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  • 11
    Electronic Resource
    Electronic Resource
    Springer
    Boundary layer meteorology 64 (1993), S. 409-420 
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The conventional calculation of heat fluxes from a vegetated surface, involving the coefficient of turbulent heat transfer, which increases logarithmically with surface roughness (commonly taken as about 0.12 of the plant height), appears inappropriate for highly structured surfaces such as desertscrub or open forest. An approach is developed here for computing sensible heat flux from sparsely vegetated surfaces, where the absorption of insolation and the transfer of absorbed heat to the atmosphere are calculated separately for the plants and for the soil. This approach is applied to a desert-scrub surface for which the turbulent transfer coefficient of sensible heat flux from the plants is much larger than that from the soil below, as shown by an analysis of plant, soil and air temperatures measured in an animal exclosure in the northern Sinai. The plant density is expressed as the sum of products (plant-height) x (plant-diameter) of plants per unit horizontal surface area (the dimensionless silhouette parameter of Lettau). The solar heat absorbed by the plants is assumed to be transferred immediately to the airflow. The effective turbulent transfer coefficientk g-eff for sensible heat from the desert-scrub/soil surface computed under this assumption increases sharply with increasing solar zenith angle, as the plants absorb a greater fraction of the incoming irradiation. The surface absorptivity (the co-albedo) also increases sharply with increasing solar zenith angle, and thus the sensible heat flux from such complex surfaces (which include open forests) is a much broader function of time of day than when computed under constantk g-eff and constant albedo assumptions. The major role that desert-fringe plants play in the genesis of convection and advection cannot be evaluated properly in the conventional calculations.
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  • 12
    Electronic Resource
    Electronic Resource
    Springer
    Boundary layer meteorology 61 (1992), S. 81-97 
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Thermal emission is modeled from a canopy/soil surface, where the soil and the leaves are at different temperatures,T g andT c respectively. The temperatureT m corresponding to a radiometer reading is given by $$B_\lambda (T_m ) = \chi B_\lambda (T_g ) + (1 - \chi )B_\lambda (T_c ) ,$$ whereB λ denotes the Planck blackbody function at wavelength λ, χ specifies the fraction of the field of view occupied by the soil at a given view direction, and an emissivity of 1.0 is assumed for the plants and the soil. The dependence of the soil-fraction χ on the view direction and the structure is expressed by the viewing-geometry parameter, which allows for concise and simple formulation. We observe from our model that at large view zenith angles, only the plants are effectively seen (that is, χ tends to zero), and thereforeT c can be determined from observations at large zenith angles, to the extent that such observations are practical. Viewing from the zenith, χ = exp(-L hc), whereL hc is the projection of the canopy leaf-area (per unit surface area) on a horizontal plane. For off-zenith observations, the soil-fraction χ depends on the distribution in the azimuth of the projected areas of various leaf categories, in addition to the dependence on the sum total of these projections,L hc.L hc, rather than the leaf-area index, emerges as the parameter characterizing the optical thickness of the canopy. Inferring bothT c andT g from observations from the zenith and from large zenith angles is possible ifL hc is known from other measurements. Drooping of leaves under water-stress conditions affects the observed temperatureT m in a complicated way because a leaf-inclination change produces a change inL hc (for the same leaf area) and also a change in the dependence of χ on the view direction. Water stress can produce an increase of the soil-fraction χ and thus tends to produce an exaggerated increase in the observed temperature compared to the actual increase in canopy temperature. These effects are analyzed for a simulated soybean canopy.
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  • 13
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
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  • 14
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The thermal-infrared (longwave) emission from a vegetated terrain is generally anisotropic, i.e., the emission temperature varies with the view direction. If a directional measurement of temperature is considered to be equal to the effective temperature of the hemispheric emission, then the estimate of the latter can be significantly in error. The view-direction (zenith angleθ eq ) at which the emission equivalence does hold is determined in our modeling study. In a two-temperature field-of-view (soil and plants),θ eq falls in a narrow range depending on plant density and canopy architecture.θ eq does not depend on soil and (uniform) plant temperatures nor on their ratio, even though the pattern of emission vs. the view direction depends crucially on this ratio. For a sparse canopy represented as thin, vertical cylindrical stalks (or vertical blades uniformly distributed in azimuth) with horizontal facets,θ eq ranges from 48 to 53° depending on the optical density of the vertical elements alone. When plant elements are modeled as small spheres,θ eq lies between 53 to 57° (for the same values of the canopy optical density). Only for horizontal leaves (a truly planophile canopy) is the temperature measured from any direction equal to the temperature of the hemispheric emission. When the emission temperature changes with optical depth within the canopy at a specified rate,θ eq depends to some extent on that rate. For practically any sparsely vegetated surface, a directional measurement at the zenith angle of 50° offers an appropriate evaluation of the hemispheric emission, since the error in the estimate will, at most, only slightly exceed 1% (around 4 W m−2). Estimates of the hemispheric emission through a nadir measurement, on the other hand, can be in error in some cases by about 10%, i.e., on the order of 40 W m−2.
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  • 15
    Electronic Resource
    Electronic Resource
    Springer
    Climatic change 1 (1977), S. 137-155 
    ISSN: 1573-1480
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The impact of man and animal on the Earth's surface albedo (reflectivity), until recently believed to be quite small, or not considered at all, is analyzed. Discussion is presented of changes in the albedo due to the heat island effects of cities on snow cover, to agricultural cultivation, irrigation, and to overgrazing; the latter of which is emphasized. In arid climates, protected steppe areas have a low albedo due to dark plant debris accumulating on the crusted soil surface, whereas the same type of terrain, when overgrazed, exhibits a high albedo of trampled, crumbled soil. Extrapolating from observed spatial differences between overgrazed terrain and natural steppe, it is suggested that anthropogenic pressures mainly due to overgrazing could have had a very significant effect on the Earth's surface albedo both regionally and as a global average during the last few thousand years. The Earth's surface albedo presently might be 0.154 whereas it might have been 0.141 about 6000 years B.P. Thus, the surface albedo could have increased by Δa = 0.013, or by nearly 10% of its value when steppe areas were in their ‘virgin’ state. The hypothesized increase would be much larger in the Northern hemisphere than in the Southern. There are uncertainties even in the second digit of the suggested value for the present day albedo, and thus certainly for the albedo in the past. Seasonal mapping of the surface albedo from LANDSAT type satellites is recommended.
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  • 16
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical and applied climatology 40 (1989), S. 67-79 
    ISSN: 1434-4483
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Description / Table of Contents: Zusammenfassung Es wird der Einfluß der Vegetation am Rande der Wüste auf den tagsüber fühlbaren Wärmefluß untersucht. Die Rechnungen basieren auf einem früher entwickelten Modell für die Oberflächenalbedo der Pflanzen-/Erdoberfläche und auf Meßwerten für den Wärmefluß von vegetationslosen und bewachsenen Flächen. Wir finden, daß der fühlbare Wärmefluß eines abgezäunten Ökosystems in der Sinai um einen Faktor 1,5 bis 2,0 größer ist (abhängig vom Sonnenstand) als der Wärmefluß von einer unbewachsenen, abgegrasten Fläche außerhalb der Einzäunung. Die Beiträge zu diesem verstärkten Fluß kommen zu gleichen Teilen von der Reduktion der Albedo und des Wärmeflusses in den Erdboden. Mehrere Studien haben gezeigt, daß ein größerer Wärmefluß die Konvektion am Tage verstärkt, zu einer höheren Grenzschicht führt und daß deshalb ein verstärkter Wärmefluß eine Zunahme der Niederschlagswahrscheinlichkeit sogar in ausgetrockneten Gebieten bewirkt, wenn die Feuchtigkeit von außen durch Advektion zugeführt wird. Das Ergebnis dieser Untersuchung zeigt, daß die Vernichtung von Vegetation am Rande der Wüste zu einer Verringerung der Niederschläge führen kann und die Austrocknung gefördert wird.
    Notes: Summary The influence of desert-fringe vegetation on the daytime sensible heat flux is examined. The calculations are based on a previously developed surface albedo model for the plants/soil surface and the soil heat flux observational data for both bare and vegetated areas. It is found that the sensible heat flux from an ecosystem in a fenced-off area (exclosure) in the Sinai is larger than that from bare soil (overgrazed areas outside the exclosure) by a factor of 1.5 to 2.0, depending on the solar zenith angle. The contributions to this enhanced flux from the albedo reduction and the soil heat flux reduction are of the same magnitude. Various studies have established that a larger heat flux increases daytime convection and boundary layer growth, and thus an enhanced flux increases the probabilities for precipitation even in a parched region when moisture is advected from outside. The results of this investigation therefore suggest that removal of desert-fringe vegetation can reduce precipitation and promote drought.
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  • 17
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical and applied climatology 43 (1991), S. 3-16 
    ISSN: 1434-4483
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary The dependence of the albedo on illumination direction is analyzed by constructing a canopy model in which the individual leaves are planar, Lambertian reflectors. Leaf transmission is treated separately, and is assumed to be proportional to the cosine of the zenith angle of the leaf normal. Effects of shading and obscuration are formulated assuming random distribution of the leaves with respect to the viewing and illumination directions. Variants of the model with different azimuthal leaf distributions are created and discussed as explicit expressions of the viewing/illumination geometry and the canopy characteristics. The canopy spectral single-scattering albedo factors, which are the conditional probabilities of a photon escaping from the canopy after a first reflection or a first transmission, are obtained by numerical integration over a hemisphere of the bidirectional reflectance and transmittance factors. Our analysis identifies the ratio of the projection of leaf area on the vertical plane perpendicular to the principal plane to that on a horizontal plane as the parameter that controls the dependence of the albedo on the solar zenith angle. The albedo factor due to the leaf transmittance generally increases with the zenith angle of illuminating beam more sharply than that due to the leaf reflectance. Model variants with various azimuthal distributions are compared with measured albedo of soybeans. Second and higher order scatterings are accounted for in a simplified way. The degree to which a model variant fits the measured albedo and its change with the solar zenith angle depends both on the leaf inclination angleand the azimuthal distribution of the leaf area.
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  • 18
    ISSN: 1434-4483
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary  The relationship between European surface temperature and winds over the eastern North Atlantic are investigated for the years 1988 to 1997. Daily Special Sensor Microwave Imager SSM/I observations are used to evaluate a monthly surface wind index that quantifies the influence of southwesterly flow. Our wind index and the monthly-mean surface-air temperatures in late winter and early spring over France and northern-latitude Europe are highly correlated. In February, the year-to-year increases/decreases match every year for France (correlation of 0.82 with the Index); and every year with just one exception for Europe (correlation with the Index of 0.76 for a longitudinal strip through Europe 45–50° N, and 0.73 for the 50–60° N strip). In March, the increases/decreases of the wind Index and of the temperatures for France also match, but the correlation with the Index is lower, 0.65. The high correlation between our Index and the large interannual fluctuations in the monthly temperature in late winter and early spring indicate that the onset of the spring conditions in Europe is significantly influenced by the wind patterns over the eastern North Atlantic. Coinciding with the fluctuations from warm-Europe/high-Index winter to the opposite conditions, we observe “seesaw” effects, fluctuations over the North Atlantic, in opposite directions in the east (25–5° W), and the west (65–45° W). In the low-Index years we find that: (a) the surface-air temperatures in the west are appreciably higher than in the east (but slightly lower in the high-Index year), and (b) the difference between the 500 mb meridional wind in the west and that in the east is positive and large, exceeding 10 m s−1 (but it becomes negative and small in the high-Index years). The “seesaw” effects suggest that a positive feedback exits between these cross-Atlantic temperature differences and the surface winds.
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  • 19
    Publication Date: 1980-01-01
    Print ISSN: 0038-092X
    Electronic ISSN: 1471-1257
    Topics: Energy, Environment Protection, Nuclear Power Engineering , Physics
    Published by Elsevier
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  • 20
    Publication Date: 1995-04-01
    Print ISSN: 0006-8314
    Electronic ISSN: 1573-1472
    Topics: Geosciences , Physics
    Published by Springer
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