Thermal Modification of Air Moving Over Melting Snow Surfaces

Takahara, Hiroshi; Hlguchi, Keiji

doi:10.1017/s0260305500010442

Cited by 13 publications

(9 citation statements)

References 6 publications

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“…The values for the exponents provided in Equations (5a)-(5c) are much smaller than those reported previously in the literature for the wind field. However, they do correspond to the internal boundary-layer height that can be inferred from the results of Takahara and Higuchi (1985). For the stated upwind roughness (0Ð006 m), Takahara and Higuchi (1985: figures 3 and 2) show that, for fetches of 9 m and 48 m, the internal boundary layer over snow has grown to heights of 0Ð6 m and approximately 2Ð0 m respectively.…”

Section: Equipment and Experimental Proceduresmentioning

confidence: 88%

“…Field observations of the internal boundary-layer growth over snow patches have been extremely rare. Takahara and Higuchi (1985) provided a very insightful experiment on the thermal modification of the air over melting snow. Their study focused on the calculation of melt; however, although they demonstrated the development of the internal boundary layer, they did not provide an analysis of its rate of growth.…”

Section: Introductionmentioning

confidence: 99%

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Boundary‐layer growth over snow and soil patches: field observations

Granger

Essery

Pomeroy

2006

Hydrological Processes

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Abstract:Much of the snowmelt season is characterized by a patchy surface; differential heating of the snow and snow-free surfaces results in a significant horizontal transport of energy that affects and contributes to the snowmelt. The calculation of the rate of energy advection requires some knowledge of the behaviour of the thermal boundary layer over the patches of snow and snow-free surfaces. We present the results from a series of field observations of the rate of growth of the thermal boundary layer over snow and snow-free patches. The results confirm that the boundary-layer growth can be described by a power function of the distance from the leading edge of the patch. For the case of the thermal boundary layer over a snow patch within a bare field, the boundary-layer growth is affected by the upwind surface roughness; the thermal boundary layer over a snow patch within a 'rough' field grows much more quickly than that in a 'smooth' field. KEY WORDS boundary layer; advection; snowmelt; sensible heat; snow patches INTRODUCTION In the calculation of the melting of a patchy snow cover, the energy advected from the adjacent bare soil to the snow surface is an important consideration. As the lower-layer air moves from the bare soil to the snow, it undergoes a considerable modification as energy is extracted. This additional flux of energy to the snow surface cannot be reliably calculated using traditional boundary-layer flux-profile relationships, since these are based on the assumption of a constant flux layer. To date, only a very few attempts have been made to provide estimates of this advective flux over a field. Shook et al. (1993) found that snow patch geometry and size frequency distributions could be described using fractal geometry. Synthetic snow-cover spatial distributions were created using the fractal sum of pulses method and in a gridded calculation; an approximation of Weisman's (1977) model of advection over snow was applied to estimate the areal melt (Shook, 1995; Shook and Gray, 1997). Liston (1995) applied a numerical atmospheric boundary-layer model, based on higher order turbulence closure assumptions, to simulate local advection during the melt of a hypothetical snow cover with simple geometrical properties; he used the model to demonstrate the increase in available melt energy as the snow cover is depleted. Essery (1999) also applied a modified boundary-layer model to assess the advection of energy with varying snow cover fraction.The boundary-layer analyses described by Weisman (1977) and Liston (1995) require the solution of a very complex set of equations describing the conservation of momentum, mass and energy, as well as the changes in atmospheric stability as a boundary layer develops. Weisman reduced his results to a simple parametric form; however, he was only able to provide a few sample values of the coefficients required to apply his

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Section: Equipment and Experimental Proceduresmentioning

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Boundary‐layer growth over snow and soil patches: field observations

Granger

Essery

Pomeroy

2006

Hydrological Processes

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“…Once the snow cover is patchy, thermal internal boundary layers (e.g. Takahara and Higuchi, 1985;Marsh and Pomeroy, 1996) develop above the heterogeneous surface. As a result, local advection of warm air from adjacent bare ground to the snow surface provides an additional source of energy contributing to snow ablation (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…Measurements of either the effect of local advection of heat or the growth of internal boundary-layers above snow patches are extremely rare, except for the studies of Takahara and Higuchi (1985) and Granger et al (2006). Essery et al (2006) applied boundary layer principles and estimated the additional sensible heat flux to melting snow.…”

Section: Introductionmentioning

confidence: 99%

Micrometeorological processes driving snow ablation in an Alpine catchment

Mott¹,

Egli²,

Grünewald³

et al. 2011

The Cryosphere

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Abstract. Mountain snow covers typically become patchy over the course of a melting season. The snow pattern during melt is mainly governed by the end of winter snow depth distribution and the local energy balance. The objective of this study is to investigate micro-meteorological processes driving snow ablation in an Alpine catchment. For this purpose we combine a meteorological boundary-layer model (Advanced Regional Prediction System) with a fully distributed energy balance model (Alpine3D). Turbulent fluxes above melting snow are further investigated by using data from eddy-correlation systems. We compare modeled snow ablation to measured ablation rates as obtained from a series of Terrestrial Laser Scanning campaigns covering a complete ablation season. The measured ablation rates indicate that the advection of sensible heat causes locally increased ablation rates at the upwind edges of the snow patches. The effect, however, appears to be active over rather short distances of about 4-6 m. Measurements suggest that mean wind velocities of about 5 m s −1 are required for advective heat transport to increase snow ablation over a long fetch distance of about 20 m. Neglecting this effect, the model is able to capture the mean ablation rates for early ablation periods but strongly overestimates snow ablation once the fraction of snow coverage is below a critical value of approximately 0.6. While radiation dominates snow ablation early in the season, the turbulent flux contribution becomes important late in the season. Simulation results indicate that the air temperatures appear to overestimate the local air temperature above snow patches once the snow coverage is low. Measured turbulent fluxes support these findings by suggesting a stable internal boundary layer close to the snow surface causing a strong decrease of the sensible heat flux towards the snow cover.Correspondence to: R. Mott (mott@slf.ch) Thus, the existence of a stable internal boundary layer above a patchy snow cover exerts a dominant control on the timing and magnitude of snow ablation for patchy snow covers.

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“…An internal boundary layer (IBL), in which air temperatures and heat fluxes differ from those at the upwind edge of the snow patch, develops and grows in depth with downwind distance. Vertical temperature profiles and IBL growth rates have been measured over snow patches by Takahara and Higuchi (1985) and Granger et al (2006).…”

Section: Introductionmentioning

confidence: 99%

Boundary‐layer growth and advection of heat over snow and soil patches: modelling and parameterization

Essery

Granger

Pomeroy

2006

Hydrological Processes

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Abstract:Melting snow is generally patchy; upward sensible heat fluxes from patches of snow-free ground warm the air and contribute energy for snowmelt. A simple model is presented for advection of heat over partial snow covers and compared with measurements of temperature profiles over snow and snow-free ground. Approximations for flux and temperature profiles in the internal boundary layers over snow patches are used to develop parameterizations for local and average surface fluxes into the snow. In comparison with results from the advection model for regular patterns of alternating snow patches and snow-free ground, a tile model is found to give a good parameterization for average heat fluxes over the whole surface, but it does not match the local fluxes into snow and snow-free ground separately. An extended tile model that gives better results is developed from the flux profile parameterization. For complex snowcover patterns with a fractal distribution of patch sizes, average fluxes are found to be close to those obtained for a regular pattern with an effective patch size linearly related to the average patch size of the complex pattern.

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Thermal Modification of Air Moving Over Melting Snow Surfaces

Cited by 13 publications

References 6 publications

Boundary‐layer growth over snow and soil patches: field observations

Boundary‐layer growth over snow and soil patches: field observations

Micrometeorological processes driving snow ablation in an Alpine catchment

Boundary‐layer growth and advection of heat over snow and soil patches: modelling and parameterization

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