Description
The unified gravity wave physics (UGWP) package has experienced numerical instability during FV3GFS Prototype 8 testing at C384 horizontal resolution. (Model crashes are documented here and here.) The source of the instabilities has been traced to the mesoscale gravity wave drag (GWD) and turbulent orographic form drag (TOFD) parameterizations contained in the CCPP module drag_suite.F90. The instability can be alleviated by taking a smaller physics time step, however, this is not a solution as it would not be amenable to operational forecasting.
Solution
The solution is to limit the tendencies calculated by the mesoscale GWD scheme in drag_suite.F90 in a similar manner as is done in the GFSv16 scheme in gwdps.f. The instability due to the TOFD parameterization was due to an earlier modification of the scheme that removed a limit on the subgrid terrain variability and caused excessively large tendencies. The limits have been re-introduced to help alleviate the instability problem.
Another change to the code was the addition of optional diagnostic outputs, controlled by the namelist option "ldiag_ugwp", which outputs the drag contributions from each of the drag components of the UGWP: mesoscale GWD, low-level blocking, small-scale GWD, TOFD and non-stationary GWD.
Description
The unified gravity wave physics (UGWP) package has experienced numerical instability during FV3GFS Prototype 8 testing at C384 horizontal resolution. (Model crashes are documented here and here.) The source of the instabilities has been traced to the mesoscale gravity wave drag (GWD) and turbulent orographic form drag (TOFD) parameterizations contained in the CCPP module drag_suite.F90. The instability can be alleviated by taking a smaller physics time step, however, this is not a solution as it would not be amenable to operational forecasting.
Solution
The solution is to limit the tendencies calculated by the mesoscale GWD scheme in drag_suite.F90 in a similar manner as is done in the GFSv16 scheme in gwdps.f. The instability due to the TOFD parameterization was due to an earlier modification of the scheme that removed a limit on the subgrid terrain variability and caused excessively large tendencies. The limits have been re-introduced to help alleviate the instability problem.
Another change to the code was the addition of optional diagnostic outputs, controlled by the namelist option "ldiag_ugwp", which outputs the drag contributions from each of the drag components of the UGWP: mesoscale GWD, low-level blocking, small-scale GWD, TOFD and non-stationary GWD.