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Turbulence Characteristics of the Stable Boundary Layer Over a Mid-Latitude Glacier. Part I: A Combination of Katabatic and Large-Scale Forcing (1998)

Smeets, C. J. P. P. ; Duynkerke, P. G. ; Vugts, H. F.

Springer

Boundary layer meteorology 87 (1998), S. 117-145

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ISSN: 1573-1472

Keywords: Stable boundary layer ; Katabatic flow ; Non-stationary flow ; Spectra ; Roughness length ; Glacio-meteorology

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract Profile and eddy-correlation (heights of 4 and 10 m) measurements performed on the Pasterze glacier (Austria) are used to study the characteristics of the stable boundary layer under conditions of katabatic and large-scale forcing. We consider cases where large-scale forcing results in a downslope (or following) ambient wind. The analysis of averaged spectra and cospectra reveals low frequency perturbations that have a large influence on the variances of temperature and horizontal wind components and also alter the cospectra of momentum and sensible heat flux. Only the spectrum of the vertical wind speed is comparable to universal spectra. The low frequency perturbations occur as brief intermittent events and result in downward entrainment of ambient air thereby producing enhanced downward sensible heat fluxes and downward as well as upward momentum fluxes with various magnitudes and timescales. After the variances were high pass filtered, the normalised standard deviations of wind speed and temperature compare favourably to findings in the literature within the range 0〉z/L〉0.5. For larger z/L they deviate as a result of an increased influence from low frequency perturbations and thus non-stationarity. In line with this, the turbulent kinetic energy budget (at 4 m height) indicates that production (shear) is in balance with destruction (buoyancy and dissipation) within the range 0〉z/L〉0.3. Non-dimensional gradients of wind speed within the range 0〉z/L〉0.3 have a slope of about 3.5. The scatter for the dimensionless temperature gradient is quite large, and the slope is comparable to that for wind speed gradients. For z/L〉0.3 the imbalance in the turbulent kinetic energy budget grows and non-dimensional gradients for wind speed and temperature deviate considerably from accepted values as a result of increased non-stationarity. Average roughness lengths for momentum and sensible heat flux derived from wind speed and temperature profiles are respectively 1 × 10-3 m and 6 × 10-5 m, consistent with the literature. The ratio (z0h/z0m) compares to those predicted by surface renewal models. A variation of this ratio with the roughness Reynolds number is not indicated by our data.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1000860406093

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Observed Wind Profiles and Turbulence Fluxes over an ice Surface with Changing Surface Roughness (1999)

Smeets, C. J. P. P. ; Duynkerke, P. G. ; Vugts, H. F.

Springer

Boundary layer meteorology 92 (1999), S. 99-121

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ISSN: 1573-1472

Keywords: Displacement height ; Ice surface roughness ; Katabatic flow ; Stable boundary layer ; Turbulence

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract Wind profile and eddy-correlation data obtained at two sites on a melting glacier surface in Iceland during the summer of 1996 are presented. Throughout the experiment the surface roughness increased rapidly from smooth to very rough, with the largest roughness element height obtained being about 1.7 m. In a layer close to the rough surface we find that the wind speed profiles were disturbed showing horizontal inhomogeneities as in a roughness sublayer. Its height was approximately two times the height of the main roughness elements (h) at both sites throughout the experiment. From the wind profiles and eddy-correlation data we calculated corrections for the displaced zero plane as a function of time and compared these with results obtained from a drag partitioning model. In general, the agreement was reasonable considering the ranges of uncertainty but the results indicate that the increasing horizontal anisotropy of the surface probably limits the use of the model. The values obtained for the roughness lengths are in good agreement with those calculated from a simple linear model, i.e., z0/h = 0.5λ with λ the frontal area index. Above the roughness sublayer the wind profiles, normalised standard deviations of wind speed, and the balance of the turbulence kinetic energy budget behaved as over an ideal homogeneous surface thereby confirming similarity of the flow.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1001899015849

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