Statistical representation of mountain shading

International audienceShadows cast by the mountains themselves have a strong influence on the surface energy balance of mountainous regions. If the influence of shadows is to be included on sub-grid scales in a surface energy balance model, a statistical representation has to be used. Slope components calculated from digital elevation models of areas in North Wales and the French Alps are found to have double-exponential distributions. From this result, expressions are developed for the fractions of the areas that will be either self-shaded or shaded by remote topography as functions of solar elevation and time of day. These expressions are in good agreement with results from a terrain shading model. Keywords: solar radiation, topography, surface energy balance, statistical parameterisatio

John Godfrey Saxe: his place among American humorists

Thesis (M.A.)--Boston University, 1936. This item was digitized by the Internet Archive

Representing moisture fluxes and phase changes in glacier debris cover using a reservoir approach

Due to the complexity of treating moisture in supraglacial debris, surface energy balance models to date have neglected moisture infiltration and phase changes in the debris layer. The latent heat flux (QL) is also often excluded due to the uncertainty in determining the surface vapour pressure. To quantify the importance of moisture on the surface energy and climatic mass balance (CMB) of debris-covered glaciers, we developed a simple reservoir parameterization for the debris ice and water content, as well as an estimation of the latent heat flux. The parameterization was incorporated into a CMB model adapted for debris-covered glaciers. We present the results of two point simulations, using both our new “moist” and the conventional “dry” approaches, on the Miage Glacier, Italy, during summer 2008 and fall 2011. The former year coincides with available in situ glaciological and meteorological measurements, including the first eddy-covariance measurements of the turbulent fluxes over supraglacial debris, while the latter contains two refreeze events that permit evaluation of the influence of phase changes. The simulations demonstrate a clear influence of moisture on the glacier energy and mass-balance dynamics. When water and ice are considered, heat transmission to the underlying glacier ice is lower, as the effective thermal diffusivity of the saturated debris layers is reduced by increases in both the density and the specific heat capacity of the layers. In combination with surface heat extraction by QL, subdebris ice melt is reduced by 3.1% in 2008 and by 7.0% in 2011 when moisture effects are included. However, the influence of the parameterization on the total accumulated mass balance varies seasonally. In summer 2008, mass loss due to surface vapour fluxes more than compensates for the reduction in ice melt, such that the total ablation increases by 4.0 %. Conversely, in fall 2011, the modulation of basal debris temperature by debris ice results in a decrease in total ablation of 2.1 %. Although the parameterization is a simplified representation of the moist physics of glacier debris, it is a novel attempt at including moisture in a numerical model of debris-covered glaciers and one that opens up additional avenues for future research

Modelled sensitivity of the snow regime to topography, shrub fraction and shrub height

Recent studies show that shrubs are colonizing higher latitudes and altitudes in the Arctic. Shrubs affect the wind transport, accumulation and melt of snow, but there have been few sensitivity studies of how shrub expansion might affect snowmelt rates and timing. Here, a three-source energy balance model (3SOM), which calculates vertical and horizontal energy fluxes – thus allowing within-cell advection – between the atmosphere, snow, snow-free ground and vegetation, is introduced. The three-source structure was specifically adopted to investigate shrub–tundra processes associated with patchy snow cover that single- or two-source models fail to address. The ability of the model to simulate the snow regime of an upland tundra valley is evaluated; a blowing snow transport and sublimation model is used to simulate premelt snow distributions and 3SOM is used to simulate melt. Some success at simulating turbulent fluxes in point simulations and broad spatial pattern in distributed runs is shown even if the lack of advection between cells causes melt rates to be underestimated. The models are then used to investigate the sensitivity of the snow regime in the valley to varying shrub cover and topography. Results show that, for domain average shrub fractional cover ≤0.4, topography dominates the pre- and early melt energy budget but has little influence for higher shrub cover. The increase in domain average sensible heat fluxes and net radiation with increasing shrub cover is more marked without topography where shrubs introduce wind-induced spatial variability of snow and snow-free patches. As snowmelt evolves, differences in the energy budget between simulations with and without topography remain relatively constant and are independent of shrub cover. These results suggest that, to avoid overestimating the effect of shrub expansion on the energy budget of the Arctic, future large-scale investigations should consider wind redistribution of snow, shrub bending and emergence, and sub-grid topography as they affect the variability of snow cover

Hidraul : suatu penyelesaian

Buku ini mengandungi beberapa bab iaitu kandungan: prakata, catatan sejarah, bab 1 ciri fizik bendalir, pendahuluan, takrifan, unit, bab 2 statik bendalir, perubahan keamatan tekanan, pengukuran tekanan, daya pada satah dan permukaan melengkung, ketimbulan, bab 3 hukum aliran bendalir, pengelasan jenis aliran, bab 4 penggunaan hukum aliran bendalir, garis tenaga dan garis gred hidraul, meter venturi, tiub pitot, bab 5 analisis bermatra, pengenalan, kaedah rayleigh, kaedah buckingham–p, bab 6 aliran dalam paip, persamaan darcy dan chezy, aliran lamina dalam paip, pengenalan kepada aliran lapisan sempadan dan gelora, bab 7 aliran seragam di saluran, seksyen ekonomis, tenaga tentu, aliran melewati suatu bonjolan di dasar, bab 8 aliran berubah bertahap di saluran, analisis aliran berubah bertahap, pengelasan lengkung air balik, analisis susuk dan titik kawalan, bab 9 model hidraul permukaan terbuka, hukum pengawalan model, penentuan skala model apabila kesasaran diambil kira, penentuan skala apabila pencetekan diambil kira, bab 10 aliran unggul boleh mampatpersamaan tenaga, halaju bunyi, tekanan dan suhu genangan, bab 11 aliran unggul dan nyata, mekanik pemisahan aliran, geseran dan hela seretan, edaran dan lif, bab 12 paluan air, penutupan injap seketika, penentuan tekanan lebih serta penutupan injap cepat dan perlahan