m_insitu.F90

! File:          m_insitu.F90
!
! Created:       6 Feb 2008
!
! Last modified: 13 Feb 2008
!
! Author:        Pavel Sakov
!                NERSC
!
! Purpose:       The code to deal with insitu observations.
!
! Description:   This module contains the following subroutines:
!                  - insitu_setprofiles
!                      breaks the measurements into profiles and returns
!                      arrays of start and end indices for each profile
!                  - insitu_writeprofiles
!                      writes profiles to a netCDF file
!                  - insitu_prepareobs
!                      sorts out the measurements within profiles so they
!                      go in surface to bottom order and thins the measurements
!                      by keeping max 1 measurements per layer of the first
!                      ensemble member
!                It also contains the following data:
!                  nprof
!                    - the number of profiles
!                  pstart(nprof)
!                    - start indices for each profile in the array "obs" of
!                      type(measurement) stored in module m_obs
!                  pend(nprof)
!                    - end indices for each profile
!
! Modifications:
!                30/7/2010 PS: added profile pivot points to profile output
!                  files (SAL.nc etc.)
!                29/7/2010 PS: some rather minor changes, including interface
!                  of insitu_writeforecast()
!                13/02/2008 PS: added insitu_writeprofiles()
!                26/02/2008 PS: put "nprof", "pstart" and "pend" as public data
!                  in this module
!                20/04/2008 PS: added insitu_QC() and insitu_writeforecast()
!                29/07/2010 PS: removed insitu_QC(). There is a generic obs QC
!                  procedure in m_obs.F90 now.

module m_insitu
  use mod_measurement
  use m_parse_blkdat
  use m_get_mod_fld
#if defined (QMPI)
   use qmpi
#else
   use qmpi_fake
#endif
  implicit none

  !
  ! public stuff
  !
  integer, allocatable, dimension(:), public :: pstart
  integer, allocatable, dimension(:), public :: pend
  integer, public :: nprof

  public insitu_setprofiles
  public insitu_prepareobs
  public insitu_writeprofiles

  !
  ! private stuff
  !

  real, parameter, private :: ONEMETER = 9806.0
  integer, parameter, private :: STRLEN = 512

  ! The portion of the layer thickness at which the variability in
  ! vertical data will be used for estimating the vertical representativeness
  ! error.
  !
  real, parameter, private :: VARCOEFF1 = 0.15
  
  ! A factor by which a calculated vertical representativeness error variance
  ! will be reduced if the data is in different layers
  !
  real, parameter, private :: VARCOEFF2 = 2.0

  ! Write information about this profile. Set to < 1 to switch off.
  !
  integer, parameter, private :: PDEBUGINFO = 0

  ! Integers used to tag the fields (to avoid parsing the string tags)
  !
  integer, parameter, private :: NONE = 0
  integer, parameter, private :: TEMPERATURE = 1
  integer, parameter, private :: SALINITY = 2

  real, parameter, private :: TEM_MIN = -2.0
  real, parameter, private :: TEM_MAX = 35.0
  real, parameter, private :: SAL_MIN = 5.0
  real, parameter, private :: SAL_MAX = 41.0

  ! Maximum allowed deviation between the observation and ensemble mean in
  ! terms of combined standard deviation.
  !
  real, parameter, private :: SAL_MAXRATIO = 10.0
  real, parameter, private :: TEM_MAXRATIO = 5.0

  ! If an observation is not considered an outlier, the observation error
  ! variance is modified so that the distance between the observation and the
  ! endemble mean is within DIST_MAX * sqrt(sigma_obs^2 + sigma_ens^2).
  ! Bigger values of DIST_MAX result in a more violent assimilation.
  !
  real, parameter, private :: DIST_MAX = 2.0

contains

  ! Work out the number of profiles, each identified by "obs % i_orig_grid"
  ! and return start id of the first and the last obs in the profile in
  ! arrays "pstart" and "pend". "pstart" and "pend" are publicly available
  ! arrays stored in this module.
  !
  subroutine insitu_setprofiles(obstag, nobs, obs)
    character(*), intent(in) :: obstag
    integer, intent(in) :: nobs
    type(measurement), dimension(:), intent(inout) :: obs

    integer, allocatable, dimension(:) :: tmp, tmp1
    integer :: o, o1, o2, p, nobsp
    type(measurement), allocatable, dimension(:) :: tmpobs

    if (nobs == 0) then
       return
    end if

    if (allocated(pstart)) then
       deallocate(pstart)
       deallocate(pend)
    end if

    ! find the very first obs of the right kind
    !
    o1 = 1
    do while (trim(obs(o1) % id) /= trim(obstag) .and. o1 <= nobs)
       o1 = o1 + 1
    end do

    if (o1 > nobs) then
       return
    end if

    ! find the very last obs of the right kind
    !
    o2 = nobs
    do while (trim(obs(o2) % id) /= trim(obstag) .and. o2 >= 0)
       o2 = o2 - 1
    end do

    nprof = 1
    do o = 2, o2
       if (obs(o) % ipiv /= obs(o - 1) % ipiv .or.&
            obs(o) % jpiv /= obs(o - 1) % jpiv .or.&
            obs(o) % date /= obs(o - 1) % date) then
          nprof = nprof + 1
       end if
    end do

    allocate(pstart(nprof))
    allocate(pend(nprof))

    ! identify profiles
    !
    ! PS: This is a tricky cycle but it seems it is doing the job. Do not
    ! meddle with it.
    !
    pend = 0
    nprof = 1
    pstart(1) = o1
    do o = o1, o2
       ! find obs from the same profile
       !
       if (trim(obs(o) % id) == trim(obstag) .and.&
            ((obs(o) % i_orig_grid > 0 .and.&
            obs(o) % i_orig_grid == obs(pstart(nprof)) % i_orig_grid) .or.&
            (obs(o) % i_orig_grid <= 0 .and.&
            obs(o) % ipiv == obs(pstart(nprof)) % ipiv .and.&
            obs(o) % jpiv == obs(pstart(nprof)) % jpiv .and.&
            obs(o) % date == obs(pstart(nprof)) % date))) then
          pend(nprof) = o
          cycle
       end if

       if (trim(obs(o) % id) /= trim(obstag)) then
          print *, 'ERROR: insitu_setprofiles(): obs id does not match processed obs tag'
          stop
       end if

       ! if there were no obs of the right type in this profile yet,
       ! then pend(nprof) has not been set yet and therefore the condition
       ! below will yield "false"
       !
       if (pend(nprof) >= pstart(nprof)) then
          nprof = nprof + 1
       end if

       if (PDEBUGINFO > 0) then
          print *, '  DEBUG: new profile #', nprof, ', o =', o, ', id =', obs(o) % i_orig_grid
       end if
       pstart(nprof) = o
       pend(nprof) = o
    end do
    if (pend(nprof) < pstart(nprof)) then
       nprof = nprof - 1
    end if

    ! truncate "pstat" and "pend" to length "nprof"
    !
    allocate(tmp(nprof))
    tmp = pstart(1 : nprof)
    deallocate(pstart)
    allocate(pstart(nprof))
    pstart = tmp
    tmp = pend(1 : nprof)
    deallocate(pend)
    allocate(pend(nprof))
    pend = tmp
    deallocate(tmp)

    ! for glider data - sort observations in each profile by increasing depth
    !
    if ( trim(obstag) == 'GSAL' .or. trim(obstag) == 'GTEM' &
    .or. trim(obstag) == 'GNIT' .or. trim(obstag) == 'GCHL' &
    .or. trim(obstag) == 'GOXY' &
       ) then
       allocate(tmp(nobs))
       allocate(tmp1(nobs))
       allocate(tmpobs(nobs))
       do p = 1, nprof
          nobsp = pend(p) - pstart(p) + 1
          do o = 1, nobsp
             tmp(o) = o
          end do
          !
          ! (using procedure from pre_local_analysis())
          !
          call order(dble(nobsp), obs(pstart(p) : pend(p)) % depth,&
               dble(nobsp), tmp, tmp1)
          tmpobs(1 : nobsp) = obs(pstart(p) : pend(p))
          do o = 1, nobsp
             obs(pstart(p) + o - 1) = tmpobs(tmp1(o))
          end do
       end do
       deallocate(tmp, tmp1, tmpobs)
    end if
  end subroutine insitu_setprofiles


  ! 1. Sort out the obs within profiles so that they are stored in order of
  !    increasing depth.
  ! 2. Thin observations by keeping a single obs within a layer using the
  !    layers from the first ensemble member
  !
  subroutine insitu_prepareobs(obstag, nobs, obs)
    character(*), intent(in) :: obstag
    integer, intent(inout) :: nobs
    type(measurement), dimension(:), intent(inout) :: obs

    ! profiles
    !
    integer, allocatable, dimension(:) :: pnow
    integer :: nobs_max

    integer :: p, o
    type(measurement), allocatable, dimension(:) :: profile

    integer, allocatable, dimension(:) :: ipiv, jpiv
    real, allocatable, dimension(:) :: a1, a2, a3, a4
    real, allocatable, dimension(:) :: z1, z2

    integer :: nrev
    integer :: ndel
    integer :: oo
    real :: rdummy
    integer :: k, nk, ni, nj
    character(STRLEN) :: fname
    integer :: tlevel
    real, allocatable, dimension(:, :) :: dz2d
    real, dimension(2, 2) :: dz_cell
    real :: dz, zcentre
    integer :: best
    logical :: isrogue

    ! As we thin the measurements within each layer, it still may be a good
    ! idea to update the obs error variance if the variability within the layer
    ! is big enough. `dmin' and `dmax' give the min and max measured values
    ! within the layer.
    !
    real :: dmin, dmax
    real :: var1, var2

    integer :: nobsnew, nobs_thistype, nobs_othertype
    
    if (master) then
       print '(a, a, a)', '   insitu_prepareobs(', trim(obstag), '):'
       print '(a, i6)', '     total # of obs = ', nobs
    end if

    if (nobs == 0) then
       return
    end if

    call insitu_setprofiles(trim(obstag), nobs, obs)

    if (master) then
       print '(a, a, a, i6)', '     # of obs of type "', trim(obstag), '" = ',&
            sum(pend(1 : nprof) - pstart(1 : nprof)) + nprof
       print '(a, i4)', '     # of profiles = ', nprof
    end if

    ! find the maximal # of obs in a single profile
    !
    nobs_max = 0
    do p = 1, nprof
       nobs_max = max(nobs_max, pend(p) - pstart(p) + 1)
    end do

    if (master) then
       print '(a, i4)', '     max # of obs in a profile before thinning = ', nobs_max
    end if

    ! reverse the obs in profiles that go from bottom to surface
    !
    allocate(profile(nobs_max))
    nrev = 0
    do p = 1, nprof
       if (obs(pstart(p)) % depth > obs(pend(p)) % depth) then
          
          profile(1 : pend(p) - pstart(p) + 1) = obs(pstart(p) : pend(p))
          do o = 0, pend(p) - pstart(p)
             obs(pstart(p) + o) = profile(pend(p) - o)
          end do
          nrev = nrev + 1
       end if
    end do
    deallocate(profile)

    if (nrev > 0 .and. master) then
       print *, '  ', nrev, ' profile(s) reversed'
    end if

    ! check for rogue obs
    !
    ndel = 0
    do p = 1, nprof
       isrogue = .false. 
       do o = pstart(p) + 1, pend(p)

          ! shift the remaining obs in this profile one obs down
          !
          if (obs(o) % depth <= obs(o - 1) % depth) then
             isrogue = .true. 
             do oo = o + 1, pend(p)
                obs(oo - 1) = obs(oo)
             end do
             ndel = ndel + 1
             pend(p) = pend(p) - 1
          end if
       end do
       if (isrogue .and. master) then 
          print *, '  a rogue obs detected in profile # ', p 
       end if  
    end do

    if (ndel > 0 .and. master) then
       print *, '  ', ndel, 'rogue obs deleted'
    end if

    !
    ! Now to the thinning of the profiles.
    !

    allocate(ipiv(nprof))
    allocate(jpiv(nprof))
    allocate(a1(nprof))
    allocate(a2(nprof))
    allocate(a3(nprof))
    allocate(a4(nprof))

    ipiv = obs(pstart(1 : nprof)) % ipiv
    jpiv = obs(pstart(1 : nprof)) % jpiv
    a1 = obs(pstart(1 : nprof)) % a1
    a2 = obs(pstart(1 : nprof)) % a2
    a3 = obs(pstart(1 : nprof)) % a3
    a4 = obs(pstart(1 : nprof)) % a4

    ! get the grid dimensions
    !
    call parse_blkdat('kdm   ','integer', rdummy, nk)
    call parse_blkdat('idm   ','integer', rdummy, ni)
    call parse_blkdat('jdm   ','integer', rdummy, nj)

    ! get the data file name
    !
    if (  trim(obstag) /= 'SAL' .and. trim(obstag) /= 'GSAL' &
    .and. trim(obstag) /= 'TEM' .and. trim(obstag) /= 'GTEM' &
    .and. trim(obstag) /= 'CHL' .and. trim(obstag) /= 'GCHL' &
    .and. trim(obstag) /= 'NIT' .and. trim(obstag) /= 'GNIT' &
    .and. trim(obstag) /= 'OXY' .and. trim(obstag) /= 'GOXY' &
    .and. trim(obstag) /= 'SIL' &
    .and. trim(obstag) /= 'PHO' &
       ) then
       print *, 'ERROR: get_S(): unknown observation tag "', trim(obstag), '"'
       stop
    end if
    fname = 'forecast001'

    allocate(z1(nprof))
    allocate(z2(nprof))
    allocate(pnow(nprof))
    allocate(dz2d(ni, nj))

    ! data thinning cycle
    !
    if (master) then
       print *, '  maximum one observation per layer will be retained after thinning'
    end if
    tlevel = 1
    z1 = 0.0
    pnow = pstart
    if (master .and. PDEBUGINFO > 0) then
       p = PDEBUGINFO
       print *, 'DEBUG dumping the info for profile #', p
       print *, 'DEBUG   p =', p, ': lon =', obs(pstart(p)) % lon, ', lat =', obs(pstart(p)) % lat
       print *, 'DEBUG now dumping the layer depths:'
    end if

    ! mark all obs of this type as bad; unmask the best obs within a layer
    !
    do o = 1, nobs
       if (trim(obs(o) % id) == trim(obstag)) then
          obs(o) % status = .false.
       end if
    end do
    do k = 1, nk
       call get_mod_fld_new(trim(fname), dz2d, 1, 'dp      ', k, tlevel, ni, nj,0)
       do p = 1, nprof
          dz_cell(:, :) = dz2d(ipiv(p) : ipiv(p) + 1, jpiv(p) : jpiv(p) + 1)
          dz = dz_cell(1, 1) * a1(p) + dz_cell(2, 1) * a2(p)&
               + dz_cell(1, 2) * a3(p) + dz_cell(2, 2) * a4(p)
          dz = dz / ONEMETER
          z2(p) = z1(p) + dz
          zcentre = (z1(p) + z2(p)) / 2.0
          best = -1
          dmin = 1.0d+10
          dmax = -1.0d+10
          if (master .and. PDEBUGINFO > 0 .and. p == PDEBUGINFO) then
             print *, 'DEBUG   p =', p, ', k =', k, ', z =', z1(p), '-', z2(p)
          end if
          do while (pnow(p) <= pend(p))
             o = pnow(p)
             
             ! check that the depth is within the layer
             !
             if (obs(o) % depth > z2(p)) then
                ! go to next profile; this obs will be dealt with when
                ! processing the next layer
                exit
             end if

             ! from this point on, the obs counter will be increased at the
             ! end of this loop

             ! store profile and layer number (overwrite the original profile
             ! id and vertical counter value)
             !
             obs(o) % i_orig_grid = p
             obs(o) % j_orig_grid = k
             obs(o) % h = z2(p) - z1(p)

             if (obs(o) % depth < z1(p)) then
                pnow(p) = pnow(p) + 1
                cycle ! next obs
             end if

             ! update `dmin' and `dmax'
             !
             dmin = min(dmin, obs(o) % d)
             dmax = max(dmax, obs(o) % d)

             if (best < 1) then
                best = o
                obs(best) % status = .true.
             else if (abs(obs(o) % depth - zcentre) < abs(obs(best) % depth - zcentre)) then
                obs(best) % status = .false. ! thrash the previous best obs
                best = o
                obs(best) % status = .true.
             end if
             pnow(p) = pnow(p) + 1
          end do ! o

          ! update the observation error variance if the difference between
          ! `dmin' and `dmax' is big enough
          !
          if (best < 1) then
             cycle
          end if

          if (.false.) then ! out for now; use the closest obs instead
             if (dmax - dmin > 0) then
                obs(best) % var = sqrt(obs(best) % var + ((dmax - dmin) / 2) ** 2)
             end if
          end if
       end do ! p
       z1 = z2
    end do ! k

    ! There are a number of ways the vertical variability can be
    ! used for updating the obs error variance.
    !
    ! Below, the following approach is used.
    !
    ! Calculate two estimates for vertical gradient using the closest data
    ! points (if available). Estimate the difference at (VARCOEFF1 * h)
    ! vertical distance from the current obs, where VARCOEFF1 is the portion
    ! of the layer thickness (typically around 0.1-0.3), and h is the layer
    ! thickness. Use the square of this difference as an estimate for the
    ! respresentation error variance. If the closest obs is in another layer
    ! -- decrease this estimate by a factor of VARCOEFF2 (typically around 2).
    ! Use the largest estimate between the two (when both are avalaible).
    !
     do p = 1, nprof
       do o = pstart(p), pend(p)
          k = obs(o) % j_orig_grid
          if (obs(o) % status) then
             var1 = -999.0
             var2 = -999.0
             if (o - 1 >= pstart(p)) then
                var1 = ((obs(o) % d - obs(o - 1) % d) /&
                     (obs(o) % depth - obs(o - 1) % depth) * obs(o) % h * VARCOEFF1) ** 2
                if (obs(o - 1) % j_orig_grid /= k) then
                   var1 = var1 / VARCOEFF2
                end if
             end if
             if (o + 1 <= pend(p)) then
                var2 = ((obs(o) % d - obs(o + 1) % d) /&
                     (obs(o) % depth - obs(o + 1) % depth) * obs(o) % h * VARCOEFF1) ** 2
                if (obs(o + 1) % j_orig_grid /= k) then
                   var2 = var2 / VARCOEFF2
                end if
             end if
             if (var1 < 0.0 .and. var2 < 0.0) then
                cycle
             end if
             obs(o) % var = obs(o) % var + max(var1, var2)
          end if
       end do
    end do

    if (master .and. PDEBUGINFO > 0) then
       p = PDEBUGINFO
       print *, 'DEBUG now dumping the obs info:'
       do o = pstart(p), pend(p)
          print *, 'DEBUG   o =', o, ', status =', obs(o) % status, &
               ', d =', obs(o) % d, ', z =', obs(o) % depth,&
               ', k =', obs(o) %  j_orig_grid, ',  h =', obs(o) % h,&
               ', var =', obs(o) % var
       end do
    end if

    deallocate(dz2d)
    deallocate(pnow)
    deallocate(z2)
    deallocate(z1)
    deallocate(a4)
    deallocate(a3)
    deallocate(a2)
    deallocate(a1)
    deallocate(jpiv)
    deallocate(ipiv)

    ! now compact the obs array
    !
    nobsnew = 0
    nobs_thistype = 0
    nobs_othertype = 0
    do o = 1, nobs
       if (obs(o) % status) then
          nobsnew = nobsnew + 1
          obs(nobsnew) = obs(o)
          if (trim(obs(o) % id) == trim(obstag)) then
             nobs_thistype = nobs_thistype + 1
          else
             nobs_othertype = nobs_othertype + 1
          end if
       end if
    end do
    obs(nobsnew + 1 : nobs) % status = .false.
    nobs = nobsnew

    ! replace the original profiles by the thinned ones
    !
    call insitu_setprofiles(trim(obstag), nobs, obs)

    if (master) then
       print *, '  thinning completed:', nobs_thistype, ' "', trim(obstag), '" obs retained'
       if (nobs_othertype > 0) then
          print *, '  ', nobs_othertype, 'obs of other type(s) retained'
       end if
    end if
  end subroutine insitu_prepareobs


  ! Write profiles to a NetCDF file
  !
  subroutine insitu_writeprofiles(fname, obstag, nobs, obs)
    use nfw_mod
    
    character(*), intent(in) :: fname
    character(*), intent(in) :: obstag
    integer, intent(inout) :: nobs
    type(measurement), dimension(:), intent(inout) :: obs

    ! profiles
    !
    integer :: p
    integer :: npoints, npoints_max

    ! I/O
    !
    integer :: ncid
    integer :: nprof_id(1), nk_id(1), dids(2)
    integer :: lat_id, lon_id, ipiv_id, jpiv_id, npoints_id, depth_id, v_id, variance_id
    character(STRLEN) :: varname

    real(8), allocatable, dimension(:, :) :: v

    if (.not. allocated(pstart)) then
       call insitu_setprofiles(trim(obstag), nobs, obs)
    end if

    call nfw_create(fname, nf_write, ncid)

    call nfw_def_dim(fname, ncid, 'nprof', nprof, nprof_id(1))
    call nfw_def_var(fname, ncid, 'lat', nf_double, 1, nprof_id, lat_id)
    call nfw_def_var(fname, ncid, 'lon', nf_double, 1, nprof_id, lon_id)
    call nfw_def_var(fname, ncid, 'ipiv', nf_int, 1, nprof_id, ipiv_id)
    call nfw_def_var(fname, ncid, 'jpiv', nf_int, 1, nprof_id, jpiv_id)
    call nfw_def_var(fname, ncid, 'npoints', nf_int, 1, nprof_id, npoints_id)
    npoints_max = maxval(pend - pstart) + 1
    call nfw_def_dim(fname, ncid, 'nk', npoints_max, nk_id(1))
    dids(1) = nk_id(1)
    dids(2) = nprof_id(1)
    call nfw_def_var(fname, ncid, 'depth', nf_double, 2, dids, depth_id)
    if (trim(obstag) == 'SAL' .or. trim(obstag) == 'GSAL') then
       varname = 'salt'
    else if (trim(obstag) == 'TEM' .or. trim(obstag) == 'GTEM') then
       varname = 'temp'
    else if (trim(obstag) == 'CHL' .or. trim(obstag) == 'GCHL') then
       varname = 'chla'
    else if (trim(obstag) == 'NIT' .or. trim(obstag) == 'GNIT') then
       varname = 'ntra'
    else if (trim(obstag) == 'OXY' .or. trim(obstag) == 'GOXY') then
       varname = 'doxy'
    else
       varname = trim(obstag)
    end if
    call nfw_def_var(fname, ncid, trim(varname), nf_double, 2, dids, v_id)
    call nfw_def_var(fname, ncid, 'variance', nf_double, 2, dids, variance_id)

    call nfw_enddef(fname, ncid)

    call nfw_put_var_double(fname, ncid, lat_id, obs(pstart) % lat)
    call nfw_put_var_double(fname, ncid, lon_id, obs(pstart) % lon)
    call nfw_put_var_int(fname, ncid, ipiv_id, obs(pstart) % ipiv)
    call nfw_put_var_int(fname, ncid, jpiv_id, obs(pstart) % jpiv)
    call nfw_put_var_int(fname, ncid, npoints_id, pend - pstart + 1)

    ! depth
    !
    allocate(v(npoints_max, nprof))
    v = -999.0
    do p = 1, nprof
       npoints = pend(p) - pstart(p) + 1
       v(1 : npoints, p) = obs(pstart(p) : pend(p)) % depth
    end do
    call nfw_put_var_double(fname, ncid, depth_id, v)
    
    ! data
    !
    v = -999.0
    do p = 1, nprof
       npoints = pend(p) - pstart(p) + 1
       v(1 : npoints, p) = obs(pstart(p) : pend(p)) % d
    end do
    call nfw_put_var_double(fname, ncid, v_id, v)
    !call nfw_put_att_double(fname, ncid, v_id, '_FillValue',nf_double,1,-999.0)
    !nfw_put_att_double(fname, ncid, varid, attname, type, length, v)
    
    ! data error variance
    !
    v = -999.0
    do p = 1, nprof
       npoints = pend(p) - pstart(p) + 1
       v(1 : npoints, p) = obs(pstart(p) : pend(p)) % var
    end do
    call nfw_put_var_double(fname, ncid, variance_id, v)

    call nfw_close(fname, ncid)

    deallocate(v)
    deallocate(pstart)
    deallocate(pend)
  end subroutine insitu_writeprofiles


  ! This subroutine appends the interpolated ensemble mean and the ensemble
  ! error variance to the assimilated profile data SAL.nc or TEM.nc. It also
  ! overwrites the observation error variance with latest values.
  !
  subroutine insitu_writeforecast(obstag, nobs, nrens, S, obs)
    use nfw_mod
    implicit none

    character(*), intent(in) :: obstag
    integer, intent(in) :: nobs
    integer, intent(in) :: nrens
    real, dimension(nobs, nrens), intent(in) :: S
    type(measurement), dimension(nobs), intent(inout) :: obs
    
    character(STRLEN) :: fname
    real, dimension(nobs) :: Smean, Svar
    integer :: i, p

    integer :: ncid
    integer :: dids(2)
    integer :: v_id, variance_id
    integer :: npoints_max, npoints
    real(8), allocatable, dimension(:, :) :: v

    ! need to set profiles for the given observation type
    !
    call insitu_setprofiles(obstag, nobs, obs)

    write(fname, '(a, ".nc")') trim(obstag)
    print *, 'Appending interpolated forecast for "', trim(obstag),&
         '" to "', trim(fname), '"'

    Smean = sum(S, DIM = 2) / nrens
    Svar = 0.0
    do i = 1, nobs
       Svar(i) = sum((S(i, :) - Smean(i)) ** 2)
    end do
    Svar = Svar / real(nrens - 1)

    call nfw_open(fname, nf_write, ncid)
    
    call nfw_inq_dimid(fname, ncid, 'nk', dids(1))
    call nfw_inq_dimid(fname, ncid, 'nprof', dids(2))

    call nfw_redef(fname, ncid)

    call nfw_def_var(fname, ncid, 'forecast', nf_double, 2, dids, v_id)
    call nfw_def_var(fname, ncid, 'forecast_variance', nf_double, 2, dids, variance_id)

    call nfw_enddef(fname, ncid)

    npoints_max = maxval(pend - pstart) + 1
    allocate(v(npoints_max, nprof))

    v = -999.0
    do p = 1, nprof
       npoints = pend(p) - pstart(p) + 1
       v(1 : npoints, p) = Smean(pstart(p) : pend(p))
    end do
    call nfw_put_var_double(fname, ncid, v_id, v)

    v = -999.0
    do p = 1, nprof
       npoints = pend(p) - pstart(p) + 1
       v(1 : npoints, p) = Svar(pstart(p) : pend(p))
    end do
    call nfw_put_var_double(fname, ncid, variance_id, v)

    ! update observation error variance
    !
    call nfw_redef(fname, ncid)
    call nfw_rename_var(fname, ncid, 'variance', 'variance_orig')
    call nfw_def_var(fname, ncid, 'variance', nf_double, 2, dids, variance_id)
    call nfw_enddef(fname, ncid)

    v = -999.0
    do p = 1, nprof
       npoints = pend(p) - pstart(p) + 1
       v(1 : npoints, p) = obs(pstart(p) : pend(p)) % var
    end do
    call nfw_put_var_double(fname, ncid, variance_id, v)

    call nfw_close(fname, ncid)

    deallocate(v)
  end subroutine insitu_writeforecast

end module m_insitu