Radiative transfer properties of the Sahara region

Egan, Walter G.

doi:10.1016/0034-4257(94)90044-2

Cited by 5 publications

(2 citation statements)

References 31 publications

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“…Dust has been measured to heights of up to 3 km, at optical depths of 1.5 (D'Almeida 1986). Egan (1994) found, using a simple radiative transfer model, that it is possible for a typical dust loading in the atmosphere to reduce top-ofatmosphere OLR by about 10 W m Ϫ2 , about the order of magnitude of the trend we see over the Sahara. The change in the amount of reflected solar radiation depends on the albedo of the surface below.…”

Section: Exploration Of Possible Physical Causesmentioning

confidence: 64%

Regional Trends inNimbus-7OLR: Effects of a Spectrally Nonuniform Albedo*

Tashima¹,

Hartmann²

1999

J. Climate

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Data from the Earth Radiation Budget wide-field-of-view sensors on board the Nimbus-7 satellite during the period 1979-87 indicate a statistically significant downward trend in daytime outgoing longwave radiation (OLR) of up to 14 W m Ϫ2 decade Ϫ1 over the Sahara desert region of Africa. Reflected solar radiation shows a positive trend of the same magnitude. No significant trends in temperature or cloudiness over the region can be demonstrated. Atmospheric desert dust aerosols increase over the same period, but the spatial pattern of the dust trend is different from that of the OLR and reflected solar trends. A plausible explanation is that these trends are instrumental in nature and are caused by degradation of the Suprasil filter dome that covers the reflected solar channels. The filter dome degradation is greater for shorter wavelengths of the solar spectrum compared to red and near-infrared wavelengths. This enhanced degradation at shorter wavelengths, combined with the increased albedo of deserts at longer wavelengths, causes the global correction applied to the reflected solar data to be too large over desert regions. The OLR is calculated by subtracting the reflected solar irradiance from the unfiltered total outgoing irradiance (reflected solar plus emitted longwave). Thus, if the total irradiance remains unchanged, an erroneous upward trend in the solar data will cause a downward trend in the OLR estimate of similar magnitude. A preliminary calculation of the magnitude of the filter degradation effect is presented. Laboratory and thematic mapper spectral reflectance data of desert sand are used to estimate the reflectivity of the desert surface as a function of wavelength. The degradation of the shortwave filter as a function of wavelength is estimated with a quadratic fit to the known deterioration rates of the reflected solar and near-infrared channels. Using these parameters, the magnitude of the trend explained by this effect is calculated to be 11 W m Ϫ2 decade Ϫ1 , which is nearly as large as the trends in the data. * Joint Institute for the Study of the Atmosphere and Ocean Contribution Number 503.

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Section: Exploration Of Possible Physical Causesmentioning

confidence: 64%

Regional Trends inNimbus-7OLR: Effects of a Spectrally Nonuniform Albedo*

Tashima¹,

Hartmann²

1999

J. Climate

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“…First, it has been shown recently that colour infrared photogra-37 phy is much less affected by dust aerosols which may hamper the transmission of light in the visible spectrum in dry areas such as the Sahel (Chavez, 1992;Egan, 1994). Colour infrared images may also offer much higher contrasts between plant canopies and the surrounding soil than do true colour pictures (Fig.…”

Section: Discussionmentioning

confidence: 99%

Soil productivity management and plant growth in the Sahel: Potential of an aerial monitoring technique

Buerkert

Mahler²,

Marschner

1996

Plant Soil

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Spatial variability of plant growth has been a major problem for plant physiologists, agronomists, agro-foresters and soil scientists comparing or modelling treatment effects on acid sandy soils in the Sahel. While aerial photographs from aeroplanes or satellite images may provide valuable information for the surveying of large areas, their use for individual small experiments or farmers' fields has been limited due to high costs, restricted availability, and unsatisfactory resolution. As a simple alternative, a commercially available Zeppelin-type balloon, dragged on a rope by a camel, was fitted with an standard 35 mm camera and a remote control system. Flight altitude varied from 20 m to 500 m above ground. A ground-based camera mounted vertically on a tripod was used to center the Zeppelin over the target area. The high-resolution true colour negatives and colour infrared slides obtained by this device were used to unravel the history of farmers' management strategies for maintaining soil productivity, to monitor treatment effects and crop growth variability in an on-station experiment, and to visualize light absorption by photosynthesis in crops and trees. Such non-destructively collected data may serve as quick but reliable references for ground measurements in a wide range of experiments with loosely-spaced crops, bushes, and trees.

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Evidence of Saharan mineral aerosols transport to the Mediterranean inside well-defined layers

Hamonou

Chazette

1998

Journal of Aerosol Science

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Radiative transfer properties of the Sahara region

Cited by 5 publications

References 31 publications

Regional Trends inNimbus-7OLR: Effects of a Spectrally Nonuniform Albedo*

Regional Trends inNimbus-7OLR: Effects of a Spectrally Nonuniform Albedo*

Soil productivity management and plant growth in the Sahel: Potential of an aerial monitoring technique

Evidence of Saharan mineral aerosols transport to the Mediterranean inside well-defined layers

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