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  • 1
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The problem of generating realistic boundary values of temperature and humidity in mesoscale models is discussed as well as the importance of allowing for non-linear interactions between the state of the surface and the atmosphere. An energy balance scheme, based on a bulk parameterisation of vegetation and simple prognostic equations for temperature and humidity of the soil interface, is presented. It is implemented in a hydrostatic, higher order closure model for the meso-γ-scale. Some tentative results are presented and critically evaluated. The presented parameterisation is very simple in its description of the interaction between plant and atmosphere, but shows the importance of such effects. Regarded as a building block in a mesoscale model, however, it is concluded that the present scheme is rather complicated. It is concluded that, although the results presented are realistic and show the importance of variable soil and vegetation types, it is highly doubtful if the presented parameterisation is useful in a wider perspective. Some of the parameterisations are less well founded from a physical point of view, and even if it were not so, the modeller will be hard put to specify all specifiable parameters and initialize all variables for any arbitrary area and event. It is thus concluded that this problem may still have to be dealt with in a simpler fashion. The question is raised if it should not be possible to obtain comparatively good results, similar to those in this paper, by simpler means.
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  • 2
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Daily and zonal (latitudinal belt) averages of heat and momentum fluxes were computed using bulk aerodynamic formulae, from the meteorological parameters measured onboard ‘M. S. Thuleland’ during the sixth Indian scientific expedition to Antarctica (26th November, 1986 to 22nd March, 1987). Both estimates showed significant variations, the momentum flux showing the largest variation. The maximum values of sensible and latent heat fluxes were observed over the 30°–40° S and 10°–20° S zones during the southern summer and fall respectively while the minimum values of latent heat flux were observed in the 60°–70° S zone for both seasons. The sensible heat flux minimum was observed in the 50°°60° S and 60°–70° S zones for summer and fall, respectively. Higher momentum flux values over the 40°–50° S zone in summer shifted to the 50°–60° S zone during fall.
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  • 3
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A diagnostic deposition model based on generally accepted micrometeorological ideas on the transfer of momentum, sensible heat and matter near the Earth's surface is presented. The parameterization of fluxes is based on the flux-gradient relationships in the turbulent region of the surface layer and the sublayer Stanton number as well as the Reynolds analogy between concentration, temperature and wind velocity distributions in the underlying sublayer. The model requires only vertical profile data of wind velocity, dry- and wet-bulb temperatures and trace gas concentrations from the turbulent part of the surface layer. The method has been applied to vertical profile data collected in field experiments such as the GREIV I 1974 project and the Great Plains Turbulence Project. In order to illustrate the way in which the model can be used to evaluate deposition fluxes and velocities of reactive trace gases, it has been applied to observed concentrations of NO, NO2 and ozone.
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  • 4
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A logarithmic + polynomial approximation is proposed for the vertical temperature profile in a neutrally or stably stratified planetary boundary layer (PBL) in conditions of quasi-stationarity. Using this approximation with the asymptotic logarithmic + linear law of the Monin-Obukhov similarity theory for the near-surface layer and with the Zilitinkevich formula for the PBL thickness allows one to derive an analytical expression for the function C in the heat transfer law, which permits simple parameterization of the thermal interaction between the atmosphere and the underlying medium in terms of external parameters, such as the geostrophic wind velocity and the temperature difference across the PBL.
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  • 5
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Wind speed was measured at a height of 1 cm above the ground and at several other heights in and above a canopy of tall fescue grass (Festuca arundinacea) using single hot-wire and triple hot-film anemometers. The plant area density in the canopy was concentrated close to the ground, with ∼75% of the plant area standing belowz=15 cm, wherez is height above the ground. The frequency distributions of horizontal wind speeds,s, were sharply skewed towards positive values at all measurement heights, but were most highly skewed near the ground where the coefficient of skewness ranged from 1.6 to 2.9. Above mid-canopy height, the frequency distribution ofs was described reasonably well by a Gumbel extreme value distribution. Average wind speed,S, decreased exponentially with depth into the canopy with an exponential scale length of abouth/2.8, whereh is the height of the canopy. Atz=1 cm, the value ofS was about 11% of the surface-layeru *. The standard deviation of the fluctuations of the vertical and horizontal components of the wind speed also decreased exponentially with depth inside the canopy with a scale length of abouth/2.5. Inside the canopy, the Eulerian integral time scales for the vertical (Γ w ) and horizontal (Γ u ) components of wind speed were about 0.1 s and 1.0 s, respectively, and were approximately constant with height. Above the canopy, these time scales increased sharply and, atz=2.25h, Γ w and Γ u were approximately 1.0 and 3.0s, respectively. Turbulence length scales in the vertical and downwind directions, Γ u and Γ w ·U, respectively, were approximately 1 cm for heights between 1 to 10 cm above the ground inside the canopy, while atz=2.25h, they were about 55 cm and 277 cm. Relatively quiescent periods (lulls) in the air close to the ground were interrupted frequently by gusts. The frequency of occurrence of gusts appears to be correlated with the value of the local shear near the top of the canopy.
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  • 6
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Mean wind velocity profiles were measured by means of radio-windsondes over the Landes region in southwestern France, which consists primarily of pine forests with scattered villages and clearings with various crops. Analysis of neutral profiles indicated the existence of a logarithmic layer between approximately z − d 0 = 67(±18)z 0 and 128(+-32)z 0 (z is the height above the ground, z 0 the surface roughness and d 0 the displacement height). The upper limit can also be given as z − d 0 = 0.33 (±0.18)h, where h is the height of the bottom of the inversion. The profiles showed that the surface roughness of this terrain is around 1.2 m and the displacement height 6.0 m. Shear stresses derived from the profiles were in good agreement with those obtained just above the forest canopy at a nearby location with the eddy correlation method by a team from the Institute of Hydrology (Wallingford, England).
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  • 7
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Horizontal turbulence measurements obtained from 22 wind sensors located on 9 towers in a mountainous coastal area are described and categorized by stability and terrain. Vector wind time series are high-pass filtered, and lateral and longitudinal wind speed variance is calculated for averaging times ranging from 15 s to 2 h. Parameterizations of the functional dependence of variance on averaging time are discussed, and a modification of Panofsky's (1988) uniform terrain technique applicable to complex terrain is presented. The parameterization is applied to the data and shown to be more realistic than a less complicated power law technique. The parameter values are shown to be different than the flat terrain cases of Kaimal et al. (1972), and are primarily a function of sensor location within the complex terrain. The parameters are also examined in terms of their dependence upon season, stability, marine boundary-layer height, and measurement height.
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  • 8
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The air pollution meteorology of a typical sea breeze day is investigated using the Colorado State University Mcsoscale Model. Results are qualitatively compared with observations and reveal a complex wind field characterised by migratory sea breeze convergence zones. Associated with these features, the model predicts enhanced upward vertical velocities and ‘doming’ of the planetary boundary layer (PBL). The diurnal variation in PBL depth is shown to vary markedly at different locations and is dependent on position in relation to the migratory convergence zones. These complex spatial and temporal variations in the wind and PBL depth have important implications for air quality in Auckland and confirm that simple Gaussian or box trajectory approaches are inappropriate for air quality assessment in such environments. The inclusion in the model of variable surface properties, a dynamic synoptic state and improved PBL parameterisations, as well as coupling with a Lagrangian particle model, are recommended if the model is to be used as a tool for further air quality studies in the Auckland area.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Boundary layer meteorology 49 (1989), S. 77-98 
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A numerical case study with a second-order turbulence closure model is proposed to study the role of urban canopy layer (UCL) for the formation of the nocturnal urban boundary layer (UBL). The turbulent diffusion coefficient was determined from an algebraic stress model. The concept of urban building surface area density is proposed to represent the UCL. Calculated results were also compared with field observation data. The height of the elevated inversion above an urban center was simulated and found to be approximately twice the average building height. The turbulent kinetic energy k, energy dissipation rate ε, and turbulence intensities 〈u 2〉 and 〈w 2〉 increase rapidly at the upwind edge of the urban area. The Reynolds stress 〈uw〉 displayed a nearly uniform profile inside the UBL, and the vertical sensible heat flux 〈wθ〉 had a negative value at the inversion base height. This indicates that the downward transport of sensible heat from the inversion base may play an important role in the formation of the nocturnal UBL.
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  • 10
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Atmospheric stability classes were determined by two atmospheric stability classification methods for the Taipei municipality in this study. In addition, meteorological conditions associated with neutral stability were investigated for the causes of the high frequency of cases of neutral stability. More than 80% of the occurrences in the neutral stability class were associated with the high cloud amounts according to the two methods.
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