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The cumulative differences of net solar radiation, expressed as phase change mass per unit area, between self- and horizon-shading algorithms for the spring period from April 1, 2011 to June 14, 2011 were considered for the entire simulation domain (206580 triangles). For this period, 99% of the differences are less than 500 kg·m⁻², with a peak difference of 1600 kg·m⁻² predicted over a small fraction of the basin.

The mean phase change mass per unit area for the basin was 14.4 kg·m⁻².

In Figure 4.8, the seasonal (October–June) cumulative difference of net solar radiation between the two shading algorithms is shown, expressed as phase change mass per unit area. For the majority of this period, low solar angles cast large shadows onto the south-facing slopes, providing the bulk of the difference. The south-facing slope of Mt. Collembola, the slope where South Meadow is located, is a frequent recipient of horizon-shadows from Fisera Ridge. The south-facing slopes of the Mt. Allan cirque, shaded from the surrounding ridge line, are also areas where the irradiance is over-predicted by the self-shading algorithm. The right side of the large ‘V’ notch on Mt. Collembola is strongly over-predicted using the self-shading algorithm, and timelapse camera observations showed that this large shadow was associated with an abrupt transition between snow and snow-free areas in the notch.

In the top left (NW) corner is an area outside of the MCRB. It is a ‘U’ shaped valley that is almost always in the shadow cast from Mt. Collembola. This topography resulted in a large over-prediction of irradiance throughout the melt period if horizon-shadows are not considered — in fact, most of the largest differences in cumulative difference of net solar radiation between the shadowing models are found in this valley. The south face of the southern ridge line (approximately 6.27 × 10⁶ m E) is shown to have some of the largest differences due to horizon-shadows from the ridge line. The largest differences in net shortwave radiation between shading models for the MCRB are observed to be on east- and south-facing slopes.

For the melt period of April–June 2011, Figure 4.9 shows the cumulative differ-ence of net solar radiation between self- and horizon-shading models, expressed as

phase change mass per unit area. South-facing slopes are again predicted to be areas of large cumulative difference in net solar radiation. The south-facing slope of South Meadow is shown to have a large cumulative difference of net solar radiation, and in general the basin exhibits a similar pattern to that of the seasonal period. However, a notable change is that the north-facing southern ridge (at about 6.27 × 10⁶ m E) has a large cumulative difference of net solar radiation. Generally this area should be self-shaded; however, due to the higher solar angles the triangle faces are not fully facing away from the sun. Although this results in triangles with low values of cos Θ_sn, these values are not so low as to generate self-shadows. Thus during the morning as the sun passes through to the south, the direct-beam irradiances calcu-lated using self-shading are not 0 W·m⁻², and thus there is a large over-prediction of net solar radiation. Because of these accumulated differences, higher-than-estimated snowmelt would likely be observed in this valley if only self-shading algorithms were used, and the persistence of late-winter snowpacks would not be appropriately mod-elled. However, these results come with a caveat: the MCRB is dominated by tree-cover, and as such, the sub-canopy irradiance is significantly reduced. Ellis and Pomeroy (2007) demonstrated that mean daily canopy transmissivity values were as low as 0.09 on north-facing slopes and 0.36 on south-facing slopes. Because this study assumes a bare-earth, some of the locations may be impacted less by ignoring horizon-shadows. However, Ellis and Pomeroy (2007) indicated that the forest-cover on south-facing slopes had the greatest variance with respect to incident irradiance.

Thus the south-face slopes that are dominated by horizon-shadows may significantly impact the sub-canopy energy balance.

Figure 4.10 depicts the cumulative difference of net solar radiation between the self- and horizon-shading models expressed as phase change mass per unit area for the entire clear-sky day February 1, 2011. This demonstrates the significant effect of the differences in algorithms for even a single day. The deep southern valley at about 6.27 × 10⁵ m East is a source of large net solar radiation differences with average phase change mass values of about 5 kg·m⁻². Overall, the valley bottoms have over estimated net solar radiation and in many cases greatly so. The South Meadow site

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Figure 4.8: Cumulative seasonal (October 17, 2010–June 14, 2011) difference of net solar radiation between self- and horizon-shading al-gorithms expressed as phase change mass per unit area. An albedo of 0.8 is assumed. Note the logarithmic scale. The Marmot Creek Re-search Basin is outlined in black and contour lines are every 100 m of elevation.

is located such that although it has a prominent shadow crossing, its accumulated difference is only approximately 2 kg·m⁻². The north and south facing sites are areas where limited difference in net solar radiation would be expected, and on this day that remains the case.

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Figure 4.9: Cumulative spring (April 1, 2011–June 14, 2011) differ-ence of net solar radiation between self- and horizon-shading algorithms expressed as phase change mass per unit area. An albedo of 0.8 is as-sumed. Note the logarithmic scale. The Marmot Creek Research Basin is outlined in black and contour lines are every 100 m of elevation.

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Figure 4.10: Cumulative daily (February 1, 2011) difference of net solar radiation between self- and horizon-shading algorithms expressed as phase change mass per unit area. An albedo of 0.8 is assumed. Note the logarithmic scale. The MCRB is outlined in black and contour lines are every 100 m of elevation.