Calculation of vertical velocity and preparation of output files¶
Two methods are provided in
flex_extract for the calculation of the vertical velocity for
from the horizontal wind field,
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,
since the computational cost (measured in ECMWF HPC units) is reduced by 90-95% at
T799. The extraction time, which depends heavily also on the performance of
generally reduced by 50% as well. The
MARS parameter 77 is then multiplied by
give a vertical velocity in Pa/s as needed by
Calculation from the horizontal wind field is still required for historical case studies using ERA-40, ERA-Interim or operational data prior to September 2008.
Calculation of the vertical velocity from the horizontal wind using the continuity equation¶
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
dependent on the specification of parameters
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 (
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:
For 0.5 degree grids:
Calculation on the lat/lon grid is not recommended for less than the operational (T1279) resolution.
GAUSSis set to 1, only the following choices are possible for
RESOLon 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
RESOLcan be any value below the operational resolution.
SMOOTH, any resolution lower than
RESOLis possible. If no smoothing is desired,
SMOOTH=0should be chosen.
SMOOTHhas no effect if the vertical velocity is calculated on a lat/lon grid (
The on-demand scripts send an error message for settings where
SMOOTH(if set) and
RESOLare 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
LEFTis equal to
GRIDor is equal to
Finally, model and flux data as well as the vertical velocity computed are written to files
flex_extractoutput files) If the parameters
OMEGADIFFare set, also files
OMEGAyymmddhhare created, containing the pressure vertical velocity (omega) and the difference between omega from
MARSand from the surface pressure tendency.
OMEGADIFFshould be set to zero except for debugging, since it triggers expensive calculations on the Gaussian grid.
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.
FLEXPART in its current version requires
deta/dt on model half levels, multiplied by
flex_extract, the program
calc_etadot assumes that parameter 77 is available if the
ETA is set to 1.
It is recommended to use the pre-calculated parameter 77 by setting
ETA to 1 whenever possible.
Setting the parameter
ETA to 1 disables calculation of vertical velocity from the horizontal wind field, which saves a lot of computational time.
However, the calculations on the Gaussian grid are avoided only if both
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.
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).