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- Author or Editor: Michel Legrand x
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Abstract
The data of the ECLATS experiment (at Niamey, 1980) are used to study the daily variations of the sensitivity of ground temperature to atmospheric dustiness. In the presence of dust, ground temperature is lower during daytime and higher than normal during nighttime. During night, the influence of dust on ground temperature prevails over other factors. The sensitivity of temperature to dustiness is evaluated and used jointly with ECLATS aircraft soundings to assess the impact of atmospheric dust on the radiance measured by the METEOSAT-I thermal infrared sensor. The results obtained demonstrate the ability of METEOSAT to detect dust clouds during nighttime. A method of optimized imaging is described and applied to a case study.
Abstract
The data of the ECLATS experiment (at Niamey, 1980) are used to study the daily variations of the sensitivity of ground temperature to atmospheric dustiness. In the presence of dust, ground temperature is lower during daytime and higher than normal during nighttime. During night, the influence of dust on ground temperature prevails over other factors. The sensitivity of temperature to dustiness is evaluated and used jointly with ECLATS aircraft soundings to assess the impact of atmospheric dust on the radiance measured by the METEOSAT-I thermal infrared sensor. The results obtained demonstrate the ability of METEOSAT to detect dust clouds during nighttime. A method of optimized imaging is described and applied to a case study.
Abstract
A method for computing the ground surface heat flux density is tested at two places in West Africa during the rainy season and during the dry season. This method is based upon the Fourier analysis of the experimental ground surface temperature. The only required parameter is the soil thermal inertia. The results of these calculations agree with the measurements. This method avoids the use of empirical formulas relating the ground heat flux density to other terms of the surface energy budget. It is shown that these relations are not universal.
Abstract
A method for computing the ground surface heat flux density is tested at two places in West Africa during the rainy season and during the dry season. This method is based upon the Fourier analysis of the experimental ground surface temperature. The only required parameter is the soil thermal inertia. The results of these calculations agree with the measurements. This method avoids the use of empirical formulas relating the ground heat flux density to other terms of the surface energy budget. It is shown that these relations are not universal.
Abstract
The use of the mesoscale model described and qualified in Part I is arranged with radiative transfer codes for the simulation of the thermal infrared response of Meteosat from a Sahelian target. The sensitivity of the satellite response to various atmosphere and surface parameters, either relevant or extraneous to dustiness, is analyzed and physically interpreted throughout the daily cycle, considering especially the thermal impact of the dust at the ground surface. The most significant parameters, according to this criterion of sensitivity, are the amount of dust in the atmosphere and its radiative characteristics, and the ground surface emissivity in the satellite channel. If neglected, the atmospheric water vapor content may be a large source of error for the retrieval of dustiness from the satellite data. The theoretical results are discussed and compared with earlier published experimental work.
Abstract
The use of the mesoscale model described and qualified in Part I is arranged with radiative transfer codes for the simulation of the thermal infrared response of Meteosat from a Sahelian target. The sensitivity of the satellite response to various atmosphere and surface parameters, either relevant or extraneous to dustiness, is analyzed and physically interpreted throughout the daily cycle, considering especially the thermal impact of the dust at the ground surface. The most significant parameters, according to this criterion of sensitivity, are the amount of dust in the atmosphere and its radiative characteristics, and the ground surface emissivity in the satellite channel. If neglected, the atmospheric water vapor content may be a large source of error for the retrieval of dustiness from the satellite data. The theoretical results are discussed and compared with earlier published experimental work.
Abstract
Simulations are carded out to verify a mesoscale model in order to perform sensitivity tests of satellite response to atmospheric dust content. The model chosen is the mesoscale model of Colorado State University with a modified radiation parameterization in order to take atmospheric dust content into account. Downward and upward longwave irradiances are estimated using a 25-interval model. The shortwave pan of the spectrum is processed by a very fast, highly parameterized, single-interval code. Tests using experimental data gathered during the Etude de la Couche Limite Atmosphérique Tropicale Sèche (ECLATS) experiment performed during the 1980 dry season near Niamey (Niger, West Africa) prove that dust content is satisfactorily handled. Three 24-h simulations performed under various meteorological and turbidity conditions show that ground surface energy exchanges are satisfactorily described, so that surface temperature is predicted with a standard deviation of about 1°C. Vertical profiles of computed air temperature and shortwave and longwave irradiances are also realistic.
Abstract
Simulations are carded out to verify a mesoscale model in order to perform sensitivity tests of satellite response to atmospheric dust content. The model chosen is the mesoscale model of Colorado State University with a modified radiation parameterization in order to take atmospheric dust content into account. Downward and upward longwave irradiances are estimated using a 25-interval model. The shortwave pan of the spectrum is processed by a very fast, highly parameterized, single-interval code. Tests using experimental data gathered during the Etude de la Couche Limite Atmosphérique Tropicale Sèche (ECLATS) experiment performed during the 1980 dry season near Niamey (Niger, West Africa) prove that dust content is satisfactorily handled. Three 24-h simulations performed under various meteorological and turbidity conditions show that ground surface energy exchanges are satisfactorily described, so that surface temperature is predicted with a standard deviation of about 1°C. Vertical profiles of computed air temperature and shortwave and longwave irradiances are also realistic.
Abstract
The performances of the new conveyable low-noise infrared radiometer for measurements of atmosphere and ground surface targets, or CLIMAT, are presented for in situ measurements. For this, quantitative analyses were carried out on measurements performed with a prototype during various field experiments. The accuracy of the radiometric measurements controlled by using a field blackbody is estimated for severe environmental conditions. Two modes of operation and two types of targets are described. Ground-based measurements of the sky radiance are compared to radiative transfer calculations that use atmospheric profiles from radiosoundings as input parameters. Sea surface temperatures estimated from airborne CLIMAT measurements are compared to satellite retrievals. These experiments constitute a first set of quantitative tests of the CLIMAT radiometer for ground-based and airborne remote sensing applications. They demonstrate that CLIMAT can be considered for future studies on clouds and aerosols, sea water, and surface such as ice, vegetation, bare soil, and rocks.
Abstract
The performances of the new conveyable low-noise infrared radiometer for measurements of atmosphere and ground surface targets, or CLIMAT, are presented for in situ measurements. For this, quantitative analyses were carried out on measurements performed with a prototype during various field experiments. The accuracy of the radiometric measurements controlled by using a field blackbody is estimated for severe environmental conditions. Two modes of operation and two types of targets are described. Ground-based measurements of the sky radiance are compared to radiative transfer calculations that use atmospheric profiles from radiosoundings as input parameters. Sea surface temperatures estimated from airborne CLIMAT measurements are compared to satellite retrievals. These experiments constitute a first set of quantitative tests of the CLIMAT radiometer for ground-based and airborne remote sensing applications. They demonstrate that CLIMAT can be considered for future studies on clouds and aerosols, sea water, and surface such as ice, vegetation, bare soil, and rocks.
Abstract
The differential absorption technique for estimating columnar water vapor values from the analysis of sunphotometric measurements with wide- and narrowband interferential filters centered near 0.94 μm is discussed and adapted. Water vapor line and continuum absorption calculations are obtained through the procedures included in the radiative transfer code LOWTRAN-7. The accuracy of these calculations is assessed through comparisons with a line-by-line code associated with an updated version of the Clough–Kneizys–Davies continuum. The effects of aerosols and aging of filters on the wide-to-narrow measurements ratio is analyzed. Columnar water vapor retrievals are based upon a search inside a 4D table that contains wide-to-narrow ratios corresponding to combinations of values assumed by four independent variables: sun zenith angle, spectral dependence of aerosol extinction, turbidity level, and the columnar water vapor itself. This table is calculated by taking into account the wide and narrow filter transmission functions, as previously recorded with spectrophotometers. The overall strategy is applied by obtaining columnar water vapor estimates from sunphotometric measurements performed during two field experiments in the Sahel, one of which is the Hydrologic Atmospheric Pilot Experiment in the Sahel. The agreement between these estimates and independent evaluations resulting from radiosoundings is satisfactory, demonstrating the consistency of the authors’ approach.
Abstract
The differential absorption technique for estimating columnar water vapor values from the analysis of sunphotometric measurements with wide- and narrowband interferential filters centered near 0.94 μm is discussed and adapted. Water vapor line and continuum absorption calculations are obtained through the procedures included in the radiative transfer code LOWTRAN-7. The accuracy of these calculations is assessed through comparisons with a line-by-line code associated with an updated version of the Clough–Kneizys–Davies continuum. The effects of aerosols and aging of filters on the wide-to-narrow measurements ratio is analyzed. Columnar water vapor retrievals are based upon a search inside a 4D table that contains wide-to-narrow ratios corresponding to combinations of values assumed by four independent variables: sun zenith angle, spectral dependence of aerosol extinction, turbidity level, and the columnar water vapor itself. This table is calculated by taking into account the wide and narrow filter transmission functions, as previously recorded with spectrophotometers. The overall strategy is applied by obtaining columnar water vapor estimates from sunphotometric measurements performed during two field experiments in the Sahel, one of which is the Hydrologic Atmospheric Pilot Experiment in the Sahel. The agreement between these estimates and independent evaluations resulting from radiosoundings is satisfactory, demonstrating the consistency of the authors’ approach.
Abstract
The new infrared radiometer (conveyable low-noise infrared radiometer for measurements of atmosphere and ground surface targets, or CLIMAT) is a highly sensitive field instrument designed to measure brightness temperatures or radiances in the infrared, from the ground level, or from an aircraft. It can be equipped with up to six channels in the 8–14-μm range. This instrument is characterized by its portability (total mass less than 5 kg), its self-sufficiency, and its automated operation. It can be operated either manually or automatically. The optical head of the instrument contains an objective lens and a condenser mounted according to the Köhler design, providing a uniform irradiation on the detector and a well-delimited field of view. The radiation is measured by a low-noise fast thermopile whose responsivity is slightly temperature dependent. The radiometric noise expressed as an equivalent brightness temperature is on the order of 50 mK for a 1-μm bandwidth at room temperature. The application of a thermal shock reveals no noticeable degradation of the measurements, even though the cavity of the thermopile is not stabilized in temperature.
Abstract
The new infrared radiometer (conveyable low-noise infrared radiometer for measurements of atmosphere and ground surface targets, or CLIMAT) is a highly sensitive field instrument designed to measure brightness temperatures or radiances in the infrared, from the ground level, or from an aircraft. It can be equipped with up to six channels in the 8–14-μm range. This instrument is characterized by its portability (total mass less than 5 kg), its self-sufficiency, and its automated operation. It can be operated either manually or automatically. The optical head of the instrument contains an objective lens and a condenser mounted according to the Köhler design, providing a uniform irradiation on the detector and a well-delimited field of view. The radiation is measured by a low-noise fast thermopile whose responsivity is slightly temperature dependent. The radiometric noise expressed as an equivalent brightness temperature is on the order of 50 mK for a 1-μm bandwidth at room temperature. The application of a thermal shock reveals no noticeable degradation of the measurements, even though the cavity of the thermopile is not stabilized in temperature.
