diff --git a/cloudnetpy/concat_lib.py b/cloudnetpy/concat_lib.py
index c471f344..5990a0cf 100644
--- a/cloudnetpy/concat_lib.py
+++ b/cloudnetpy/concat_lib.py
@@ -5,24 +5,30 @@
 from cloudnetpy import utils
 
 
-def update_nc(old_file: str, new_file: str):
-    """Appends data to daily netCDF file.
+def update_nc(old_file: str, new_file: str) -> int:
+    """Appends data to existing netCDF file.
 
     Args:
-        old_file: Filename of a daily netCDF file.
+        old_file: Filename of a existing netCDF file.
         new_file: Filename of a new file whose data will be appended to the end.
 
+    Returns:
+        1 = success, 0 = failed to add new data.
+
     Notes:
         Requires 'time' variable with unlimited dimension.
 
     """
+    success = 0
     nc_old = netCDF4.Dataset(old_file, 'a')
     nc_new = netCDF4.Dataset(new_file)
     valid_ind = _find_valid_time_indices(nc_old, nc_new)
     if len(valid_ind) > 0:
         _update_fields(nc_old, nc_new, valid_ind)
+        success = 1
     nc_new.close()
     nc_old.close()
+    return success
 
 
 def concatenate_files(filenames: list,
diff --git a/tests/unit/test_concat_lib.py b/tests/unit/test_concat_lib.py
index 6e5bd55f..23409b83 100644
--- a/tests/unit/test_concat_lib.py
+++ b/tests/unit/test_concat_lib.py
@@ -24,7 +24,8 @@ def run_before_and_after_tests(self):
 
     def test_does_append_to_end(self):
         concat_lib.concatenate_files(self.files[:2], self.filename, concat_dimension='profile')
-        concat_lib.update_nc(self.filename, self.files[2])
+        succ = concat_lib.update_nc(self.filename, self.files[2])
+        assert succ == 1
         nc = netCDF4.Dataset(self.filename)
         time = nc.variables['time'][:]
         assert len(time) == 3 * 12
@@ -33,7 +34,8 @@ def test_does_append_to_end(self):
 
     def test_does_not_append_to_beginning(self):
         concat_lib.concatenate_files(self.files[1:3], self.filename, concat_dimension='profile')
-        concat_lib.update_nc(self.filename, self.files[0])
+        succ = concat_lib.update_nc(self.filename, self.files[0])
+        assert succ == 0
         nc = netCDF4.Dataset(self.filename)
         time = nc.variables['time'][:]
         assert len(time) == 2 * 12
@@ -43,7 +45,8 @@ def test_does_not_append_to_beginning(self):
     def test_does_not_append_to_middle(self):
         files = [self.files[0], self.files[2]]
         concat_lib.concatenate_files(files, self.filename, concat_dimension='profile')
-        concat_lib.update_nc(self.filename, self.files[1])
+        succ = concat_lib.update_nc(self.filename, self.files[1])
+        assert succ == 0
         nc = netCDF4.Dataset(self.filename)
         time = nc.variables['time'][:]
         assert len(time) == 2 * 12