, led to further improvement. The results are compared with previous measurements at Mace Head, and elsewhere in the remote marine boundary layer.
Miguel O Roman - One of the best experts on this subject based on the ideXlab platform.
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the modis collection v005 brdf albedo product assessment of spatial representativeness over forested landscapes
Remote Sensing of Environment, 2009Co-Authors: A. Strahler, Crystal B Schaaf, Miguel O Roman, Curtis E Woodcock, Xiaoyuan Yang, Rob H Braswell, P S Curtis, Kenneth J Davis, Danilo DragoniAbstract:A new methodology for establishing the spatial representativeness of tower albedo measurements that are routinely used in validation of satellite retrievals from global land surface albedo and reflectance anisotropy products is presented. This method brings together knowledge of the intrinsic biophysical properties of a measurement site, and the surrounding landscape to produce a number of geostatistical attributes that describe the overall variability, spatial extent, strength of the spatial correlation, and spatial structure of surface albedo patterns at separate seasonal periods throughout the year. Variogram functions extracted from Enhanced Thematic Mapper Plus (ETM+) retrievals of surface albedo using multiple spatial and temporal thresholds were used to assess the degree to which a given point (tower) measurement is able to capture the intrinsic variability of the immediate landscape extending to a satellite pixel. A validation scheme was implemented over a wide range of forested landscapes, looking at both deciduous and coniferous sites, from tropical to boreal ecosystems. The experiment focused on comparisons between tower measurements of surface albedo acquired at Local Solar Noon and matching retrievals from the MODerate Resolution Imaging Spectroradiometer (MODIS) (Collection V005) Bidirectional Reflectance Distribution Function (BRDF)/albedo algorithm. Assessments over a select group of field stations with comparable landscape features and daily retrieval scenarios further demonstrate the ability of this technique to identify measurement sites that contain the intrinsic spatial and seasonal features of surface albedo over sufficiently large enough footprints for use in modeling and remote sensing studies. This approach, therefore, improves our understanding of product uncertainty both in terms of the representativeness of the field data and its relationship to the larger satellite pixel.
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The MODIS (Collection V005) BRDF/albedo product: Assessment of spatial representativeness over forested landscapes
Remote Sensing of Environment, 2009Co-Authors: Miguel O Roman, Crystal B Schaaf, Alan H. Strahler, Curtis E Woodcock, Xiaoyuan Yang, Rob H Braswell, P S Curtis, Kenneth J Davis, Danilo Dragoni, Michael L. GouldenAbstract:A new methodology for establishing the spatial representativeness of tower albedo measurements that are routinely used in validation of satellite retrievals from global land surface albedo and reflectance anisotropy products is presented. This method brings together knowledge of the intrinsic biophysical properties of a measurement site, and the surrounding landscape to produce a number of geostatistical attributes that describe the overall variability, spatial extent, strength of the spatial correlation, and spatial structure of surface albedo patterns at separate seasonal periods throughout the year. Variogram functions extracted from Enhanced Thematic Mapper Plus (ETM+) retrievals of surface albedo using multiple spatial and temporal thresholds were used to assess the degree to which a given point (tower) measurement is able to capture the intrinsic variability of the immediate landscape extending to a satellite pixel. A validation scheme was implemented over a wide range of forested landscapes, looking at both deciduous and coniferous sites, from tropical to boreal ecosystems. The experiment focused on comparisons between tower measurements of surface albedo acquired at Local Solar Noon and matching retrievals from the MODerate Resolution Imaging Spectroradiometer (MODIS) (Collection V005) Bidirectional Reflectance Distribution Function (BRDF)/albedo algorithm. Assessments over a select group of field stations with comparable landscape features and daily retrieval scenarios further demonstrate the ability of this technique to identify measurement sites that contain the intrinsic spatial and seasonal features of surface albedo over sufficiently large enough footprints for use in modeling and remote sensing studies. This approach, therefore, improves our understanding of product uncertainty both in terms of the representativeness of the field data and its relationship to the larger satellite pixel.
Danilo Dragoni - One of the best experts on this subject based on the ideXlab platform.
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the modis collection v005 brdf albedo product assessment of spatial representativeness over forested landscapes
Remote Sensing of Environment, 2009Co-Authors: A. Strahler, Crystal B Schaaf, Miguel O Roman, Curtis E Woodcock, Xiaoyuan Yang, Rob H Braswell, P S Curtis, Kenneth J Davis, Danilo DragoniAbstract:A new methodology for establishing the spatial representativeness of tower albedo measurements that are routinely used in validation of satellite retrievals from global land surface albedo and reflectance anisotropy products is presented. This method brings together knowledge of the intrinsic biophysical properties of a measurement site, and the surrounding landscape to produce a number of geostatistical attributes that describe the overall variability, spatial extent, strength of the spatial correlation, and spatial structure of surface albedo patterns at separate seasonal periods throughout the year. Variogram functions extracted from Enhanced Thematic Mapper Plus (ETM+) retrievals of surface albedo using multiple spatial and temporal thresholds were used to assess the degree to which a given point (tower) measurement is able to capture the intrinsic variability of the immediate landscape extending to a satellite pixel. A validation scheme was implemented over a wide range of forested landscapes, looking at both deciduous and coniferous sites, from tropical to boreal ecosystems. The experiment focused on comparisons between tower measurements of surface albedo acquired at Local Solar Noon and matching retrievals from the MODerate Resolution Imaging Spectroradiometer (MODIS) (Collection V005) Bidirectional Reflectance Distribution Function (BRDF)/albedo algorithm. Assessments over a select group of field stations with comparable landscape features and daily retrieval scenarios further demonstrate the ability of this technique to identify measurement sites that contain the intrinsic spatial and seasonal features of surface albedo over sufficiently large enough footprints for use in modeling and remote sensing studies. This approach, therefore, improves our understanding of product uncertainty both in terms of the representativeness of the field data and its relationship to the larger satellite pixel.
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The MODIS (Collection V005) BRDF/albedo product: Assessment of spatial representativeness over forested landscapes
Remote Sensing of Environment, 2009Co-Authors: Miguel O Roman, Crystal B Schaaf, Alan H. Strahler, Curtis E Woodcock, Xiaoyuan Yang, Rob H Braswell, P S Curtis, Kenneth J Davis, Danilo Dragoni, Michael L. GouldenAbstract:A new methodology for establishing the spatial representativeness of tower albedo measurements that are routinely used in validation of satellite retrievals from global land surface albedo and reflectance anisotropy products is presented. This method brings together knowledge of the intrinsic biophysical properties of a measurement site, and the surrounding landscape to produce a number of geostatistical attributes that describe the overall variability, spatial extent, strength of the spatial correlation, and spatial structure of surface albedo patterns at separate seasonal periods throughout the year. Variogram functions extracted from Enhanced Thematic Mapper Plus (ETM+) retrievals of surface albedo using multiple spatial and temporal thresholds were used to assess the degree to which a given point (tower) measurement is able to capture the intrinsic variability of the immediate landscape extending to a satellite pixel. A validation scheme was implemented over a wide range of forested landscapes, looking at both deciduous and coniferous sites, from tropical to boreal ecosystems. The experiment focused on comparisons between tower measurements of surface albedo acquired at Local Solar Noon and matching retrievals from the MODerate Resolution Imaging Spectroradiometer (MODIS) (Collection V005) Bidirectional Reflectance Distribution Function (BRDF)/albedo algorithm. Assessments over a select group of field stations with comparable landscape features and daily retrieval scenarios further demonstrate the ability of this technique to identify measurement sites that contain the intrinsic spatial and seasonal features of surface albedo over sufficiently large enough footprints for use in modeling and remote sensing studies. This approach, therefore, improves our understanding of product uncertainty both in terms of the representativeness of the field data and its relationship to the larger satellite pixel.
S. C. Smith - One of the best experts on this subject based on the ideXlab platform.
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Concentrations of OH and HO2 radicals during NAMBLEX: measurements and steady state analysis
Atmospheric Chemistry and Physics, 2006Co-Authors: S. C. Smith, J. D. Lee, W. J. Bloss, G. P. Johnson, T. Ingham, D. E. HeardAbstract:OH and HO2 concentrations were measured simultaneously at the Mace Head Atmospheric Research Station in the summer of 2002 during the NAMBLEX (North Atlantic Marine Boundary Layer EXperiment) field campaign. OH was measured by laser-induced fluorescence employing the FAGE (Fluorescence Assay by Gas Expansion) technique, with a mean daytime detection limit of 2.7×105 molecule cm?3 (5 min acquisition period; signal-to-noise ratio = 1). HO2 was detected as OH following its chemical conversion through addition of NO, with a mean detection limit of 4.4×106 molecule cm?3. The diurnal variation of OH was measured on 24 days, and that of HO2 on 17 days. The Local Solar Noon OH concentrations ranged between (3?8)×106 molecule cm?3, with a 24 h mean concentration of 9.1×105 molecule cm?3. The Local Solar Noon HO2 concentrations were (0.9?2.1)×108 molecule cm?3 (3.5?8.2 pptv), with a 24 h mean concentration of 4.2×107 molecule cm?3 (1.6 pptv). HO2 radicals in the range (2?3)×107 molecule cm?3 were observed at night. During NAMBLEX, a comprehensive suite of supporting measurements enabled a detailed study of the behaviour of HOx radicals under primarily clean marine conditions. Steady state expressions are used to calculate OH and HO2 concentrations and to evaluate the effect of different free-radical sources and sinks. The diurnally averaged calculated to measured OH ratio was 1.04±0.36, but the ratio displays a distinct diurnal variation, being less than 1 during the early morning and late afterNoon/evening, and greater than 1 in the middle of the day. For HO2 there was an overprediction, with the agreement between calculated and measured concentrations improved by including reaction with measured IO and BrO radicals and uptake to aerosols. Increasing the concentration of IO radicals included in the calculations to above that measured by a DOAS instrument with an absorption path located mainly over the ocean, reflecting the domination of the inter-tidal region as an iodine source at Mace Head, led to further improvement. The results are compared with previous measurements at Mace Head, and elsewhere in the remote marine boundary layer.
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Concentrations of OH and HO<sub>2</sub> radicals during NAMBLEX: measurements and steady state analysis
Atmospheric Chemistry and Physics Discussions, 2005Co-Authors: S. C. Smith, J. D. Lee, W. J. Bloss, G. P. Johnson, D. E. HeardAbstract:Abstract. OH and HO2 concentrations were measured simultaneously at the Mace Head Atmospheric Research Station in the summer of 2002 during the NAMBLEX (North Atlantic Marine Boundary Layer EXperiment) field campaign. OH was measured by laser-induced fluorescence employing the FAGE (Fluorescence Assay by Gas Expansion) technique, with a mean daytime detection limit of 2.7×105 molecule cm−3 (5 min acquisition period; signal-to-noise ratio = 1). HO2 was detected as OH following its chemical conversion through addition of NO, with a mean detection limit of 4.4×106 molecule cm−3. The diurnal variation of OH was measured on 24 days, and that of HO2 on 17 days. The Local Solar Noon OH concentrations ranged between (3–8)×106 molecule cm−3, with a 24 h mean concentration of 9.1×105 molecule cm−3. The Local Solar Noon HO2 concentrations were (0.9–2.1)×108 molecule cm−3 (3.5–8.2 pptv), with a 24 h mean concentration of 4.2×107 molecule cm−3. HO2 radicals in the range (2–3)×107 molecule cm−3 were observed at night. During NAMBLEX, a comprehensive suite of supporting measurements enabled a detailed study of the behaviour of HOx radicals under primarily clean marine conditions. Case study periods highlight the typical radical levels observed under different conditions. Steady state expressions are used to calculate OH and HO2 concentrations and to evaluate the effect of different free-radical sources and sinks. The diurnally averaged calculated to measured OH ratio was 1.04±0.36, but the ratio displays a distinct diurnal variation, being less than 1 during the early morning and late afterNoon/evening, and greater than 1 in the middle of the day. For HO2 there was an overprediction, with the agreement between calculated and measured concentrations improved by including reaction with measured IO and BrO radicals and uptake to aerosols. Increasing the concentration of IO radicals included in the calculations to above that measured by a DOAS instrument with an absorption path located mainly over the ocean, reflecting the domination of the inter-tidal region as an iodine source at Mace Head, led to further improvement. The results are compared with previous measurements at Mace Head, and elsewhere in the remote marine boundary layer.
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Concentrations of OH and HO2 radicals during NAMBLEX: measurements and steady state analysis
Atmospheric Chemistry and Physics Discussions, 2005Co-Authors: S. C. Smith, J. D. Lee, W. J. Bloss, G. P. Johnson, D. E. HeardAbstract:OH and HO2 concentrations were measured simultaneously at the Mace Head Atmospheric Research Station in the summer of 2002 during the NAMBLEX (North Atlantic Marine Boundary Layer EXperiment) field campaign. OH was measured by laser-induced fluorescence employing the FAGE (Fluorescence Assay by Gas Expansion) technique, with a mean daytime detection limit of 2.7×105 molecule cm?3 (5 min acquisition period; signal-to-noise ratio = 1). HO2 was detected as OH following its chemical conversion through addition of NO, with a mean detection limit of 4.4×106 molecule cm?3. The diurnal variation of OH was measured on 24 days, and that of HO2 on 17 days. The Local Solar Noon OH concentrations ranged between (3?8)×106 molecule cm?3, with a 24 h mean concentration of 9.1×105 molecule cm?3. The Local Solar Noon HO2 concentrations were (0.9?2.1)×108 molecule cm?3 (3.5?8.2 pptv), with a 24 h mean concentration of 4.2×107 molecule cm?3. HO2 radicals in the range (2?3)×107 molecule cm?3 were observed at night. During NAMBLEX, a comprehensive suite of supporting measurements enabled a detailed study of the behaviour of HOx radicals under primarily clean marine conditions. Case study periods highlight the typical radical levels observed under different conditions. Steady state expressions are used to calculate OH and HO2 concentrations and to evaluate the effect of different free-radical sources and sinks. The diurnally averaged calculated to measured OH ratio was 1.04±0.36, but the ratio displays a distinct diurnal variation, being less than 1 during the early morning and late afterNoon/evening, and greater than 1 in the middle of the day. For HO2 there was an overprediction, with the agreement between calculated and measured concentrations improved by including reaction with measured IO and BrO radicals and uptake to aerosols. Increasing the concentration of IO radicals included in the calculations to above that measured by a DOAS instrument with an absorption path located mainly over the ocean, reflecting the domination of the inter-tidal region as an iodine source at Mace Head, led to further improvement. The results are compared with previous measurements at Mace Head, and elsewhere in the remote marine boundary layer.
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Concentrations of OH and HO 2 radicals during NAMBLEX: measurements and steady state analysis
Atmospheric Chemistry and Physics, 2005Co-Authors: S. C. Smith, J. D. Lee, W. J. Bloss, G. P. Johnson, T. Ingham, Dwayne HeardAbstract:OH and HO2 concentrations were measured si- multaneously at the Mace Head Atmospheric Research Sta- tion in the summer of 2002 during the NAMBLEX (North Atlantic Marine Boundary Layer EXperiment) field cam- paign. OH was measured by laser-induced fluorescence employing the FAGE (Fluorescence Assay by Gas Expan- sion) technique, with a mean daytime detection limit of 2.7◊10 5 molecule cm 3 (5 min acquisition period; signal-to- noise ratio = 1). HO2 was detected as OH following its chem- ical conversion through addition of NO, with a mean detec- tion limit of 4.4◊10 6 molecule cm 3 . The diurnal variation of OH was measured on 24 days, and that of HO2 on 17 days. The Local Solar Noon OH concentrations ranged between (3- 8)◊10 6 molecule cm 3 , with a 24 h mean concentration of 9.1◊10 5 molecule cm 3 . The Local Solar Noon HO2 concen- trations were (0.9-2.1)◊10 8 molecule cm 3 (3.5-8.2 pptv), with a 24 h mean concentration of 4.2◊10 7 molecule cm 3 (1.6 pptv). HO2 radicals in the range (2-3)◊10 7 molecule cm 3 were observed at night. During NAMBLEX, a com- prehensive suite of supporting measurements enabled a de- tailed study of the behaviour of HOx radicals under primar- ily clean marine conditions. Steady state expressions are used to calculate OH and HO2 concentrations and to eval- uate the effect of different free-radical sources and sinks. The diurnally averaged calculated to measured OH ratio was 1.04±0.36, but the ratio displays a distinct diurnal variation, being less than 1 during the early morning and late after- Noon/evening, and greater than 1 in the middle of the day. For HO2 there was an overprediction, with the agreement be- tween calculated and measured concentrations improved by including reaction with measured IO and BrO radicals and uptake to aerosols. Increasing the concentration of IO radi- cals included in the calculations to above that measured by a DOAS instrument with an absorption path located mainly over the ocean, reflecting the domination of the inter-tidal region as an iodine source at Mace Head, led to further im- provement. The results are compared with previous measure- ments at Mace Head, and elsewhere in the remote marine boundary layer.
Crystal B Schaaf - One of the best experts on this subject based on the ideXlab platform.
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the modis collection v005 brdf albedo product assessment of spatial representativeness over forested landscapes
Remote Sensing of Environment, 2009Co-Authors: A. Strahler, Crystal B Schaaf, Miguel O Roman, Curtis E Woodcock, Xiaoyuan Yang, Rob H Braswell, P S Curtis, Kenneth J Davis, Danilo DragoniAbstract:A new methodology for establishing the spatial representativeness of tower albedo measurements that are routinely used in validation of satellite retrievals from global land surface albedo and reflectance anisotropy products is presented. This method brings together knowledge of the intrinsic biophysical properties of a measurement site, and the surrounding landscape to produce a number of geostatistical attributes that describe the overall variability, spatial extent, strength of the spatial correlation, and spatial structure of surface albedo patterns at separate seasonal periods throughout the year. Variogram functions extracted from Enhanced Thematic Mapper Plus (ETM+) retrievals of surface albedo using multiple spatial and temporal thresholds were used to assess the degree to which a given point (tower) measurement is able to capture the intrinsic variability of the immediate landscape extending to a satellite pixel. A validation scheme was implemented over a wide range of forested landscapes, looking at both deciduous and coniferous sites, from tropical to boreal ecosystems. The experiment focused on comparisons between tower measurements of surface albedo acquired at Local Solar Noon and matching retrievals from the MODerate Resolution Imaging Spectroradiometer (MODIS) (Collection V005) Bidirectional Reflectance Distribution Function (BRDF)/albedo algorithm. Assessments over a select group of field stations with comparable landscape features and daily retrieval scenarios further demonstrate the ability of this technique to identify measurement sites that contain the intrinsic spatial and seasonal features of surface albedo over sufficiently large enough footprints for use in modeling and remote sensing studies. This approach, therefore, improves our understanding of product uncertainty both in terms of the representativeness of the field data and its relationship to the larger satellite pixel.
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The MODIS (Collection V005) BRDF/albedo product: Assessment of spatial representativeness over forested landscapes
Remote Sensing of Environment, 2009Co-Authors: Miguel O Roman, Crystal B Schaaf, Alan H. Strahler, Curtis E Woodcock, Xiaoyuan Yang, Rob H Braswell, P S Curtis, Kenneth J Davis, Danilo Dragoni, Michael L. GouldenAbstract:A new methodology for establishing the spatial representativeness of tower albedo measurements that are routinely used in validation of satellite retrievals from global land surface albedo and reflectance anisotropy products is presented. This method brings together knowledge of the intrinsic biophysical properties of a measurement site, and the surrounding landscape to produce a number of geostatistical attributes that describe the overall variability, spatial extent, strength of the spatial correlation, and spatial structure of surface albedo patterns at separate seasonal periods throughout the year. Variogram functions extracted from Enhanced Thematic Mapper Plus (ETM+) retrievals of surface albedo using multiple spatial and temporal thresholds were used to assess the degree to which a given point (tower) measurement is able to capture the intrinsic variability of the immediate landscape extending to a satellite pixel. A validation scheme was implemented over a wide range of forested landscapes, looking at both deciduous and coniferous sites, from tropical to boreal ecosystems. The experiment focused on comparisons between tower measurements of surface albedo acquired at Local Solar Noon and matching retrievals from the MODerate Resolution Imaging Spectroradiometer (MODIS) (Collection V005) Bidirectional Reflectance Distribution Function (BRDF)/albedo algorithm. Assessments over a select group of field stations with comparable landscape features and daily retrieval scenarios further demonstrate the ability of this technique to identify measurement sites that contain the intrinsic spatial and seasonal features of surface albedo over sufficiently large enough footprints for use in modeling and remote sensing studies. This approach, therefore, improves our understanding of product uncertainty both in terms of the representativeness of the field data and its relationship to the larger satellite pixel.
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Assessment of global climate model land surface albedo using MODIS data
Geophysical Research Letters, 2003Co-Authors: Keith W. Oleson, Crystal B Schaaf, Yufang Jin, Feng Gao, Gordon B. Bonan, Alan H. StrahlerAbstract:[1] Land surface albedo from the Community Land Model is compared to white-sky (diffuse) and black-sky albedo (direct at Local Solar Noon) from MODIS. Generally, comparisons are more favorable in summer than winter, for visible waveband than near-infrared in regions without snow cover, and for black- than white-sky. In regions with extensive snow cover, the model overestimates white- and black-sky albedo by up to 20% absolute. The snow-free visible and near-infrared black-sky albedo is simulated quite well with biases within ±5% over most of the land surface. However, a large negative model bias was found for the Sahara Desert and Arabian Peninsula, particularly in the near-infrared. The poorer simulation of white- compared to black-sky albedo in vegetated areas implies that the model may be overestimating the increase of albedo with Solar zenith angle. These results identify several areas that should have priority in further evaluating and improving albedo in the model.
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IGARSS - Global albedo, BRDF and nadir BRDF-adjusted reflectance products from MODIS
IEEE International Geoscience and Remote Sensing Symposium, 2002Co-Authors: Crystal B Schaaf, A. Strahler, E. Tsvetsinskaya, Wolfgang Lucht, J P Muller, Xiaowen Li, Xiaoyang Zhang, P LewisAbstract:Annual sequences of the first reprocessed albedo, bidirectional reflectance distribution function (BRDF), and nadir BRDF-adjusted surface reflectance (NBAR) products are being evaluated. BRDF model parameters (or weights) are used to compute black sky albedo at Local Solar Noon and white sky albedo and to compute surface reflectance at a common nadir geometry. In addition to these standard resolution albedo, BRDF, and NBAR products, which are provided in the integerized sinusoidal grid projection, the BRDF parameters, black sky albedos (at Local Solar Noon), and white sky albedos are now also being operationally produced in a global geographic projection known as the Climate Modeling Grid (CMG). These are currently available at a 0.25 degree spatial resolution, although there is community interest in a 0.05 degree resolution. In addition to the operational CMGs, coarser 0.5 degree and 1 degree resolution versions of the reprocessed albedo data have also been produced for the ISLSCP-II initiative. The global distribution of these albedos (as they vary throughout the year) are presented, as well as discussions of the most recent evaluations of the quality of the standard products.
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How does snow impact the albedo of vegetated land surfaces as analyzed with MODIS data
Geophysical Research Letters, 2002Co-Authors: Yufang Jin, Crystal B Schaaf, Feng Gao, Alan H. Strahler, Xubin Zeng, Robert E. DickinsonAbstract:[1] Albedo derived from MODIS observations is found to be very stable during November 2000–January 2001. We analyze shortwave albedo under snow and snow-free conditions by IGBP land cover types. Snow changes the spectral property of the surface reflectivity and causes high heterogeneity in the surface albedo between and within land types. The mean black sky (or direct beam) albedo at Local Solar Noon for snow-covered forests is less than 0.30 in the shortwave (0.3–5.0 μm), but it reaches 0.57 for snow-covered grassland and barren. Although we are unable to further separate within-class albedos with fractional tree cover, we find that the normalized difference snow index (NDSI) is highly correlated with surface albedo and hence can be taken as a measure of snow, soil and canopy fraction.
