1.6 solrad.aml

An additional feature of this AML is the incorporation of elevational lapse rates for transmittance. The information of lapse rates has to be prepared in a second file. The elevational lapse rate can be held constant or can be varied according to the seasonal characteristics of the atmosphere. The DAYMET program (Thornton et al., 1997), a derivative of the MT-CLIM routine (Hungerford et al., 1989), incorporates a constant (standard) elevational lapse rate for transmittance throughout the year (0.000008/m). We have analyzed multiple SAMSON stations data within the same climate region and over 25 years to derive average seasonal values for elevational lapse rates. The analysis showed that lapse rates for transmittance tend to be higher in the wet season. This is most likely due to the overall higher humidity of the air under such weather conditions.

The AML allows the user to choose from 4 different approaches to cope with actual transmittivity of the atmosphere. These 4 approaches are based on four transmittance values provided in the output file from SUM_TRAN. In this program, the ratio between extraterrestrial and ground-measured radiation intensity is calculated in order to obtain the actual transmittivity of the atmosphere. The first measure is the maximum transmittance observed during the analysis period (1 day or 14 days), the second value is the actually observed transmittance based on radiation as measured at a horizontal surface, while the third value is the actually observed transmittance based on radiation as measured perpendicular to the solar beam (= direct normal radiation). Since all of these values are “as is”, they are theoretically only valid for the average sun-altitude angle of this specific time period, since transmittance is affected by the average perturbation of the atmosphere as well as by the sun-altitude angle (see discussion of shortwave.aml). In order to get a transmittance value that is unlinked from the sun-altitude a fourth value is provided, which represents the theoretical vertical transmittance based on a transformation of the direct normal radiation values. For use in the solrad.aml it is strongly recommended to use this fourth corrected transformation value as basis for generating actual direct radiation calculations. All other measures will generally result in too low calculations of the direct solar energy input.

Both files (the lapse rate file and the transmittance file) need to have an identical ID-value in the first column, and the AML will report an error message if the ID-values do not match. Even though the SUM_TRAN output is not discussed in detail here, I give a brief description of the minimum information required in the two files in order to run the solrad.aml. Both files can be generated in free format, space or tab delimited. Only the order of the variables (columns) is important, and should conform to the following example (including a header file, even though the variable names are not important).

Transmittance file:
    id tmp1  tmp2  trnmx  trnhr  trndr  trndrc  tba  tmp3  tmp4  mxdnet
     1  15  0.970  0.653  0.163  0.154  0.281  0.196  923  1371  1414.0
     2  15  0.923  0.668  0.327  0.276  0.434  0.302  942  1301  1411.0
     3  15  0.885  0.682  0.184  0.154  0.251  0.175  959  1245  1407.0
     4  11  0.839  0.673  0.199  0.164  0.249  0.173  941  1173  1398.0
    ..  ..    ...    ...    ...    ...    ...    ...  ...   ...     ...

Lapse rate file:
    id     lpsrt
     1  0.000103
     2  0.000108
     3  0.000093
     4  0.000081
    ..       ...

where id = identifier of the period the data row is valid for (day, biweek, month, etc.); tmp1,…,tmp4 = variables generated by SUM_TRAN that are unused by solrad.aml; trnmx = maximum transmittance observed during the analysis period; trnhr = actual transmittance based on radiation as measured at a horizontal surface; trndr = actual transmittance based on radiation as measured perpendicular to the solar beam; trndrc = sun-altitude angle corrected transmittance based on a transformation of the direct normal radiation values (represents transmittance of a vertical solar beam); tba = alpha-coefficient for the sun-altitude angle correction of the actual transmittance (is used for option 4; see eqn. below); mxdnet = is the maximum “observed” direct normal extraterrestrial radiation (which can be used in the AML instead of the cosine function based on the solar constant); lpsrt = the elevational lapse rate for transmittance.

The variables tmp1 through tmp4 are not used in the AML, but are explained in the description of sum_tran.for. However, they need to be present in a transmittance file (consisting of a character at minimum). The id-values are important to determine the appropriate Julian day and is therefore crucial. If daily values are presented in the two parameter-files then the id should stand for the appropriate Julian day. If, however, the data is averaged over a specific period (e.g. 14 days) then the id should indicate what period number the values are valid for. The AML will ask for the number of days the parameter values are averaged for. It then calculates the Julian day for the middle day of the period (e.g. Julian day 8 for the 1st bi-week of the year; Julian day 22 for the second bi-week of a year, etc.), and calculates an output grid for this mid-term date.

The AML can be started in ARC or in GRID, and the user will be prompted for the following information:

• Name of the file with transmittance data
• Name of the file with lapse rate data
• Name of the DEM of the study site
• Name of the radiation output grid
• Latitude in degrees of the study site
• Time interval for incrementing daily solar path
• Reference elevation the transmittance data is valid for
• Number of days the transmittance/lapse rate data are averaged for

where Tau = the transmittance of the atmosphere; and M = the air mass (mass of the atmosphere) for a given sun-altitude angle (see shortwave.aml for more information on air mass calculations), and the factor alpha governs the asymptotic maximum of a transmittance at a sun-altitude angle of 90º (an alpha-coefficient of 0.56 results in a vertically measured transmittance of 0.8).

In order to enable solar flux calculations for actual transmittance the SUM_TRAN program determines the daily or biweekly transmittance, and corrects the respective transmittance to a value as if the solar beam would pass vertically through the atmosphere. Since the corrections are based on the same formula as presented above, the program writes not only the corrected transmittance values to the output file, but also the associated alpha-coefficient (termed tba in the transmittance file header displayed above). Based on t his value, the solrad.aml is enabled to calculate the appropriate attenuation of the beam radiation due to lower sun-altitude angles (reduction of transmittivity due to a longer path through the atmosphere).
 
 
 

General specifications of the AML:

References:

Hungerford, R.D., Nemani, R.R, Running, S.W. and Coughlan, J.C. 1989. MTCLIM: A mountain microclimate simulation model. USDA Forest Service, Research paper INT-414, Intermountain Research Station, Ogden.

Kreith, F., and Kreider, J.F. 1978. Principles of Solar Engineering. McGraw-Hill, New York, NY.

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.

Thornton, P. E., S. W. Running, M. A. White (1997). Generating surfaces of daily meteorological variables over large regions of complex terrain. Journal of Hydrology 190(3-4): 214-251
 

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