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  • 1
    Electronic Resource
    Electronic Resource
    Atmospheric Response To Spatial Variations In Concentration And Size Of Polynyas In The Southern Ocean Sea-Ice Zone (2000)
    Springer
    Boundary layer meteorology 94 (2000), S. 65-88 
    Details
    ISSN: 1573-1472
    Keywords: Boundary layer ; Numerical modelling ; Sea ice ; Multiple polynyas ; Heat flux
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Although it is well known that sea-ice regions are important components of the Earth's climate system, the exchanges of energy between ocean, ice and atmosphere are not well understood. The majority of past observational and modelling studies of atmosphere-surface interactions over sea-ice regions were primarily concerned with airflow over a single, isolated area of open water. The more realistic situations of multiple polynyas within a sea-ice field and different areal concentrations of sea ice were studied here. Spatial structure of the atmospheric boundary layer in response to this surface was simulated using a high-resolution numerical model. A sea-ice concentration of 80%, typical of the Southern Ocean sea-ice zone, was maintained within a 100-km wide domain. The effects of three polynya characteristics were assessed: their horizontal extent; local concentration of sea ice (LCI); and their arrangement with ice floes. Over polynyas of all sizes distinct plumes of upward heat flux, their width and height closely linked to polynya width, resulted in mixed layers 600 to 1000 m deep over and downwind of the polynyas, their depth increasing with polynya width. Mean surface heat flux (MSHF) increased with size in polynyas less than 30 km wide. The air-to-ice MSHF over the first 10 km of sea-ice downwind of each polynya and the domain-average surface heat flux increased linearly with polynya width. Turbulent kinetic energy plumes occurred over all polynyas, their heights and widths increasing with polynya widths. Downward flux of high momentum air in the plumes caused increased wind speeds over polynyas in the layer from about 300–1000 m above the surface, the depth varying directly with polynya width. MSHFs decreased as LCIs increased. The arrangement of polynyas had relatively little effect on the overall depth of the modified layer but did influence the magnitude and spatial structure of vertical heat transfer. In the two-polynya case the MSHF over the polynyas was larger when they were closer together. Although the MSHF over the sea ice between the polynyas decreased in magnitude as their separation increased, the percentage of the polynya-to-air heat recaptured by this ice floe increased fivefold.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1002442212593
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  • 2
    Electronic Resource
    Electronic Resource
    Orographic and stability effects on valley-side drainage flows (1995)
    Springer
    Boundary layer meteorology 75 (1995), S. 403-428 
    Details
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The effects of orography and stability on valley-side drainage winds were investigated with the aid of a numerical model. The model is three-dimensional, non-hydrostatic, cast in terrain-following co-ordinates, has a surface energy budget and a 1.5 order TKE closure scheme. Experiments were conducted over a schematic three-dimensional valley to assess the influences on airflow of valley-side slope magnitude, valley cross-section shape, tilt of the valley floor and stability. In drainage flow, magnitudes of horizontal and vertical velocities and heights of their maxima are directly related to slope angle. The velocities are either insensitive to, or slightly inversely related to stability. The cooling which drives the flows is strongest over steep slopes and in large stabilities. The depth of the cooled layer, whilst increasing over steeper slopes, is inversely related to the stability. TKE increases with slope angle and decreases with increasing stability. In the downslope direction, the near-surface cooled layer significantly increases whereas the inversion intensity decreases by about 20%. These two features are due to mixing between the drainage flow and the overlying air. Tha drainage flow accelerates down the slope until it reaches the accumulated pool of cold air in the valley bottom, whereupon it slows down markedly and is accompanied by uplift over the centre of the valley. The cross-valley circulation is influenced by valley-side slope angle, valley cross-section shape and tilt of the valley floor, in addition to the effects of stability. For a given shape, the circulation is a direct function of the valley-side slope and an inverse function of the ambient stability. This relationship is described mathematically.V-shaped valleys generate stronger flows than doU-shaped valleys and a tilted valley floor also leads to a significant increase in velocities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00712271
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  • 3
    Electronic Resource
    Electronic Resource
    Stability and wind shear effects on meso-scale cellular convection (1995)
    Springer
    Boundary layer meteorology 75 (1995), S. 263-285 
    Details
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A three-dimensional numerical model has been used to assess the effects of vertical stability and wind shear on the nature and form of meso-scale cellular convection (MCC). The model was shown to be capable of simulating a real occasion of MCC before it was used in idealised cases. These cases revealed different regimes in MCC: open cells, longitudinal bands and closed cells/transverse bands. Open cells were favoured by the existence of instability in the surface layer and a lack of wind shear in the Ekman layer. Longitudinal bands were favoured by similar conditions in the surface layer plus wind shear in the Ekman layer. A near-neutral surface layer favoured the occurrence of closed cells/transverse bands. The depth of convection in the longitudinal bands was a function of the stability in both the surface and Ekman layers and of the wind shear in the Ekman layer. The regimes are related to the instability and shear through bulk Richardson numbers in the surface and Ekman layers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00712697
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  • 4
    Electronic Resource
    Electronic Resource
    Transition Regimes in Valley Airflows (1999)
    Springer
    Boundary layer meteorology 91 (1999), S. 385-411 
    Details
    ISSN: 1573-1472
    Keywords: Boundary layer ; Slope airflow ; Numerical model ; Transition ; Valley airflow
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A three-dimensional, non-hydrostatic model was used to examine the dynamical characteristics of morning and evening transition periods in the atmosphere over four idealised valleys. The simulations provided detailed structure over full diurnal cycles of the valley-wind system. An essentially two-dimensional simulation (Case 1) clearly showed valley-side slope flows, driven by pressure gradients and modulated by vertical diffusion and Coriolis effects. The rotation of the wind was clockwise on both valley sides, contrary to most observations in nature. Three-dimensional simulations (Cases 2–4) rectified this feature and that for Case 4 satisfactorily modelled the valley-plain wind system throughout the diurnal cycle. Three types of transition were identified with the aid of different tools: hodographs; space-time evolution of the wind fields; and the evolution of the forcing terms in the momentum and temperature equations. Whichever type or Case was considered, the evening transition was longer than the morning one and the along-valley transition followed the along-slope one. In Cases 1 and 4 the evening transition started up to 2 h before sunset and the morning transition started up to 2.5 h after sunrise. In the three-dimensional cases the evening transition began at about 1700 and ended at about 2400, starting at the bottom of the valley and propagating up both valley sides, but at different speeds. It also started at the ground and propagated vertically. The morning transition began at about 0900 and ended at about 1100, also starting at the bottom of the valley and propagating both vertically and up the valley sides, albeit with different regimes on the two sides. The along-valley transition lagged that on the slopes by about 1.5 h. In Case 1 the forcing terms were dominated by the pressure gradient and the vertical diffusion, with the Coriolis effects introducing an along-valley component to the slope flows. The three dimensional cases were more complex, with not only the addition of the effects of advection and horizontal diffusion but also more temporal variation of more of the forcings than in Case 1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1001846005338
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Orographic and stability effects on day-time, valley-side slope flows (1994)
    Springer
    Boundary layer meteorology 68 (1994), S. 275-300 
    Details
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The effects of orography and initial stability upon the magnitude and configuration of daytime, valley-side slope flows were investigated. A three-dimensional, time-dependent, non-hydrostatic numerical model provided simulations over a range of idealised valley forms for a range of vertical stabilities. The model's short-wave radiation scheme was improved and the runs were for a virtually dry atmosphere. Airflow over the valley is influenced by two distinct stability regimes, separated by a sharp threshold value of 0.37°C km−1. At lower stabilities, flow is strong and predominantly downward. Above the threshold, uplift occurs for all stabilities, decreasing in magnitude with increasing stability. Cross-valley flow increases in the stability range 0.06°C–0.6°C/100 m and decreases at higher stabilities. For a given stability above the threshold value, vertical velocities are directly related to slope angle. Horizontal velocities increase with slope at low angles but there is a suggestion that they decrease with increasing slope angle at high angles. The effect of valley half-width is much smaller than that of slope; greater valley width leads to a weaker cross-valley circulation. Conditions for the development of valley-slope flow configuration in harmony with the underlying orography are derived. A quantitative relationship between the magnitude of the average flow and the average slope and the initial stability is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00705601
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    Paper (German National Licenses)
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