pro zensun,day,time,lat,lon,zenith,azimuth,solfac ;+ ; ROUTINE: zensun ; ; USEAGE: zensun,day,time,lat,lon,zenith,azimuth,solfac ; ; INPUT: ; day Julian day (positive scalar or vector) ; ; time Universal Time in hours (scalar or vector) ; ; lat geographic latitude of point on earth's surface (degrees) ; ; lon geographic longitude of point on earth's surface (degrees) ; ; OUTPUT: ; ; zenith solar zenith angle (degrees) ; ; azimuth solar azimuth (degrees) ; Azimuth is measured counter-clockwise from due south ; ; solfac Solar flux multiplier. ; SOLFAC=cosine(ZENITH)/RSUN^2 ; where rsun is the current earth-sun distance in ; astronomical units. ; ; NOTE: SOLFAC is negative when the sun is below the horizon ; ; PURPOSE: Compute the solar zenith and azimuth angles and solar flux ; multiplier for a given location, time and day. ; ; EXAMPLE: Compute the solar flux at Palmer Station for day 283 ; ; time=findgen(1+24*60)/60 ; zensun,283,time,-64.767,-64.067,z,a,sf ; solflx=sf*s ; plot,time,solflx ; ; where s is the TOA solar irradiance at normal incidence: ; ; s=1618.8 ; W/m2/micron for AVHRR1 GTR100 ; s= 976.9 ; W/m2/micron for AVHRR2 GTR100 ; s=1685.1 ; W/m2/micron for 410nm GTR100 ; s= 826.0 ; W/m2/micron for 936nm GTR100 ; s=1.257e17 ; photons/cm2/s PAR GTR100 ; s=1372.9 ; w/m2 total broadband ; ; PROCEDURE: ; 1. Calculate the subsolar point latitude and longitude, based on ; DAY and TIME. Since each year is 365.25 days long the exact ; value of the declination angle changes from year to year. For ; precise values consult THE AMERICAN EPHEMERIS AND NAUTICAL ; ALMANAC published yearly by the U.S. govt. printing office. ; The subsolar coordinates used in this code were provided by ; a program written by Jeff Dozier. ; ; 2. Given the subsolar latitude and longitude, spherical geometry is ; used to find the solar zenith, azimuth and flux multiplier. ; ; AUTHOR: Paul Ricchiazzi 23oct92 ; Earth Space Research Group, UCSB ; ; SOURCE: This routine was adapted from the LOWTRAN (vers7) subroutine ; SUBSOL.F. Some corrections were required ; ;- ; eqt = equation of time (minutes) ; solar longitude correction = -15*eqt ; dec = declination angle (degrees) = solar latitude ; ; LOWTRAN v7 data (25 points) ; The LOWTRAN solar position data is characterized by only 25 points. ; This should predict the subsolar angles within one degree. For ; increased accuracy add more data points. ; ;nday=[ 1., 9., 21., 32., 44., 60., 91., 121., 141., 152.,$ ; 160., 172., 182., 190., 202., 213., 244., 274., 305., 309.,$ ; 325., 335., 343., 355., 366.] ; ;eqt=[ -3.23, -6.83,-11.17,-13.57,-14.33,-12.63, -4.2, 2.83, 3.57, 2.45,$ ; 1.10, -1.42, -3.52, -4.93, -6.25, -6.28,-0.25, 10.02, 16.35, 16.38,$ ; 14.3, 11.27, 8.02, 2.32, -3.23] ; ;dec=[-23.07,-22.22,-20.08,-17.32,-13.62, -7.88, 4.23, 14.83, 20.03, 21.95,$ ; 22.87, 23.45, 23.17, 22.47, 20.63, 18.23, 8.58, -2.88,-14.18,-15.45,$ ; -19.75,-21.68,-22.75,-23.43,-23.07] ; ; Analemma information from Jeff Dozier ; This data is characterized by 74 points ; if n_params() eq 0 then begin doc_library,'zensun' return endif nday=[ 1.0, 6.0, 11.0, 16.0, 21.0, 26.0, 31.0, 36.0, 41.0, 46.0,$ 51.0, 56.0, 61.0, 66.0, 71.0, 76.0, 81.0, 86.0, 91.0, 96.0,$ 101.0, 106.0, 111.0, 116.0, 121.0, 126.0, 131.0, 136.0, 141.0, 146.0,$ 151.0, 156.0, 161.0, 166.0, 171.0, 176.0, 181.0, 186.0, 191.0, 196.0,$ 201.0, 206.0, 211.0, 216.0, 221.0, 226.0, 231.0, 236.0, 241.0, 246.0,$ 251.0, 256.0, 261.0, 266.0, 271.0, 276.0, 281.0, 286.0, 291.0, 296.0,$ 301.0, 306.0, 311.0, 316.0, 321.0, 326.0, 331.0, 336.0, 341.0, 346.0,$ 351.0, 356.0, 361.0, 366.0] eqt=[ -3.23, -5.49, -7.60, -9.48,-11.09,-12.39,-13.34,-13.95,-14.23,-14.19,$ -13.85,-13.22,-12.35,-11.26,-10.01, -8.64, -7.18, -5.67, -4.16, -2.69,$ -1.29, -0.02, 1.10, 2.05, 2.80, 3.33, 3.63, 3.68, 3.49, 3.09,$ 2.48, 1.71, 0.79, -0.24, -1.33, -2.41, -3.45, -4.39, -5.20, -5.84,$ -6.28, -6.49, -6.44, -6.15, -5.60, -4.82, -3.81, -2.60, -1.19, 0.36,$ 2.03, 3.76, 5.54, 7.31, 9.04, 10.69, 12.20, 13.53, 14.65, 15.52,$ 16.12, 16.41, 16.36, 15.95, 15.19, 14.09, 12.67, 10.93, 8.93, 6.70,$ 4.32, 1.86, -0.62, -3.23] dec=[-23.06,-22.57,-21.91,-21.06,-20.05,-18.88,-17.57,-16.13,-14.57,-12.91,$ -11.16, -9.34, -7.46, -5.54, -3.59, -1.62, 0.36, 2.33, 4.28, 6.19,$ 8.06, 9.88, 11.62, 13.29, 14.87, 16.34, 17.70, 18.94, 20.04, 21.00,$ 21.81, 22.47, 22.95, 23.28, 23.43, 23.40, 23.21, 22.85, 22.32, 21.63,$ 20.79, 19.80, 18.67, 17.42, 16.05, 14.57, 13.00, 11.33, 9.60, 7.80,$ 5.95, 4.06, 2.13, 0.19, -1.75, -3.69, -5.62, -7.51, -9.36,-11.16,$ -12.88,-14.53,-16.07,-17.50,-18.81,-19.98,-20.99,-21.85,-22.52,-23.02,$ -23.33,-23.44,-23.35,-23.06] ; ; compute the subsolar coordinates ; dd=((fix(day)+time/24.-1.) mod 365.25) +1. eqtime=interpol(eqt,nday,dd)/60. decang=interpol(dec,nday,dd) latsun=decang lonsun=-15.*(time-12.+eqtime) ; ; compute the solar zenith, azimuth and flux multiplier ; t0=(90.-lat)*!dtor ; colatitude of point t1=(90.-latsun)*!dtor ; colatitude of sun p0=lon*!dtor ; longitude of point p1=lonsun*!dtor ; longitude of sun zz=cos(t0)*cos(t1)+sin(t0)*sin(t1)*cos(p1-p0) ;} normalized solar yy=sin(t1)*sin(p1-p0) ;} coodinates in rotated xx=cos(t0)*sin(t1)*cos(p1-p0)-sin(t0)*cos(t1) ;} coordinate system azimuth=-atan(yy,xx)/!dtor ; solar azimuth zenith=acos(zz)/!dtor ; solar zenith ; rsun=1.-0.01673*cos(.9856*(dd-2.)*!dtor) ; earth-sun distance in AU solfac=zz/rsun^2 ; flux multiplier return end