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Clamping the acos argument to [-1,1] to avoid NaN #1732

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Merged
BenjaminRuston merged 1 commit into from
Dec 11, 2025
Merged

Clamping the acos argument to [-1,1] to avoid NaN #1732

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19 changes: 16 additions & 3 deletions src/ncar/obs2ioda/src/hsd.f90
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Original file line number Diff line number Diff line change
Expand Up @@ -969,6 +969,8 @@ subroutine calc_solar_zenith_angle(xlat, xlon, gmt, minute, julian, solzen)
real(r_single) :: slon ! longitude of the sun
real(r_single) :: declin ! declination of the sun
real(r_single) :: hrang, da, eot, xt, tloctm, rlat
real(r_single) :: asin_arg, coszen_arg

! Check for valid hour (gmt), minute, and Julian day
if (gmt < 0 .or. gmt > 23) then
solzen = ieee_value(solzen, ieee_quiet_nan)
Expand All @@ -992,7 +994,11 @@ subroutine calc_solar_zenith_angle(xlat, xlon, gmt, minute, julian, solzen)
if ( julian >= 80 ) slon = (julian - 80 ) * deg_per_day
if ( julian < 80 ) slon = (julian + 285) * deg_per_day

declin = asin(sin(obliq*deg2rad)*sin(slon*deg2rad)) ! in radian
! compute ASIN argument and clamp to [-1,1] to prevent NaN
asin_arg = sin(obliq*deg2rad) * sin(slon*deg2rad)
if (asin_arg > 1.0) asin_arg = 1.0
if (asin_arg < -1.0) asin_arg = -1.0
declin = asin(asin_arg) ! in radian

da = 6.2831853071795862*(julian-1)/365.
eot = (0.000075+0.001868*cos(da)-0.032077*sin(da) &
Expand All @@ -1003,8 +1009,15 @@ subroutine calc_solar_zenith_angle(xlat, xlon, gmt, minute, julian, solzen)
tloctm = xt + xlon/15.0
hrang = 15.0*(tloctm-12.0) * deg2rad
rlat = xlat * deg2rad
solzen = acos( sin(rlat)*sin(declin) + &
cos(rlat)*cos(declin)*cos(hrang) )

coszen_arg = sin(rlat)*sin(declin) + &
cos(rlat)*cos(declin)*cos(hrang)

! clamp acos argument to [-1,1] to avoid NaN from tiny floating-point overshoot
if (coszen_arg > 1.0) coszen_arg = 1.0
if (coszen_arg < -1.0) coszen_arg = -1.0

solzen = acos(coszen_arg)
solzen = solzen * rad2deg

return
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