Simulation of surface-hoar layers for snow-cover models

Abstract

During two winters, typical meteorological conditions (temperature, humidity, wind, radiation) and snow-surface conditions (snow surface and snow temperatures) were measured to simulate the formation and ablation processes (mainly sublimation) of surface hoar on two level snow plots, situated at 2500 and 1500 m a.s.l. In order to verify the simulated deposition/ablation rates, the surface-hoar mass, thickness and occasionally density were also measured. The evaluation shows that, using the aerodynamic bulk method for the half-hourly simulation periods, >90% of the day/night periods could be rated as either hoar-formation or ablation periods. The simulated sublimation rates (deposition/ablation) deviate in the mean by not more than 10% from the measured amounts! However, some larger deviations are present, mainly during ablation periods with heavy melting. Finally a method is shown for transforming net deposited vapour amounts into surface-hoar layers of a corresponding height in order to produce “layers” which might be integrated into snow-cover simulation models.

Numerical simulation of extreme snowmelt observed at the SIGMA-A site, northwest Greenland, during summer 2012

“…Vertical axis (relative snow depth) denotes the height above the bottom thick ice layer. Characters and colors indicating snow grain shape follow the definition by Fierz et al (2009). In sequence from the left they denote precipitation particles, decomposing and fragmented precipitation particles, rounded grains, faceted crystals, depth hoar, surface hoar, melt forms, and ice layer.…”

Spatial distribution of surface hoar crystals in sparse forests

“…This explains the success of bulk transfer models (Colbeck, 1988;Hachikubo and Akitaya, 1997;Höller, 1998;Föhn, 2001;Lehning et al, 2002): wind can provide moisture to the snow surface via bulk transfer. However, humidity does not always serve as the main determining factor.…”

“…Many factors affect surface hoar growth, from wind (Hachikubo and Akitaya, 1997;Hachikubo, 2001;Föhn, 2001), to radiation balance (Cooperstein, 2008), to air-snow temperature gradients (Lang et al, 1984;Hachikubo and Akitaya, 1997), to humidity (Feick et al, 2007). At the most simple physics level, one might say surface hoar grows when we have cold ice, warm air, and lots of water vapour.…”

“…Some studies found light wind to be beneficial to surface hoar growth (Föhn, 2001;Hachikubo and Akitaya, 1997;Colbeck, 1988). Yet, there are recorded examples of wind (with relative humidity close to that of vapour pressure over ice) sublimating ice rather than depositing vapour upon it (Hachikubo, 2001;Feick et al, 2007).…”

“…Yet, there are recorded examples of wind (with relative humidity close to that of vapour pressure over ice) sublimating ice rather than depositing vapour upon it (Hachikubo, 2001;Feick et al, 2007). In addition, the physical necessity of wind in the deposition process itself remains unresolved, with conflicting opinions in Lang (1985) compared with Hachikubo and Akitaya (1997) and Föhn (2001). At the very least, high (>3 m s −1 ) wind speeds at the snow surface will affect the shape of deposition into something other than surface hoar (Cheng and Shiu, 2002), and low wind speeds will present large barriers to bulk transfer of adequate vapour.…”