THE PHYSICAL CLIMATOLOGY OF ALPINE TUNDRA, SCOUT-MOUNTAIN, BRITISH-COLUMBIA, CANADA

Eleven months of radiation budget measurements from an alpine tundra s ite in southern British Columbia encapsulated a wide range of atmosphe ric and surface conditions and provide insights into the controlling f actors affecting energy and mass exchanges in the high mountain enviro nment. The seasonal snow cover exerts a strong surface control on ener gy exchange, the high albedo suppressing solar radiation absorption an d the cold surface temperatures limiting net longwave radiation losses . Turbulent energy fluxes during winter were very small and driven by the negative net radiation. The very thin snowpack that persisted thro ughout the winter meant that energy storage changes within the snow we re negligible and the snowmelt period was very brief. When the tundra is snow-free, cloud cover is the most important determinant of the rad iation budget through its influence on atmospheric transmissivity and longwave radiation fluxes. Local orographic cloud development provided an important negative feedback which countered the large solar irradi ances that would otherwise be expected in high mountains during the su mmer. Both energy- and moisture-limiting evapotranspiration regimes oc curred during both field seasons, governed largely by the frequency, r ather than the magnitude, of precipitation events. In the absence of p recipitation, the tundra desiccated rapidly due to the efficient drain age, limited soil moisture storage capacity, and the presence of quick -drying rock surfaces. Surface resistance measurements demonstrated th e lack of efficiency in the transfer of subsurface moisture to the atm osphere during dry periods, and show that the change from a wet-surfac e evaporative regime to a dry-surface one is achieved very quickly.