Feature Formation on the Snow Surface due to Interactions with the Atmospheric Surface Layer

Exchange processes with the atmosphere lead to the formation and decay of crusts and surface deposits. Surface hoar crystals are hexagonal plate-type crystals which form by deposition of water vapour onto the snow surface. These surface layers are important to accurately simulate snow cover evolution, particularly in the context of weak layer formation and stability evaluation. They are of interest to avalanche researchers, due to their lack of inter crystalline bonding once buried and weak attachment to the original snow surface.

A better knowledge of the formation processes and the corresponding meteorological conditions will also increase our understanding of snow – atmosphere interactions, which are important in many other applications such as hydrology or climatology.

Whereas some attempts have been made to unveil the subtle micro-meteorological processes involved in surface hoar formation, no study so far concentrated on this processes by measuring sensible and latent heat fluxes with high time resolution and in the alpine environment. Even less well understood than surface hoar formation is the formation of thin breakable crusts at the snow surface. These crusts are often assigned to wind action but very little is known about the formation processes.

In this study we propose to address the formation of these surface features focusing on the correlation between observation and measurements of the turbulent fluxes involved, particularly the latent heat flux and the water vapour flux. Therefore sonic anemometers and Li-cors were installed close to the measurement site. The results of the observed surface hoar formation and the other snow parameters will also be compared to the modeled surface hoar formation by SNOWPACK.

March 16^th, 2007 Franziska Stössel. Surface hoar crystal from the SLF study plot at Weissfluhjoch. Stored in isooctane and processed with a binocular eyepiece. The crystal length is 1.8 mm.

March 16^th, 2007 Franziska Stössel. Surface hoar layer on the snow surface, where the above showed crystal comes from. The area of the picture is 11.3 cm² (2.9x3.9 cm).

May 16th, 2007 Franziska Stössel. Sonic Anemometer with Li-cor installed on the study plot at Weissfluhjoch.