Shortwave radiation

The global short-wave radiation was modelled in the following way.

where lsc is the solar constant or 1367 Wm2; r2 is the reciprocal of the square of the radius vector of the earth, or correction for the eccentricity of the earth's orbit, which is calculated using Fourier series derived by Spencer (1971); and Ti represents atmospheric transmittance functions, both for diffuse and direct radiation, which take into account Rayleigh scattering, transmittance by ozone, by uniformly mixed gases, by water vapour and by aerosols, and are computed following a parametric model by Iqbal (1983). The t -functions incorporate the relative optical path length and pressure corrected air mass, depending on solar zenith angle and altitude. Further updates to Iqbal's model are introduced for the calculation of precipitable water, following Prata (1996) and for ozone layer thickness, which is taken from the NASA Total Ozone Mapping Spectrometer dataset (TOMS-EP 2001). The F-factors are corrections for direct radiation with respect to its angle of incidence (Ft), for diffuse radiation (Fsk), multiple scattering (Fms) and reflected radiation by both snow-covered and snow-free surrounding terrain (Fsn). The F-terms take into account the horizon obstruction or sky view factor (fv, Equation 3.6). They are computed in a slightly modified way from Greuell et al. (1997) as explained by Corripio (2004), with terrain and solar parameters such as vector normal to the surface, shading, horizon configuration and solar vector calculated after Corripio (2003b).

The results of the model, compared with measured radiation on a clear day, are shown in Figure 3.5, where the differences between modelled and measured data were smaller than the nominal pyranometer accuracy (10%). In this case, the valley is uniformly covered in snow and runs east to west, for different configurations there is a small error introduced by the necessary simplification and symmetry assumption of the terrain configuration parameters.

The albedo at the upper station was fairly constant, with an average value of 0.44 and a standard deviation of 0.07. Its decrease was only 4% over a month. An unusual pattern was observed at the upper AWS in the last hours of the afternoon, when the albedo value rose sharply to almost 1.0. This could be an artifact due to differential shading. Another possible explanation for this behaviour is an increase in reflected diffuse radiation as the sun hit the penitentes' wall from the west at a very low angle. The fact that the increase in albedo happens after 18:00 h, when the solar azimuth enters the south-western

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__I Diffuse modelled

__I Reflected modelled

--I Total modelled

__Albedo

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Julian Day: 37

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Julian Day: 37

Figure 3.5 (Plate 2) Energy fluxes on a clear day on Loma Larga glacier, 4667 m a.s.l. DOY 37, 6th of February. Note the increasing albedo in the afternoon, an explanatory hypothesis is given in the text

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Figure 3.5 (Plate 2) Energy fluxes on a clear day on Loma Larga glacier, 4667 m a.s.l. DOY 37, 6th of February. Note the increasing albedo in the afternoon, an explanatory hypothesis is given in the text quadrant, adds support to this hypothesis, although the causes are not clear yet.

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