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-                      rb1674ed
+                      rf20342a
 *******************
 Vertical Coordinate
+Vertical wind
 *******************
 Calculation of vertical velocity and preparation of Output-files
+Calculation of vertical velocity and preparation of output files
 ================================================================
 ``flex_extract`` has two ways to calculate the vertical velocity for ``FLEXTRA``/``FLEXPART``:
+Two methods are provided in ``flex_extract`` for the calculation of the vertical velocity for ``FLEXTRA``/``FLEXPART``:
     (i) from the horizontal wind field,
     (ii) from the MARS parameter 77, which is available for operational forecasts and analyses since September 2008 and for reanalysis datasets **ERA5** and **CERA-20C**.
+    (ii) from the MARS parameter 77, which is available for operational forecasts and analyses since September 2008 and for reanalysis datasets **ERA5** and **CERA-20C**, which contains the vertical velocity directly in the eta coordinate system of the ECMWF model.
 Especially for high resolution data, use of the ``MARS`` parameter 77 is recommended,
 …
 Calculation of vertical velocity from horizontal wind using the continuity equation
+Calculation of the vertical velocity from the horizontal wind using the continuity equation
 ===================================================================================
+The vertical velocity is computed by the FORTRAN90 program ``calc_etadot`` in the ECMWF
+vertical coordinate system by applying the equation of continuity and thereby ensuring mass consistent 3D wind fields. A detailed description of ``calc_etadot`` can be found in the
+The vertical velocity in the ECMWF's eta vertical coordinate system is computed by the Fortran program ``calc_etadot``, using the continuity equation and thereby ensuring mass-consistent 3D wind fields. A detailed description of ``calc_etadot`` can be found in the
 documents v20_update_protocol.pdf, V30_update_protocol.pdf and
 V40_update_protocol.pdf. The computational demand and accuracy of ``calc_etadot`` is highly
 …
 following guidance can be given for choosing the right parameters:
     * For very fine output grids (0.25 degree or finer) the full resolution T799 or even T1279 of the operational model is required (``RESOL=799``, ``SMOOTH=0``). The highest available resolution (and the calculation of vertical velocity on the Gaussian grid (``GAUSS=1``) is, however, rather demanding and feasible only for resolutions up to T799. Higher resolutions are achievable on the HPC. If data retrieval at T1279  needs to be performed on *ecgate*, the computation of the vertical velocity is feasible only on the lat/lon grid (``GAUSS=0``), which also yields very good results. Please read document v20_update_protocol.pdf-v60_update_protocol.pdf to see if the errors incurred are acceptable for the planned application.
+    * For very fine output grids (0.25 degree or finer), the full resolution T799 or even T1279 of the operational model is required (``RESOL=799``, ``SMOOTH=0``). The highest available resolution (and the calculation of vertical velocity on the Gaussian grid (``GAUSS=1``) is, however, rather demanding and feasible only for resolutions up to T799. Higher resolutions are achievable on the HPC. If data retrieval at T1279  needs to be performed on *ecgate*, the computation of the vertical velocity is feasible only on the lat/lon grid (``GAUSS=0``), which also yields very good results. Please read document v20_update_protocol.pdf-v60_update_protocol.pdf to see if the errors incurred are acceptable for the planned application.
     * For lower resolution (often global) output grids, calculation of vertical velocities with lower than operational spectral resolution is recommended. For global grids the following settings appear optimal:
         - For 1.0 degree grids: ``GAUSS=1``, ``RESOL=255``, ``SMOOTH=179``
         - For 0.5 degree grids: ``GAUSS=1``, ``RESOL=399``, ``SMOOTH=359``
         - Calculation on the lat/lon grid is not recommended for less than the operational (T1279) resolution.
         - If ``GAUSS`` is set to 1, only the following choices are possible for ``RESOL`` on *ecgate*: 159,255,319,399,511,799, (on the HPC also 1279, 2047 in future models). This choice is restricted because a reduced Gaussian grid is defined in then ECMWF EMOSLIB only for these spectral resolutions. For ``GAUSS=0``, ``RESOL`` can be any value below the operational resolution.
         - For ``SMOOTH`` any resolution lower than ``RESOL`` is possible. If no smoothing is desired, ``SMOOTH=0`` should be chosen. ``SMOOTH`` has no effect if vertical velocity is calculated on lat\/lon grid (``GAUSS=0``).
     * The on demand scripts send an error message for settings where ``SMOOTH`` (if set) and ``RESOL`` are larger than 360./``GRID``/2, since in this case, the output grid cannot resolve the highest wave numbers. The scripts continue operations, however.
+        - If ``GAUSS`` is set to 1, only the following choices are possible for ``RESOL`` on *ecgate*: 159,255,319,399,511,799, (on the HPC also 1279; 2047 in future model versions). This choice is restricted because a reduced Gaussian grid is defined in the ECMWF EMOSLIB only for these spectral resolutions. For ``GAUSS=0``, ``RESOL`` can be any value below the operational resolution.
+        - For ``SMOOTH``, any resolution lower than ``RESOL`` is possible. If no smoothing is desired, ``SMOOTH=0`` should be chosen. ``SMOOTH`` has no effect if the vertical velocity is calculated on a lat\/lon grid (``GAUSS=0``).
+    * The on-demand scripts send an error message for settings where ``SMOOTH`` (if set) and ``RESOL`` are larger than 360./``GRID``/2, since in this case, the output grid cannot resolve the highest wave numbers. The scripts continue operations, however.
     * Regional grids are not cyclic in zonal directions, but global grids are. The software assumes a cyclic grid if ``RIGHT``-``LEFT`` is equal to ``GRID`` or is equal to ``GRID``-360.
     * Finally, model and flux data as well as the vertical velocity computed are written to files ``<prefix>yymmddhh`` for application in ATM modelling. If the parameters ``OMEGA`` or ``OMEGADIFF`` are set, also files ``OMEGAyymmddhh`` are created, containing the pressure vertical velocity (omega) and the difference between omega from ``MARS`` and the surface pressure tendency. ``OMEGADIFF`` should be zero except for debugging, since it triggers expensive calculations on the Gaussian grid.
+    * Finally, model and flux data as well as the vertical velocity computed are written to files ``<prefix>yymmddhh`` (the standard ``flex_extract`` output files) If the parameters ``OMEGA`` or ``OMEGADIFF`` are set, also files ``OMEGAyymmddhh`` are created, containing the pressure vertical velocity (omega) and the difference between omega from ``MARS`` and from the surface pressure tendency. ``OMEGADIFF`` should be set to zero except for debugging, since it triggers expensive calculations on the Gaussian grid.
 Calculation of vertical velocity from pre-calculated MARS parameter 77
+Calculation of the vertical velocity from the pre-calculated MARS parameter 77
 ======================================================================
 Since November 2008, the parameter 77 (deta/dt) is stored in ``MARS`` on full model levels. ``FLEXTRA``/``FLEXPART`` in its current version requires ``deta/dt`` on model half levels, multiplied by ``dp/deta``. In ``flex_extract``, the program ``calc_etadot`` assumes that this parameter is available if the ``CONTROL`` parameter ``ETA`` is set to 1.
+Since November 2008, the parameter 77 (deta/dt) is stored in ``MARS`` on full model levels. ``FLEXTRA``/``FLEXPART`` in its current version requires ``deta/dt`` on model half levels, multiplied by ``dp/deta``. In ``flex_extract``, the program ``calc_etadot`` assumes that parameter 77 is available if the ``CONTROL`` parameter ``ETA`` is set to 1.
 It is recommended to use the pre-calculated parameter 77 by setting ``ETA`` to 1 whenever possible.
 Setting parameter ``ETA`` to 1 normally disables calculation of vertical velocity from the horizontal wind field, which saves a lot of computational time.
+Setting the parameter ``ETA`` to 1 disables calculation of vertical velocity from the horizontal wind field, which saves a lot of computational time.
 .. note::
    However, the calculation on the Gaussian grid are avoided only if both ``GAUSS`` and ``ETADIFF`` are set to 0. Please set ``ETADIFF`` to 1 only if you are really need it for debugging since this is a very expensive option. In this case ``ETAyymmddhh`` files are produced that contain the vertical velocity from horizontal winds and the difference to the pre-calculated vertical velocity.
+   However, the calculations on the Gaussian grid are avoided only if both ``GAUSS`` and ``ETADIFF`` are set to 0. Please set ``ETADIFF`` to 1 only if you are really need it for debugging since this is a very expensive option. In this case, ``ETAyymmddhh`` files are produced that contain the vertical velocity from horizontal winds and the difference to the pre-calculated vertical velocity.
 The parameters ``RESOL``, ``GRID``, ``UPPER``, ``LOWER``, ``LEFT``, ``RIGHT`` still apply. As for calculations on the Gaussian grid, the spectral resolution parameter ``RESOL`` should be compatible with the grid resolution (see previous subsection).

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