Tentative investigations on surface hoar in mountain forests : Snow and Avalanches

1998

S. 31-34

Bandaufführung

9217S

105128

This paper deals with air temperature, net-radiation and relative-humidity measurements which we performed in Austria in different zones of mountain forests in recent years. Our aim using such measurements was to approximate the general conditions for the formation of surface-hoar layers in low-density forest stands near the timber line. These layers are a necessary condition for snow-slab release and avalanche formation. Our investigations showed that the total radiation balance was negative but less negative in a forest with a close canopy (on clear nights about -30 to -40W m-¬) than within a small clearing (6 m by 6 m). Moreover, the total radiation balance within the forest clearing was almost the same as that in an open field. There we measured -60 to -80 W m-¬ on clear nights. Approximations of surface temperature (ts) showed that ts was below the dewpoint (tE) in an open field but did not reach tE in the forest; moreover, ts was very close to tE within the forest clearing. Water-vapor flux was positive only in open terrain (i.e. condensation beween 0.1 x 10-6 and 0.9x10-6 kg m-¬s-Ü) and evaporation occurred within the clearing and in the forest.