Energy budget at the surface

Differences in surface albedo between the perturbed and the control simulations are remarkable over glaciers (LC1 =1). There is also a significant effect where the proportion of glacier declines in the transition zones in the periphery of the glaciers. This is the particularly the case in the Bernese Alps. Over that region (486 km2 approx.), the spectrally averaged albedo, a, has risen from 0.38 in the control to 0.76, and from 0.38 in the control to 0.74 in the perturbed simulation average over glaciers (164 km2 approx. ) and over the transition zone (322 km2 approx.), respectively, during the period. This contributes to a decrease in the net solar radiation by more than 300 W m"2 locally over the glacier in the perturbed simulation as shown in Fig. 2.

Figure 2. Net solar radiation differences at 1200 UTC on February 27, 1990 in southern Switzerland simulated with CRCM at 1 km resolution ("perturbed"-"control"). The solid line is the Swiss-Italian border, contour intervals is 100 Wm'2, negative values are shaded, and the box indicates the location where the analyses over Bernese Alps are performed in the text

Figure 2. Net solar radiation differences at 1200 UTC on February 27, 1990 in southern Switzerland simulated with CRCM at 1 km resolution ("perturbed"-"control"). The solid line is the Swiss-Italian border, contour intervals is 100 Wm'2, negative values are shaded, and the box indicates the location where the analyses over Bernese Alps are performed in the text

Since the upper atmospheric conditions are similar in both simulations, it may be assumed that the incident solar radiation, S, is similar in both simulations (AS1 < 15 %). Thus, the net solar radiation differences are mostly due to the surface albedo effects. It can be seen in Fig. 2 that remarkable effects are located in the Bernese Alps ice field north of the Rhone valley. Since the main interest is in evaluating the effects of the land-cover change on the surface conditions, most of the analysis found in this text is carried out mainly over this region that includes glacier areas as well as transition zones in the periphery of the glaciers. In Fig. 2, the region under study is identified on the grid, that is glacier where LCI = LC2 = 1 (¿w > 75 %), and the transition zone where LCI = 1, LC2 = 24 (25 % < 5giac < 75 %).

The differences in radiation budget components between the perturbed and the control simulations in the Bernese Alps are shown in Fig. 3a. The net all-wave radiation differences at the surface (AQ*; Q* = (I-a) S + FL) is dominated by the effects of the reflection of the solar radiation, averaged over both glaciers and transition zone, where the net solar radiation is reduced by more than 175 W m2 compared to the control simulation. The net infrared longwave differences are slightly negative (-2 and -1 W m"2) over glaciers and transition zone, respectively, and have a negligible effect on the net radiation budget differences during February 27, 1990. The differences in energy budget components over the Bernese Alps are shown in Fig. 3b. While the differences in the heat flux entering the ground is slightly negative on the daily average, AG = -0.3 and -0.7 W m2 for glaciers and transition zone respectively, the behaviour of the sensible and latent heat fluxes is somewhat different. The sensible heat flux magnitude is significantly reduced in the perturbed simulation. The direction of the fluxes is preserved (positive from the surface to the atmosphere) but over glaciers, it decreases by whereas it decreases only by in the transition zone compared to the control simulation on the daily average. The downward latent heat flux (negative from the atmosphere to the surface) is decreased (negatively) by more than 200 W m"2 over glaciers on the daily average but only by 15 W m"2 in the transition zone compared to the control simulation on the daily average. There is also a significant diurnal influence in the sensible and latent heat fluxes in the transition zone. The downward sensible heat flux is reduced more during the day than during the night, the downward latent heat flux is reduced during the day but increases during the night. The changes in both sensible and latent heat fluxes are balanced within a few W m"2 by the decrease in the net radiative flux at the surface; the result is a slight decrease of energy available for the underlying surface. Note that the energy change associated with the freezing of soil moisture and snow, M, is zero since there is no liquid water available in the soil and surface temperature is maintained below freezing. Table 3 summarises the effects and shows the energy budget components (using hourly means), spatially averaged over the glaciers and transition zone in the Bernese Alps, and averaged for the day of February 27, 1990.

Table 3. Daily averages of radiation and energy balance components in Wmarea averaged over the Berneese Alps in the control simulation and averaged over transition zone and glacier respectively in the perturbed simulation. f)(atm) is the downward atmospheric infrared flux and F/isfc) (= <JTg) is the upward terrestrial infrared flux where Fj = F/(atm) - F/(sfc)

Table 3. Daily averages of radiation and energy balance components in Wmarea averaged over the Berneese Alps in the control simulation and averaged over transition zone and glacier respectively in the perturbed simulation. f)(atm) is the downward atmospheric infrared flux and F/isfc) (= <JTg) is the upward terrestrial infrared flux where Fj = F/(atm) - F/(sfc)

Energy component

"Control"

"Perturbed"

area averaged over Bernese Alps

Bcmcsc Alps

25% < < 75% 4;nc > 75%

S

141.3

163.2

154.9

aS

53.3

120.5

118.2

FJatm)

199.7

196.7

198.0

FL(sfi)

271.6

272.9

268.9

a,

459.8

324.2

279.7

LES

-448.8

-366.4

-314.9

G

-3.1

-3,9

-3.2

M

0,0

0,0

0.0

ftadlnicn budget coupon »rata In beineH Alps region, V— »il* RatHalwri budgaC component» tn tunin Alpa i igk) n, 21 * * < 7B % __|p*nwbMt -cpi*Bl|j ___ ____^«tnM-CWrtfoQ .

i ioe ftadlnicn budget coupon »rata In beineH Alps region, V— »il* RatHalwri budgaC component» tn tunin Alpa i igk) n, 21 * * < 7B % __|p*nwbMt -cpi*Bl|j ___ ____^«tnM-CWrtfoQ .

Figure 3. Differences in surface fluxes, variables and coefficients between the "perturbed" and the "control" simulations area averaged over the Bernese Alps glaciers and transition zone regions from 0100 to 2300 UTC on February 27, 1990
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