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- Author or Editor: P. Y. Deschamps x
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Abstract
Day–night surface temperature differences measured in the infrared (10.5–12.5 μm channel) by the HCMR satellite experiment frequently show large diurnal heating (several °C) of the upper layer of the ocean during the summer months in the Mediterranean Sea when the wind speed is low. When observed in the 0.5–1.1 μm channel, glitter reflectance—i.e., direct solar radiation specularly reflected towards the sensor—correlates with diurnal heating. Glitter reflectance has been modeled to retrieve an equivalent wind speed. Observed diurnal heating (ΔT) do not exceed 5°C, in agreement with the limit value calculated from the heat transfer equation assuming thermal diffusivity is only molecular. The influence of wind speed can be approximately described by ΔT=0.4U −1+0.5 (in °C for U in m s−1), for U less than 2 m s−1. A mean diurnal heating of nearly 1°C is calculated for the marine coastal areas of southern France. During this period, satellite observations should be restricted to night and early morning orbits, or to periods of high wind speed (U>5 m s−1).
Abstract
Day–night surface temperature differences measured in the infrared (10.5–12.5 μm channel) by the HCMR satellite experiment frequently show large diurnal heating (several °C) of the upper layer of the ocean during the summer months in the Mediterranean Sea when the wind speed is low. When observed in the 0.5–1.1 μm channel, glitter reflectance—i.e., direct solar radiation specularly reflected towards the sensor—correlates with diurnal heating. Glitter reflectance has been modeled to retrieve an equivalent wind speed. Observed diurnal heating (ΔT) do not exceed 5°C, in agreement with the limit value calculated from the heat transfer equation assuming thermal diffusivity is only molecular. The influence of wind speed can be approximately described by ΔT=0.4U −1+0.5 (in °C for U in m s−1), for U less than 2 m s−1. A mean diurnal heating of nearly 1°C is calculated for the marine coastal areas of southern France. During this period, satellite observations should be restricted to night and early morning orbits, or to periods of high wind speed (U>5 m s−1).
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Abstract
An accuracy budget of the surface reflectance determination from Meteosat geostationary satellite data is performed. Error analysis allows identification of three main problems: calibration uncertainty of the Meteosat instrument, atmospheric corrections, and surface effects (spectral and directional). Calibration accuracy is 10%, leading to a 10% relative uncertainty on reflectance. Spectral effects of the surface lead to a maximum bias of 0.01 for a vegetated surface as sensed by Meteosat, while directional effects can lead to a bias of 0.035 between two measurements taken at two different sun zenith and azimuth angles at the same view angle over savannas. The maximum error due to the atmosphere is estimated to be of the order of 0.03 in reflectance for a surface reflectance of 0.40 and 0.01 for, a surface reflectance of 0.10. Validation with in situ measurement is within the expected error over savanna. But the difference is still high over the southwest France site of HAPEX-MOBILHY, certainly due to the joint spectral and directional errors. Comparisons with surface albedo maps from literature show the same spatial and spatial evolutions with a better spatial and temporal determination in our results.
Abstract
An accuracy budget of the surface reflectance determination from Meteosat geostationary satellite data is performed. Error analysis allows identification of three main problems: calibration uncertainty of the Meteosat instrument, atmospheric corrections, and surface effects (spectral and directional). Calibration accuracy is 10%, leading to a 10% relative uncertainty on reflectance. Spectral effects of the surface lead to a maximum bias of 0.01 for a vegetated surface as sensed by Meteosat, while directional effects can lead to a bias of 0.035 between two measurements taken at two different sun zenith and azimuth angles at the same view angle over savannas. The maximum error due to the atmosphere is estimated to be of the order of 0.03 in reflectance for a surface reflectance of 0.40 and 0.01 for, a surface reflectance of 0.10. Validation with in situ measurement is within the expected error over savanna. But the difference is still high over the southwest France site of HAPEX-MOBILHY, certainly due to the joint spectral and directional errors. Comparisons with surface albedo maps from literature show the same spatial and spatial evolutions with a better spatial and temporal determination in our results.
Abstract
A multispectral/multiangular procedure is proposed to calibrate the infrared channel of METEOSAT-2 IR 1 (760–980 cm−1), using the radiances of NOAA-7 AVHRR channels 4 (870–980 cm−1) and 5 (795–885 cm−1). The METEOSAT radiance can be successfully simulated by using one or both AVHRR infrared channels. The calibration coefficients are strongly influenced by the airmass (propagation paths) or incident zenith angles (θ i ). With one AVHIRR channel, the best results are obtained at θM<θN for channel 4 and at θM>θN for channel 5. With both AVHRR channels, the best results are found at θM = θN; however, an overall accuracy of less than 0.2 K is achievable at almost any viewing geometry. Application of the procedure to real data shows that the calibration factor for the METEOSAT-2 IR 1 channel can be accurately estimated from the NOAA AVHRR data on the same date and zone. The method can easily be automated and used to calculate or adjust the calibration factors for METEOSAT-2.
Abstract
A multispectral/multiangular procedure is proposed to calibrate the infrared channel of METEOSAT-2 IR 1 (760–980 cm−1), using the radiances of NOAA-7 AVHRR channels 4 (870–980 cm−1) and 5 (795–885 cm−1). The METEOSAT radiance can be successfully simulated by using one or both AVHRR infrared channels. The calibration coefficients are strongly influenced by the airmass (propagation paths) or incident zenith angles (θ i ). With one AVHIRR channel, the best results are obtained at θM<θN for channel 4 and at θM>θN for channel 5. With both AVHRR channels, the best results are found at θM = θN; however, an overall accuracy of less than 0.2 K is achievable at almost any viewing geometry. Application of the procedure to real data shows that the calibration factor for the METEOSAT-2 IR 1 channel can be accurately estimated from the NOAA AVHRR data on the same date and zone. The method can easily be automated and used to calculate or adjust the calibration factors for METEOSAT-2.
Abstract
Satellite infrared data have been used to investigate the mesoscale variability of the SST (sea surface temperature) field. A statistical analysis of the SST field has been performed by means of the structure function. Results give the equivalent power-law exponent n of the spatial variance density spectrum E(k) ∼ k −h . The exponent n was found to vary from 1.5 to 2,3 with a mean value of 1.8 in the ]range of scales 3–100 km which is in agreement with previous one-dimensional analysis from shipborne and airborne measurements. These observed values of n are discussed and compared with the values predicted by turbulence theories.
Abstract
Satellite infrared data have been used to investigate the mesoscale variability of the SST (sea surface temperature) field. A statistical analysis of the SST field has been performed by means of the structure function. Results give the equivalent power-law exponent n of the spatial variance density spectrum E(k) ∼ k −h . The exponent n was found to vary from 1.5 to 2,3 with a mean value of 1.8 in the ]range of scales 3–100 km which is in agreement with previous one-dimensional analysis from shipborne and airborne measurements. These observed values of n are discussed and compared with the values predicted by turbulence theories.
Abstract
A physically based model is used to derive downward solar irradiance at the surface of the earth and surface albedo from Meteosat satellite measurements in the wavelength range between 0.40 and 1. 1 0 μm. The model takes into account Rayleigh and Mie scattering, water vapor and ozone absorption. No threshold setting is necessary to distinguish between clear and cloudy conditions, thereby avoiding the problem of its arbitrary nature and to some extent allowing quicker and easier data processing. Comparison of noontime satellite estimates with analogous hourly solar irradiance measurements obtained from the French pyranometer network shows a correlation coefficient of 0.92 and a rms error of 109 W m−2 (20% of the mean solar irradiance). The maximum error occurs for values of insulation around 300 W m−2 and can be mostly attributed to the different spatial and temporal samplings of the systems being compared. Mean monthly estimates of the solar irradiance between 1100 and 1300 UT, at different locations, fit pyranometer measurements with a rms error of 37 W m−2 (6%). The method is finally used to product maps of solar irradiance albedo and net solar flux at the surface over France. These maps can be easily derived from the whole Meteosat frame, with resolution ranging from a few kilometers to several dozens of kilometers, which are scales suitable for agrimeteorology and climate studies respectively.
Abstract
A physically based model is used to derive downward solar irradiance at the surface of the earth and surface albedo from Meteosat satellite measurements in the wavelength range between 0.40 and 1. 1 0 μm. The model takes into account Rayleigh and Mie scattering, water vapor and ozone absorption. No threshold setting is necessary to distinguish between clear and cloudy conditions, thereby avoiding the problem of its arbitrary nature and to some extent allowing quicker and easier data processing. Comparison of noontime satellite estimates with analogous hourly solar irradiance measurements obtained from the French pyranometer network shows a correlation coefficient of 0.92 and a rms error of 109 W m−2 (20% of the mean solar irradiance). The maximum error occurs for values of insulation around 300 W m−2 and can be mostly attributed to the different spatial and temporal samplings of the systems being compared. Mean monthly estimates of the solar irradiance between 1100 and 1300 UT, at different locations, fit pyranometer measurements with a rms error of 37 W m−2 (6%). The method is finally used to product maps of solar irradiance albedo and net solar flux at the surface over France. These maps can be easily derived from the whole Meteosat frame, with resolution ranging from a few kilometers to several dozens of kilometers, which are scales suitable for agrimeteorology and climate studies respectively.
Abstract
A simple three-layer model of the earth-atmosphere system, including the ground, troposphere and stratosphere, with their interactions, is developed. The model permits the radiative characteristics of both the troposphere and stratosphere to be separately adjusted to describe any atmospheric state. The accuracy is tested against the more precise computations of Herman et al. (1976), which take into account the aerosol profile and the angular variation of reflectance at the top of the troposphere. Analytical expressions are obtained for the albedo variation due to a thin stratospheric aerosol layer. The physical procedures are outlined, as well as the influence of the main parameters: aerosol optical thickness, single scattering albedo and asymmetry factor, and sublayer albedo.
The method is applied to compute the variation of the zonal albedo and the planetary radiation balance due to a stratospheric aerosol layer of background H2SO4 droplets and of volcanic ash. The resulting ground temperature perturbation is evaluated, using a Budyko type climate model.
Abstract
A simple three-layer model of the earth-atmosphere system, including the ground, troposphere and stratosphere, with their interactions, is developed. The model permits the radiative characteristics of both the troposphere and stratosphere to be separately adjusted to describe any atmospheric state. The accuracy is tested against the more precise computations of Herman et al. (1976), which take into account the aerosol profile and the angular variation of reflectance at the top of the troposphere. Analytical expressions are obtained for the albedo variation due to a thin stratospheric aerosol layer. The physical procedures are outlined, as well as the influence of the main parameters: aerosol optical thickness, single scattering albedo and asymmetry factor, and sublayer albedo.
The method is applied to compute the variation of the zonal albedo and the planetary radiation balance due to a stratospheric aerosol layer of background H2SO4 droplets and of volcanic ash. The resulting ground temperature perturbation is evaluated, using a Budyko type climate model.
