Progress in understanding of land surface/atmosphere exchanges at high latitudes

This paper summarises some of the key results from two European field progr ammes, WINTEX and LAPP, undertaken in the Boreal/Arctic regions in 1996-98. Both programmes have illustrated the very important role that snow plays w ithin these areas, not only in the determination of energy, water and carbo n fluxes in the winter, but also in controlling the length of the summer ac tive season, and hence the overall carbon budget. These studies make a cons iderable advance in our knowledge of the fluxes from snow-covered landscape and the interactions between snow and vegetation. Also some of the first m easurements of greenhouse gas fluxes (carbon dioxide and methane) are repor ted for the European arctic and sub-arctic. The measurements show a conside rable variability across the arctic, with very high instantaneous values fr om sub-arctic birch and fen areas and extremely low fluxes reported from th e polar desert in the high arctic. The overall annual budgets are everywher e limited by the very short active season in these regions. The heat flux o ver a high latitude boreal forest during late winter was found to be high. At low solar angles the forest shades most of the snow surface, therefore a n important part of the radiation never reaches the snow surface but is abs orbed by the forest. This indicates that in areas with sparse vegetation an d low solar angles, absorption of direct solar radiation is due to an appar ent vegetation cover, which is much greater than the actual one. The first steps are taken in using these measurements to improve models, bo th point soil/vegetation/atmosphere transfer schemes and 3D meteorogical mo dels. The results are encouraging; increasing the realism progressively imp roves the representation of the fluxes. A start is made in developing lands cape, or catchment scale models. There seems to be some hope that comparati vely simple relationships between evaporation and photosynthesis and leaf a rea may be sufficiently robust to allow the use of remotely sensed images t o investigate the areally averaged exchanges. It is suspected that high latitude regions will experience considerable cli matic and environmental change in the coming decades. A well found predicti on of how these regions will respond requires a comprehensive knowledge of how vegetation will respond and how the changed vegetation will interact wi th the snow cover and the atmosphere. The studies from the LAPP and WINTEX programmes presented in this volume are an important contribution to this u nderstanding and provide a useful foundation for future research.