Mountainous areas

In experiment MOD2, the new SCF parameterization (Eq. 7) for regions with mountains was compared with the old one given in Eq. 2.

The new parameterization considerably improves the calculation of the surface albedo for mountains with snow cover, which predominate in regions as the Himalayan or Rockies. For the Himalayan example in Fig. 4, a marked difference between the satellite derived (SRB) and simulated (CTRL)

monthly mean surface albedos is evident. While the ECHAM4/T42 experiment of the current climate simulates a pronounced annual cycle with an amplitude of about 0.2, the SRB data varies only slightly.

In Fig. 4, the curve labelled USAF1 was computed from the USAF snow depth climatology and the following algorithms from the ECHAM4: (i) the parameterization of the snow albedo, (ii) the calculation of the background albedos and surface temperatures, (iii) the transformation equation to compute the SCF and (iv) the forest fraction. The USAF1 albedo is approximately 0.05 lower than the control simulation in winter. This is due to the fact that SWE in the control simulation exceeds the observation.

The curve labeled USAF2 in Fig. 4 is calculated as USAF1 except that the SCF is calculated using the new parameterization (Eq. 7). The difference is due to the subgrid orography not accounted for by the old parameterization.

During summer, where the USAF climatology provides snow free conditions, the SRB albedo is significantly higher than in USAF1 and USAF2. This may be due to the problem that the measurements sites used for compiling the snow depth climatology do not represent the very high mountain regions with permanent snow cover.

The influence of the new SCF parameterization for regions with mountains is summarized by the sensitivities in Table 3. To exclude regions that are not of interest, the averaging was again limited to land grid boxes with snow cover in February. The decreased SCF leads to a lower albedo and stronger heating of the surface due to increased shortwave and, thus, net radiation. The higher surface temperature and higher wind speeds lead to enhanced turbulent heat fluxes and, therefore, to higher precipitation. This may be related to an intensification of the hydrological cycle.

Table 3. As Table 2, but for simulation MOD2

Parameter

Unit

MOD2

CTRL

Diff.

Diff. (%)

Snow cover

%

28.6

33.6

-5.0

-14,9

Snow water eq.

cm

3.14

3,23

-0,09

-2.9

Surface albedo

0.29

0.31

-0.02

-8.0

NetSW, surface

Wm-2

97,5

95.8

1.7

1.8

Global radiation

Wm-2

126.3

127.2

-0.9

-0.7

SW, up, surface

Wm-2

-28.7

-31.4

2.7

-8.5

Net LW, surface

Wm-2

-51.5

-51.2

-0.3

0.6

2-m temperature

K

273.23

272.95

0.32

Latent heat flux

Wnr2

-32.4

-31.7

0.6

2.0

Evapotranspiration

mm/day

1.09

1.07

-0.02

-1.9

Sensible heat flux

Wm-2

-2.4

-1.6

-0.76

46.7

Rcl. soil moisture

%

73.3

73.4

-0.09

-0.1

Precipitation

mm/day

1.70

1.69

0.01

0.8

10 m windspeed

ms1

3.65

3.62

0.03

0.8

It is of some interest to compare surface climate sensitivities of MOD2 and MOD1. The responses are of opposite sign but overall changes in MOD2 are two to three times larger than in MOD1. This indicates that the combined effect of the modification introduced in MOD1 and MOD2 is to reduce the snow cover fraction and thus the surface albedo in the average.

5.3 Forested areas

The model run with modified surface albedos of snow-covered forests as described in Sections 4.3, 4.4 and 4.5, MOD3, significantly changes the surface climate. Annual means of the Northern Hemisphere (with measurable snow cover in February) are given in Table 4. Considerable differences are found for SWE, SCF, surface albedo and surface temperature. The decreasing albedo leads to higher temperatures and, thus, to less snowfall and earlier snow melt in spring when the higher temperatures reduce stability and, therefore, produce larger turbulent heat fluxes. This increases the vertically integrated water vapour (and cloud water) by about 2%, which leads to a slight increase in precipitation. Nevertheless, the snowfall rate decreases due to higher temperatures, mainly during the transient seasons. Again, the above discussion is based on 1 -dimensional considerations. Local processes are, however, also affected by large-scale advective processes such as moisture convergence.

Table 4. As Table 2, but for simulation MOD3

Parameter

Unit

MOD3

CTRL

Diff.

Diff. (%)

Snow cover

%

32.4

33.6

-1.2

-3.5

Snow water eq.

cm

3.10

3.23

-0.13

-4.1

Surface albedo

0.29

0.31

-0.02

-7.5

Net SW, surface

Wtrr2

98.2

95.8

2.4

2.6

Net LW, surface

Wnr-

126,6

127,2

-0.6

1.1

2-m temperature

K

273.55

272.95

0.6

Evapotranspiration

mm/day

1,09

1.07

0 02

2.3

Sensible + latent heat flux

Wm-2

-35,2

-33,7

-1,5

4.3

Rel. soil moisture

%

72.6

73.4

-0.8

-1.2

Snow fall

mm/day

0.636

0.650

-0,014

-2.3

Precipitation

mm/day

1.71

1.69

0.02

1.0

Cloud cover

%

62.4

62.7

-0.3

-0.5

Higher deviations between the surface climate in MOD 3 and CTRL are found on smaller scales as, e.g., the boreal forests. The difference between MOD 3 and CTRL can be compared with the effect of deforestation on the (surface) climate. Their responses show equal tendencies on the surface climate: Thomas and Rowntree (1992) showed that the removal of the boreal forests increases the land surface albedo and snow depth but decreases air temperature, surface net radiation, sensible heat flux, latent heat flux and precipitation during the months of March, April and May. The impact caused by changes in the surface albedo is likely to be larger when including the oceanic feedback instead of prescribing the sea surface temperature (Bonan et al., 1995): colder winter climates increase the extent of sea ice, thereby reinforcing the cooling caused by higher ocean albedos.

The largest deviations between MOD3 and CTRL occur in spring: Fig. 7 shows the long-term differences for the surface albedo and 2-m temperatures in March. The pronounced decrease in the surface albedo leads to substantially higher shortwave net radiation and, consequently, to enhanced surface temperatures. Fig. 7c indicates that the differences are statistically significant on the 95% level, using the statistical t-test. The substantial warming leads to an accelerated snow melt in late spring which reduces the overestimated snow cover in late spring in ECHAM4. Based on daily model output, it is found that the retreat of the simulated snow line is approximately 5 days later between mid-April and mid-June than in CTRL (not shown). The largest albedo differences are found over the boreal forests in both the higher latitudes of Eurasia and North America. This feature is mainly attributed to Eq. 9 which assumes the albedo of snow covered forests to be 0.2. This value has been confirmed as realistic by several authors (Verseghy, 1991; Harding and Pomeroy, 1996; Pomeroy and Dion, 1996, and others). A crude estimate of the maximum surface albedo of snow covered evergreen forests, as calculated with CLASS, leads to a value of approximately 0.25, which is in agreement with observational studies (Pomeroy and Dion, 1996 and Betts and Ball, 1997). This estimate is based on the following assumptions: (i) sky view factor SVF = 5-6% (Eq. 8 for needleleaf trees with LAI = 6), and (ii) snow on the ground with as = 0.8. The albedo of 0.25 obtained with CLASS is distinctly lower than the albedos suggested in ECHAM4, being 0.4 for boreal forests in winter and surface temperatures below -10°C.

The impact of changes in the forest albedo on the absorbed shortwave radiation and surface temperature from December through February is small due to low sun and, hence, low global radiation.

Figure 7. Mean differences between MOD3 and CTRL for March (10-year means), a) Surface albedo, b) 2-m-temperature, c) t-statistic test for surface albedo - dark shaded areas differ on the 95% level. Areas shaded in bright grey depict the area with measurable snow in February

Figure 7. Mean differences between MOD3 and CTRL for March (10-year means), a) Surface albedo, b) 2-m-temperature, c) t-statistic test for surface albedo - dark shaded areas differ on the 95% level. Areas shaded in bright grey depict the area with measurable snow in February

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