Get rid of DRIPCG

[CoryMartin-NOAA]

A little (tall?) birdie has informed us that we apparently should not be using DRIPCG, and seems we are using DRIPCG in our tests. This PR removes that. Note that the global-workflow will likely need modified in config.atmanl to use the different YAML name.

[RussTreadon-NOAA]

Changes are straight forward. Yes, g-w parm/config.gfs/config.atmanl will need to replace

export ATMVARYAML=${HOMEgfs}/sorc/gdas.cd/parm/atm/variational/3dvar_dripcg.yaml

with

export ATMVARYAML=${HOMEgfs}/sorc/gdas.cd/parm/atm/variational/3dvar_pcg.yaml

One question. Why do we switch to PCG instead of DRPCG. I thought the problem was with the I (inexact) part of the PCG. What do JCSDA and GMAO use for the variational DA runs?

[CoryMartin-NOAA]

Good question @RussTreadon-NOAA , and I have no good answer! I don't even know why we started with DRIPCG besides that is what a lot of the JCSDA test YAMLs use.

[RussTreadon-NOAA]

@shoyokota may have some insights regarding which minimizer, among various options, we should consider using. Of course, we should test various minimizers in single obs, stand-alone, and cycled modes. Doing so is not a pre-requisite for this PR.

[CoryMartin-NOAA]

@DavidNew-NOAA is going to test a few in the next week or so for his AMS poster. But that will be single analysis testing and not cycling. I think GMAO is using DRPCG, or at least Ricardo has a PR to OOPS to fix DRPCG and DRIPCG.

[andytangborn]

Seems simple enough. I can run a single analysis time experiment to see what the impact is going to be for aerosol DA.

[RussTreadon-NOAA]

FYI: Ricardo's OOPS issue is #2475. The corresponding PR is #2476.

[DavidNew-NOAA]

Seems pretty straightforward. I'll get some data on algorithm performance next week with the experiments for my AMS poster.

[CoryMartin-NOAA]

@DavidNew-NOAA found that PCG is slow, and NRL is apparently using DRPCG, so I've updated to use DRPCG.

[RussTreadon-NOAA]

Looks good to me. Approve.

[RussTreadon-NOAA]

g-w PR #2225 was just opened to update the GDASApp hash cloned by g-w. If this PR is merged into develop before #2225 is acted upon, I'll update the GDASApp hash to include changes from this PR.

[CoryMartin-NOAA]

Thanks @RussTreadon-NOAA you beat me to it! I think there may be some minor changes that will need to be made to g-w config files when this PR is merged. Like the 3dvar YAML template for example.

I'll merge this now.