First temperature scenario

The first temperature scenario is prescribed by a seasonally differentiated rise of the mean temperature (Table 19.2). The model is driven over the same period using the same overall initial conditions and the same stations as in the control integration.

At D88-89, the snow season decreases by 34 days and its maximum depth decreases by 30 cm (Table 19.3, Figure 19.6). A rise in Q* is observed because larger values of L* were simulated. Recalling that Q* = K* + L*, where K* is the net solar and L*, the net infrared energy flux at the surface, AQ* is determined by AL* when there is no change in K*. Ts also increases with the same consequences as described in Section 19.3.1. The cumulated AQm only reaches 10MJm-2, whereas for the control run, three times more was calculated than what the much smaller snow depth can explain. The runoff peak is simulated 31 days earlier than in the control runs (Table 19.3).

At D98 -99, the snow cover duration undergoes a reduction of 27 days and its depth thins by 22 cm. For the radiation and energy fluxes, the same changes are observed as for D88 -89. Furthermore, a part of the precipitation falls as liquid instead of solid compared to the control run. On the one hand, this induces snowmelt (by absorption of energy by the snowpack) and on the other hand, it prevents the pack from accumulating. The runoff peak is computed for the end of March, which is 11 days before the control run (Table 19.3; Figure 19.5).

0 140 120 100 80 60 40 20 0

450 1 400 350 300 250 200 150 100 50 0

900 800 700 600 500 400 300 200 100 0

Disentis 88-89

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Days of year

Figure 19.6 Evolution of the snowpack under the three temperature change scenarios: obs = observed snow depth. Note the different scales

Days of year

Figure 19.6 Evolution of the snowpack under the three temperature change scenarios: obs = observed snow depth. Note the different scales

At S88-89, the snow cover duration reduces by 31 days and its maximum depth decreases by 81cm. With regard to the energy and radiation fluxes, the same changes are observed as for Disentis, but their magnitude is greater. Q* enhances owing to a decrease of a because of more liquid precipitation, which keeps a at its lower asymptotic value (Table 19.1). If the precipitation falls as snow, a is increased, which diminishes Q*. At this high-elevation station, the turbulent fluxes Qh and Qe reach cumulated values of -45 MJm-2 and 20MJm-2 respectively. Qh contributes to the melting of the pack and Qe depletes the snowpack by sublimating the snow. The runoff peaks 26 days earlier, at the end of June, compared to the control run.

Days of year

Figure 19.7 Evolution of Q*, Qe, Qh, and AQm at Santis 98-99. The curves stop when the snowpack has melted

At S98-99, the snow cover decreases by 51 days and its maximum depth decreases by 177 cm. All the above-mentioned changes of the energy and radiation fluxes are also observed except that the direction of the Qe flux is changed and its negative sign indicates a solid condensation, which adds energy to the snow surface (Figure 19.7). The peak runoff occurs in the middle of June, 52 days earlier than in the control run (Table 19.3).

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