Difference between revisions of "Effects of Shading and Aspect"

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GSSHA currently employs methods to account for both longwave and shortwave radiation in each cell.  Longwave radiation is mainly a function of temperature, clouds, and atmospheric emissivity, while the shortwave radiation calculations take into consideration albedo, topographic shading, aspect of the terrain in relation to the sun, albedo of snow as it ages, atmospheric absorption and reflection, clouds, and vegetation.  The calculated longwave and shortwave radiation values are then used within the EB and HY models to simulate the melting of snow.  For the TI method of snow melt an effective temperature (Teff) is calculated, which incorporates the differences in net radiation affecting each cell.
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'''Topographic Shading'''<br>
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For every cell within the GSSHA model a search is ran every 2 weeks to determine which hours of the day the cell is shaded from surrounding topography.  The hourly Solar Azimuth Angle (SAA) and Solar Elevation Angle (SEA) in combination with basic geometry on the structured grid are used to determine if any cell within the GSSHA domain blocks the direct line between the solar location and the cell.  If the direct line is blocked during an hour, Kt,x=0.0 for that hour (simulating complete shading), otherwise Kt,x=1.0 for that hour (simulating no shading).  Kt,x is a reduction factor that is used to reduce the amount of shortwave radiation affecting each cell.  Having Kt,x be a value of 0.0 or 1.0 is the same assumption that is made in the GEOTOP model (Zanotti, Endrizzi et al. 2004).
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'''Aspect Angle'''<br>
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Due to several factors including location of the sun and the slope of land surface, the solar radiation emitted from the sun often does not hit the land surface on a perpendicular plane, resulting in less radiation per unit area (Bras 1990).  To account for this, a reduction in shortwave radiation based on the aspect angle of the land surface to the location of the sun (Ks,x) is calculated hourly for each cell within the model.  In order to complete this, the location of the sun is derived in three-dimensional space (sx, sy, sz) on a unit-sphere.  For each cell a three-dimensional vector (cx, cy, cz) on a unit-sphere is then created that is perpendicular to the cell surface.  A dot product can be used between the two vectors to calculate the solar-surface incident angle (SSIA).
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An adjusted solar elevation angle (SEAadj) can then be computed, which takes into consideration the slope of the land surface within the cell.  Ks,x is then a function of SEAadj.

Latest revision as of 22:25, 28 December 2013

Topographic Shading
For every cell within the GSSHA model a search is ran every 2 weeks to determine which hours of the day the cell is shaded from surrounding topography. The hourly Solar Azimuth Angle (SAA) and Solar Elevation Angle (SEA) in combination with basic geometry on the structured grid are used to determine if any cell within the GSSHA domain blocks the direct line between the solar location and the cell. If the direct line is blocked during an hour, Kt,x=0.0 for that hour (simulating complete shading), otherwise Kt,x=1.0 for that hour (simulating no shading). Kt,x is a reduction factor that is used to reduce the amount of shortwave radiation affecting each cell. Having Kt,x be a value of 0.0 or 1.0 is the same assumption that is made in the GEOTOP model (Zanotti, Endrizzi et al. 2004).

Aspect Angle
Due to several factors including location of the sun and the slope of land surface, the solar radiation emitted from the sun often does not hit the land surface on a perpendicular plane, resulting in less radiation per unit area (Bras 1990). To account for this, a reduction in shortwave radiation based on the aspect angle of the land surface to the location of the sun (Ks,x) is calculated hourly for each cell within the model. In order to complete this, the location of the sun is derived in three-dimensional space (sx, sy, sz) on a unit-sphere. For each cell a three-dimensional vector (cx, cy, cz) on a unit-sphere is then created that is perpendicular to the cell surface. A dot product can be used between the two vectors to calculate the solar-surface incident angle (SSIA).

An adjusted solar elevation angle (SEAadj) can then be computed, which takes into consideration the slope of the land surface within the cell. Ks,x is then a function of SEAadj.

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