Conclusion

This study is intended to evaluate quantitatively the effects of land-cover change on surface fluxes and prognostic variables using the CRCM interfaced with GCMIIphysics, the latter using a simple land-surface scheme and boundary-layer parameterisation, under the atmospheric conditions particular of a stormy winter day over western Europe. Although the surface conditions were substantially modified in the perturbed simulation through the specification of alpine glaciers in southern Switzerland, it is realised that these have also an influence in surrounding areas. In this experiment, changing the landcover type from a typical vegetated surface to glaciers over areas in the Alps affect primarily the surface albedo which in turn modified the reflected solar radiation flux, the radiation budget at the surface, and subsequently the partitioning of the sensible heat flux and latent heat flux. Secondly, even though the vertical gradient of windspeed, temperature and moisture at the surface are modified, the negative change in the momentum, heat and moisture turbulent surface fluxes are mainly modulated by the ventilation coefficient, the latter being dominated by the change in the neutral drag coefficient. On average during this particular day over the Bernese Alps, the increase in anemometer-level windspeed is a result of the increased in the lowest modellevel windspeed, and the decrease of the roughness height (or of the decrease in the surface ground temperature, seen mainly during the day, is due to the decrease of the net available energy at the surface, and the increase in the surface saturation specific humidity.

This study thus helps to understand through a specific case-study that there are no simple relationships between the land-cover definition and the conditions above and emphasises the needs for the specification of land-use in high-resolution climate models. Although a sophisticated regional climate model is not absolutely necessary for this kind of study, the analysis of the simulated results emphasises the fact that conditions were also affected in the periphery of the perturbed areas. Snow accumulating on glaciers appear to be an important factor in determining the thermal behaviour of the surface. The analysis given above in the text should then be used with caution and should not be generalised to all glaciers, for all atmospheric conditions and for all seasons.

There are observational and modelling indications that the weather and climate are modulated by land-surface characteristics. Kalthoff et al. (1999) have analysed temporally and spatially, on the basis of station observations in the upper Rhine valley, the behaviour of the surface energy budget and concluded that the orography, precipitation and land-use were the main influences. Harrison (1975) studied the elevation component of soil temperature variation in Britain and concluded that the change in soil type (particularly through changes in thermal conductivity) between lowlands and uplands is a major factor affecting the seasonal distribution of temperature. Soil texture also seems to play a role in the spring moisture regime since the upland soils experienced much slower rates of drying in soil surface horizons. Ecosystems, although represented crudely in climate models, influence weather and climate over periods of seconds to years through exchanges of energy, moisture and momentum between the land surface and the atmosphere [see Pielke et al., (1998) for an overview]. Garratt (1993) provided a comprehensive summary of GCM atmospheric boundary-layer surface schemes, and the main results from sensitivity studies have shown that regional and global climate depend on albedo, surface moisture, surface roughness, and vegetation. This suggests that there is a need to account for soil and vegetation effects in such models. Segal et al. (1988) have shown that under favourable environmental conditions, vegetated areas adjacent to dry bare soil regions may provide substantial gradients of sensible heating which result in the onset of thermally induced mesoscale circulation. Seth and Giorgi (1996) have studied the organised mesoscale circulations induced by vegetation using a RCM and drawn similar conclusions for spatial scales less than 300 km. Pielke et al. (1997) demonstrated that the use of high-resolution land-cover in their RCM (RAMS) had a substantial influence on the overlying atmosphere. In addition, there is evidence that local land use practices influence regional climate in adjacent areas Stohlgren et al. (1998).

Therefore, there are clear indications that properly-resolved land-cover data (vegetation and soil types, their relative proportions over an area, and their radiative, thermal and aerodynamic properties) including glacier areas, should be taken into account in high resolution regional climate models.

The advance in the development of geographical data bases and satellite imagery now allows the definition of a high resolution land-cover (in terms of the types displayed in Table 1 for instance), and soil characteristics of the surface with much more detail than 1° resolution over Switzerland. Work is currently under way to investigate the effects of resolving the complete landcover types over Switzerland at high resolution in order to assess the seasonal effects of glaciers and other types of surface on Swiss climate, where the underlying working hypothesis is that variability in the surface climate can be generated with greater details in surface conditions.

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