add option to allow zero obs in the obsfile (#179)

src/model_specific/mom6/common_obs_mom6.f90.kdtree

@@ -621,19 +621,23 @@ END SUBROUTINE monit_dep
 !-----------------------------------------------------------------------
 ! Basic modules for observation input
 !-----------------------------------------------------------------------
-SUBROUTINE get_nobs(cfile,nrec,nn)
+SUBROUTINE get_nobs(cfile,nrec,nn,errIfNoObs)
   CHARACTER(*),INTENT(IN) :: cfile
   INTEGER,INTENT(IN) :: nrec
   INTEGER,INTENT(OUT) :: nn
+  LOGICAL,INTENT(IN),OPTIONAL :: errIfNoObs
   REAL(r_sngl),ALLOCATABLE :: wk(:) 
   INTEGER :: ios
   INTEGER :: iu,iv,it,is,issh,ieta,isst,isss,ix,iy,iz !(OCEAN)
   INTEGER :: nprof !(OCEAN)
   REAL(r_sngl) :: lon_m1, lat_m1
   INTEGER :: iunit
-  LOGICAL :: ex
+  LOGICAL :: ex, errIfNoObs_
   LOGICAL, PARAMETER :: dodebug=.false.
 
+  errIfNoObs_ = .true.
+  if (PRESENT(errIfNoObs)) errIfNoObs_ = errIfNoObs
+
   ALLOCATE(wk(nrec))
   nn = 0
   iu = 0
@@ -716,8 +720,10 @@ SUBROUTINE get_nobs(cfile,nrec,nn)
   DEALLOCATE(wk)
 
   if (nn .eq. 0) then
-    WRITE(6,*) "get_nobs:: No observations have been found. Exiting..."
-    STOP (60)
+    if (errIfNoObs_) then
+        WRITE(6,*) "get_nobs:: No observations have been found. Exiting..."
+        STOP (60)
+    endif
   endif
 
 END SUBROUTINE get_nobs

src/obs/obsop_sst_viirs.f90

@@ -108,10 +108,10 @@ PROGRAM obsop_sst_viirs
 
   if (BINARY_INPUT) then ![processed binary input]
      print *, "obsop_sst_viirs.f90:: reading binary file=",trim(obsinfile)
-     CALL get_nobs(trim(obsinfile),8,nobs)
+     CALL get_nobs(trim(obsinfile),8,nobs,errIfNoObs=.false.)
 
-     if (nobs<0) then
-        print*, "obsop_sst_viirs.f90: nobs<=0. Exit now..."
+     if (nobs<=0) then
+        print*, "obsop_sst_viirs.f90: nobs=",nobs,"<=0. Exit now..."
         STOP (0)
      endif