C **********************************************************************
C *                                                                    *
C *                                SOFTWARE LICENSING                  *
C *                                                                    *
C *                This program is free software; you can redistribute *
C *                it and/or modify it under the terms of the GNU      *
C *                General Public License as published by the Free     *
C *                Software Foundation, either Version 2 of the        *
C *                license, or (at your option) any later version.     *
C *                                                                    *
C *                This program is distributed in the hope that it     *
C *                will be useful, but without any warranty; without   *
C *                even the implied warranty of merchantability or     *
C *                fitness for a particular purpose. See the GNU       *
C *                General Public License for more details.            *
C *                                                                    *
C *                A copy of the GNU General Public License is         *
C *                available at http://www.gnu.org/copyleft/gpl.html   *
C *                or by writing to the Free Software Foundation, Inc.,*
C *                59 Temple Place - Suite 330, Boston, MA 02111, USA. *
C *                                                                    *
C **********************************************************************

      Subroutine NCLD(YDAY,ALAT,ALON,SWOBS,ATC,CLD,CUTOFF,RAD)
c
c Subroutine to calculate percent cloud cover based on observed
c shortwave radiation and day of the year.  Converted from the
c original program to a subroutine. -hlj-        June 21, 1993.
c
c ORIGINAL COMMENTS ------------------------------------------------
c n_cldfrac.f       A.Oien/A. Plueddeman May 19, 1992
c
c This program estimates the daytime cloud fraction based on the
c ratio of observed to theoretical shortwave radiation.
c No estimate can be made at night, and the cloud fraction is set
c to a flag value.  It is assumed that the nighttime values will
c later be interpolated.  The program is similar to n_clearsky
c except that here the timebase and observed shortwave (with
c nighttime bias removed) are read in from the met data file.  
c Output data is handled more nicely, written to user-defined 
c output data file.
c Values for ATC and CUTOFF are determined empirically from
c iterations of n_clearsky and this program.  See n_clearsky and
c n_cloud for further description of ATC and CUTOFF.
c ORIGINAL COMMENTS ------------------------------------------------
c
c Since this subroutine will be run concurrently with the numerical
c circulation model, cloud cover is persisted through the night using
c the last cloud cover value obtained before cutoff was reached,
c rather than interpolated over the night. -hlj-
c
c ORIGINAL COMMENTS ------------------------------------------------
c 6-23-92
c Modified from n_clearsky to use time series from met data 
c file rather than rather than calculating from start time and
c increment.  Renamed TIME(K) array to yday(k) array to avoid 
c confusion with time variable in cloud subroutine. 
c ORIGINAL COMMENTS ------------------------------------------------

c     set lat, lon of observations, compute lon-based time correction
c     ALAT = 59.5935
c     ALON = 20.9642
      COR = ALON / 360.

c     set desired ATC and CUTOFF
c     started with ATC=.83 , CUTOFF=.05 as with n_clearsky,
c     final version uses ATC=0.77, CUTOFF=0.10 
      ATC = 0.60       !changed over phone, Ji & Signell, 12/27/93
      CUTOFF = 0.05    !Ji & Signell, 12/27/93

C   be careful of sign of COR here (alon is positive W for this convention)

      TD = YDAY - (COR)

CMNT  CORRECT LOCAL TIME TO TRUE SOLAR TIME USING
CMNT  FIT TO TIME EQUATION DATA PROVIDED ON
CMNT  SMITHSONIAN MET TABLES, PAGE 495

      TEQN = -5.5 - 9. * COS((TD-45.6)*(6.283185/182.5))
      TEQN = TEQN / (24.*60.)
      TD = TD + TEQN
      Call N_CLOUD(CUTOFF,ALAT,TD,SWOBS,RAD,ATC,CLD)
      Return
      End