Aspect Is Everything

With a good coating of snow in the upper elevations, but a long stretch of warm, sunny weather in the forecast (after a system brushes by tonight and tomorrow), the next several days offer the opportunity to see the importance of aspect and solar radiation in snowpack evolution.

All aspects at upper-elevations in the Wasatch Mountains are presently
snow covered (Source: Alta Ski Area)

There energy balance at the Earth's surface is dominated by four major components: (1) net all-wave radiation, (2) ground heat flux (G), (3) sensible heat flux (H), and latent heat flux (L).  Typical values over a flat surface at midnight and noon on a June day near the Columbia River in eastern Oregon are illustrated below.

Typical values of the surface energy budget
components (Watts/square meter) at midnight and
noon (Whiteman 2000)

While all of these energy-balance components contribute to snowpack evolution, it is the net all-wave radiation (R) that has the greatest dependence on aspect.  The net all-wave radiation is comprised of two major components.  The first is radiation from the sun, which is commonly referred to as shortwave radiation because it is greatest at shorter (visible) wavelengths.  The second is radiation from the atmosphere, clouds, and snowpack, which is commonly referred to as longwave radiation because it is greatest at longer (infrared) wavelengths.  When there is more short and long wave radiation incoming that outgoing, the net all-wave radiation is positive and energy is available to warm the Earth's surface, evaporate water or ice, etc.

In the midlatitudes under clear skies, the net all-wave radiation reaches a peak near local noon when the incoming shortwave radiation is largest, but is negative overnight when the shortwave radiation is near zero and more long-wave radiation is emitted by the Earth's surface than is received from the atmosphere.  This is well-illustrated by the orange "R" line in the figure below.

Typical daily temperature cycle of the surface energy
budget (Watts/square meter) (Whiteman 2000).

The figure above is for a flat surface in June near the Columbia River in Oregon.  When we move to the snow-covered Wasatch range in October, the net all-wave radiation over a flat surface is reduced (compared to the above graph) during the day because of the lower sun angle and because the snow reflects more sunlight back to space.

What happens over mountain slopes, however, is strongly dependent on the slope aspect.  As illustrated below for north- and south-facing aspects, the amount of radiation intercepted by the Earth's surface varies with season, time of day, and slope angle (the latitude of the Wasatch Mountains roughly splits the difference between the two sites).

Source: Barry (1992)

At the winter solstice (22 Dec) the direct solar radiation incident at noon on a 30 degree north-facing aspect is 0 Watts/meter squared at 50N (i.e., it is in the shade), but >500 Watts/meter squared on a south-facing aspect.  That is a huge difference.  Further south at Tucson, the 30 degree north-facing aspect gets a little sun at noon (~100 Watts/meter squared), but the south-facing aspect gets far more, nearly 800 Watts/meter squared.

This is why aspect is everything.  In the coming days, we'll see a quick melt out on the south-facing aspects, but snow will hang on stubbornly on north-facing aspects.