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  • 1
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 2
    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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  • 3
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The heat input into the planetary boundary layer (PBL) resulting from surface-atmosphere interactions under extremely arid conditions is formulated as a linear differential equation. The forcing for this heat input is the product of the shortwave (solar) absorption at the surface and the surface-to-PBL heat transfer efficiency, η. This efficiency is determined by five variables: the turbulent heat transfer coefficient, the soil heat conductance, the surface longwave emissivity, the surface temperature, and the fraction of the longwave flux from the surface absorbed within the PBL. The first two variables may vary by orders of magnitude, while the others vary much less. If a simplifying assumption is made that these variables and the thickness of the PBL do not vary with time, and that the shortwave absorption by the surface is given by a half-sine wave, then the PBL temperature cycle can be explicitly expressed (by exponential and trigonometric functions) as dependent on only two system parameters: (i) the system time constant and (ii) the transfer efficiency η divided by the thermal capacity of the PBL. The shape of this diurnal cycle depends solely on the system time constant, which is a simple function of the thermal capacity of the PBL, the PBL temperature, and the same variables that define η. For a small time constant, the peak PBL temperature will occur near noon, while for large values it will occur close to sunset. The amplitude of this diurnal cycle is proportional to the product of η and the peak (noon) shortwave absorption at the surface, and also depends very strongly on the system time constant. A concept of trans-absorptivity, that specifies the heat input into the PBL resulting from the shortwave absorption by the surface, is introduced and discussed in terms of the governing equations. The trans-absorptivity is given as the product of the surface absorptivity (the co-albedo) and the efficiency η. It is suggested that climatic effects of surface changes, such as removal of vegetation, should be formulated in terms of changes in the trans-absorptivity.
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  • 4
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The October rains (at the onset of the rainy season that extends to April) in southern Israel have steeply increased in the last quarter century relative to the prior two decades. A less pronounced, but appreciable, increase is noted for the rest of the rainy season. This apparent reversal of desertification is attributed here to land use changes. Afforestation, increased cultivation and limitations on grazing after the establishment of the State of Israel resulted in an increased vegetation cover over the inherently high-albedo soils in this region (an area of ∼104 km2). The changes are shown in a July 1985 Landsat image of the area. The increase in precipitation is specifically attributed to intensification of the dynamical processes of convection and advection resulting from plant-induced enhancement of thedaytime sensible heat flux from the generally dry surface. This enhancement results both from the reduced surface albedo and the reduced soil heat flux (reduced day-to-night heat storage in the soil) in October when insolation is strong. Stronger daytime convection can lead to penetration of the inversions capping the planetary boundary layer (which are weaker in October than in summer) while strengthened advection (sea breeze) can provide moist air from the warm Mediterranean Sea. This suggested mechanism is consistent with previous studies showing that the autumn rains in southern Israel exhibit convective mesoscale characteristics and occur predominantly in the daytime. However, other causes, such as a shift in the synoptic-scale circulation, cannot be ruled out at this stage.
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  • 5
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    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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  • 6
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The surface albedo and the surface roughness for forested areas with snow on the ground are expressed in terms of the tree silhouette parameter s, the projection on the vertical plane of trees per unit area. The absorption of insolation (direct solar beam) is quantitatively described for a horizontal snow surface with vertical tree trunks, stressing the role of the bark at snow level as triggering the snow melt. Measurement of s by field sampling in two forested sites in central Switzerland yielded values ranging from 1.8 to 2.1.
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  • 7
    ISSN: 1573-0956
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract This paper reviews the techniques and recent results of orbital remote sensing, with emphasis on Landsat and Skylab imagery. Landsat (formerly ERTS) uses electronic sensors (scanners and television) for repetitive observations with moderate ground resolution. The Skylab flights used a wider range of electro-optical sensors and returned film cameras with moderate and high ground resolution. Data from these programs have been used successfully in many fields. For mineral resources, satellite observations have proven valuable in geologic mapping and in exploration for metal, oil, and gas deposits, generally as a guide for other (conventional) techniques. Water resource monitoring with satellite data has included hydrologic mapping, soil moisture studies, and snow surveys. Marine resources have been studied, with applications in the fishing industry and in ocean transportation. Agricultural applications, benefiting from the repetitive coverage possible with satellites, have been especially promising. Crop inventories are being conducted, as well as inventories of timber and rangeland. Overgrazing has been monitored in several areas. Finally, environmental quality has also proven susceptible to orbital remote sensing; several types of water pollution have been successfully monitored. The effects of mining and other activities on the land can also be studied. The future of orbital remote sensing in global monitoring of the Earth's resources seems assured. However, efforts to extend spectral range, increase resolution, and solve cloud-cover problems must be continued. Broad applications of computer analysis techniques are vital to handle the immense amount of information produced by satellite sensors.
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  • 8
    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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  • 9
    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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  • 10
    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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