1.2 shortwavc.aml

This AML is a correction of the shortwave.aml (Kumar et al., 1997). The original coding of L. Kumar has an error inherent when calculating the over-shadowing by adjacent terrain (using the HILLSHADE function in GRID). The solar path is not corrected for N/S latitudes, resulting in wrong sun azimuth values when hillshading a DEM in northern latitudes. Southern latitudes are calculated without error. In order to correct the AML for northern latitudes, I have corrected the calculation of the sun azimuth angle, which caused the error  when hillshading the DEM. However, I did not thoroughly test whether this correction holds in cases where the sun effectively passes north of the site. Thus, any application of the corrected version south of 23.33N has to be performed with caution.

Similarly to shortwave.aml, when starting the application the user is prompted for:

If the user wants to calculate the radiation for a single day, the same Julian day has to be submitted for start and end of calculations. Southern latitudes have to be indicated as negative values. The time interval (submitted in minutes) to increment the solar path determines how often the sun position is calculated during a day. The program then integrates the energy linearly over time. 30 to 120 minutes are reasonable intervals depending on the size of the DEM (# of pixels).

The routine takes into account overshadowing by high peaks, meaning that the routine detects pixels that are in the shadow of adjacent higher terrain for a given sun position. This can result in no direct solar radiation over the whole day for specific areas in a DEM, especially when calculating radiation for the winter solstice in northern latitudes when the maximum daily sun-altitude angle is generally low. This hillshading function has specifically been corrected in this version. See shortwave.aml for more information about this routine.
 

Modifications:

  • This AML had to be adjusted to a bug in ArcInfo 8.0.2, where the raise to fractional powers causes the program to crash. This reported bug is supposed to be fixed for AI 8.2. Calvin Tolkamp proposed an interim "fix", which is included in this version. Attention: This "fix" has been updated Sept. 14th, 2000!
  • Lattitudes exceeding ± 66° caused the original code to crash. This flaw has been fixed on Sept. 26th 2002.
  • The original code (SHORTWAVE.AML) does not properly initialize, and checks for the file instead of . Thus, if there are remaining temporary grids left in the run directory, the code crashed. The flaw has been fixed in Oct. 7th, 2003.
     
     
     

    General specifications of the AML:

    Command: &r shortwave (at GRID or ARC prompt)
    Required input: DEM
    Output units: kJ/m2/time period
    Solar constant used: 1.367 kW/m2
    Transmittance used: 0.8 (max for vertical beam)
    Minimum calculation step: 1 day
    Speed of calculations: Very fast due to parsimonious programming
    Flexibility of the routine: Medium; can’t go beyond 1 day
    User interface: Limited, but simple and easy
    Known errors:  -
    Corrections 9/14/2000: Adjustments to bug in ArcInfo 8.0.2
    Programmer L. Kumar (original coding)
    N.E. Zimmermann (coding of corrections)
    Download: shortwavc.aml       (use: "save link as")
    Contact: niklaus.zimmermann @ wsl.ch

     

    References:

    Kumar, L., Skidmore, A.K. and Knowles, E., 1997. Modelling topographic variation in solar radiation in a GIS environment. International Journal for Geographical Information Science, 11(5): 475-497.
     
     

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    Last Updated: 4/25/04
    By Niklaus E. Zimmermann