Experimental setup

In order to assess the role of land-cover types in the Swiss Alps, we simulate conditions prevailing on the particular winter of 1990 with the CRCM with and without alpine glaciers. This winter is characterised by its strong storms over the Alps. To do so, the CRCM is run in a cascade "self-nesting" mode which consists of downscaling first NCEP-NCAR reanalysis (Kalnay et al., 1996) at 60 km with 20 vertical levels, and an archival period of 6 h during a three-month period from January 1 to March 15, 1990. These results are then used to nest a simulation within the same model but now at 5 km resolution and 30 vertical levels, and an archival period of 1 h during a three-day period from February 26 to March 1, 1990. The latter finally serves to nest a simulation at 1 km with 46 vertical levels over a one-day period, i.e., February 27, 1990. Because the 5- and 1-km simulations are expensive to run in terms of computer resources, and require a large amount of memory on our local server, it was necessary to run them on a much reduced-size domain and for shorter periods compared to the 60-km simulation. The technique involves the use of several subdomains as shown in Fig. 1. The downscaling of NCEP-NCAR data is carried out on domain A. The intermediate nesting on domain B provides a better spatial scale transition, since it allows the atmospheric circulation to adapt to the complex terrain. Over domain C, the model is run at 1 km resolution with a very realistic topographic representation of the Alps. Here, numerous details are now accurately captured, the Rhone Valley is resolved as well as many of its tributary valleys, and also the appropriate areas where glaciers are located. At 60-km resolution the land-sea mask and the height of the orography are, as for the land-cover and soil types, taken from the 1° x 1° reference file. The SSTs and sea-ice coverage are taken from the l° x 1° resolution dataset of GISST (Rayner et al., 1996). At 5- and 1-km, the height of the orography is properly resolved on the model grid.

Figure J. Domains of integration. The outer domain (A) is used to downscale NCEP-NCAR reanalysis at 60 km. Intermediate domain (B) is used for the 5-km integration, and the inner domain (C), is used for the 1 -km integration

Two experiments are performed at 1 km resolution: first, a "control" experiment with uniform land-cover and soil type over southern Switzerland, and secondly, a "perturbed" experiment in which glaciers are realistically resolved on the grid.

The land-cover and soil types prescribed in the control experiment are taken from the 1° dataset of Wilson and Henderson-Sellers. The primary land-cover consists of type Nb. 13 (Table 1; short grass and forbs) uniform over the domain, the secondary land-cover is of type Nb. 4 (evergreen needleleaf tree) north of the Alps, and Nb. 24 (desert) in the Alps. The soil type is uniformly distributed throughout the domain characterised by a medium colour and an intermediate texture. The parameter values used in the control simulation and defined from the land-cover and soil types are depicted in Table 2. The soil texture defines a porosity of 48 %. The "dry" soil albedo may be reduced by 7 % for completely wet conditions. Throughout the domain the background land-cover visible and near infrared albedo are nearly constants.

Table 2. Parameter values used in the CRCM at 1-km resolutions over southern Switzerland. LC1-2 stands for primary and secondary land cover type (see Table 1); other symbols are defined in the text

Experiments

"control"

"perturbed"

Parameter

north of Alps

2 5 % < Sdm <75% <W >75%

LC\

13

13

1

1

LCI

4

24

24

1

av

0.1 i

0.11

0.66 - .43 (1-^te)

0.66

LC Ctnir

0.23

0.23

0.22 + .23(1-^)

0.22

GND ... ,„ av dry

0.23

0.23

-

-

GND .. , „ av dry

0.45

0.45

-

-

Dv (cm)

I i 6.5

83.5

-

-

WV(kg m -)

559.2

400.8

200,0

200.0

s

0.58

0,75

0.42

0.25

Smmt (Cm)

402.7

7.0

1.0

1.0

COM

0.0044

0,0045

(CMfc,„„m/ +0.0023)72

0.0023

dam

0.10

0.13

1 - 5,1a,.

0.0

In the perturbed experiment, the Swiss glaciers have been superimposed over the control land-cover and soil types fields. The original glacier dataset has been produced by the Swiss Federal Office of Statistics (OFS, 1999) at 100 m resolution compiled over the period 1979-1985. This data has then been aggregated on the CRCM 1-km grid. This field represent the fractional coverage of glaciers (Sgiac) over Switzerland and the local values are used to infer the primary and the secondary land-cover types, the fractional area of bare soil, the albedo within the two spectral bands, and the neutral drag coefficient (or the roughness height) at each CRCM grid square. It is assumed that the soil type is the same as the one defined in the control experi ment. Even though the "dry" soil albedo is the same, the resulting albedo of the bare soil fraction may well be different from the control experiment due to a different soil moisture evolution. The procedure devised to build the file containing the "perturbed" fixed lower boundary conditions is the following: the aggregated glacier coverage field is used as an input file and where the fractional coverage of the glaciers exceeds 75 % locally, it is assumed that the primary and the secondary land-cover type are both prescribed as "glacier" (Table 1; Nb. 1). The effective heat capacity of the glacier ice is fixed at 148'492.42 Jm"2 K"1 (dg ~ 13 cm) but is allowed to vary according to the snow mass. The glacier drag value, {CDM = 0.0023) has been chosen to be smoother than the one used in the control experiment, but is representative of an heterogeneous ice surface. Its associated roughness height is in the range of values typical over glaciers (Smeets et al., 1999). Where the fractional coverage of the glacier lies between 25 and 75 % locally, it is assumed that the primary land-cover type is fixed as "glacier" (Nb. 1), the secondary land-cover type is fixed as "desert" (Nb. 24), and this is considered as a transition zone between glaciers and bare soil. The visible band albedo decreases and the infrared band albedo increase linearly from their original "glacier" values as the fractional coverage of bare soil increases to values that match the local "dry" soil values. The fractional area of the grid covered by bare soil is fixed as the remaining part of that is not covered by glaciers. The neutral drag coefficient varies as the local average of the glacier drag value and that defined in the control experiment. The effective heat capacity of the glacier ice is fixed as the one described in the previous case. Finally, where the fractional coverage of the glaciers lies below 25 %, all the parameter values are kept the same as defined in the control experiment.

During both the control and the perturbed simulations, the archival period is at hourly intervals. Although the lower boundary conditions of the landcover are changed, the initial values of surface temperature, Ts„, soil wetness Wn, and snow mass, SMo, are kept the same to avoid systematic bias in the analysis.

0 0

Post a comment