Incident Radiation Model

RADIN4 is a radiation subroutine that is called ONLY if the hourly input information does not contain all necessary radiation flux values for visible light, NIR and thermal radiation separately. It fills in the values with estimates. In all cases that RADIN4 is called, the hourly input variable RADTOP(2,IHR) contains the measured values of actual total solar radiation (otherwise this variable contains the hourly actual total NIR radiation).

View definitions of variables used in this subroutine:

• in sequence encountered in subroutine
• in alphabetical order

Code outline:

   1)  An estimate for the amount of atmosphere that solar radiation 
       has to pass before it hits the earth surface, AIRMAS, is 
       calculated.  AIRMAS is a function of the cosine of the solar 
       zenith angle, which determines the length of the pathway and 
       is corrected for the curvature of the atmosphere and the 
       refraction of light.

   2)  The potential direct beam radiation at the earth's surface is
       calculated in a Beer's law relationship with the following assumptions:

   600 W/m2 of ingoing visible light with an extinction coefficient of 0.160
 
   720 W/m2 of ingoing NIR with an extinction coefficient of 0.050.

   From the potential direct beam radiation of the NIR, a term 
   WATABS is subtracted. WATABS is an estimate for the amount of 
   NIR solar radiation that is absorbed by atmospheric water. The 
   estimate is based on an empirical equation, as a function of 
   the cosine of the solar zenith angle.

   3)  The amount of diffuse radiation is calculated from an 
       estimation of the percentage of intercepted radiation that is 
       scattered downwards.  This percentage is assumed to be in a 
       constant relation to the cosine of the solar zenith angle.
       For the visible wavelengths a fraction of 0.4*cos(ZENANG) is 
       assumed and for NIR the fraction is 0.54*cos(ZENANG).
       With this information the potential total visible radiation
       and NIR (POTVIS and POTNIR) can be calculated.

   4)  The ratio of total actual solar radiation and the potential 
       solar radiation, RATIO, can now be calculated and, assuming 
       that the wavelength distribution remains the same for the 
       potential and the actual radiation, the contribution of 
       visible wavelengths in the total actual radiation can be 
       estimated (RADABV).

  5)  Alternatively, instead of calculating RATIO for each hour, a 
      fixed input value RATIOD can be assigned to RATIO (index IPOT=1!)
      and RADABV for visible light and NIR calculated from this value.

  7)  Finally, the fraction of direct beam radiation in the total 
      radiation for the visible light and NIR is calculated for 
      potential radiation (FB1 and FB2), as well as for the actual 
      radiation ( FBEAM1(1,IHR) and FBEAM1(2,IHR)) using an 
      empirical equation that is a function of FB1 and FB2, and 
      RATIO.