part of cam6_4_179: moving mountain gw bug fix when ZM is off

[adamrher]

Fixes #1565

[jimmielin]

Thanks @adamrher, this looks good. I am planning to bring it in together with ESCOMP/atmospheric_physics#407 which fixes the ZM parentheses bug.

I will also edit this so if use_gw_convect_dp is on but ttend_dp is unavailable it aborts since I think that is the intended behavior, but please let me know if you want to allow the deep convection scheme to run with zero tendencies, too.

[adamrher]

Thanks! There is build-namelist logic that sets use_gw_convect_dp=.false. if deep_scheme='off', so I'll leave up to you if you want to an add'l check in gw_drag.F90.

CAM/bld/build-namelist

Lines 3976 to 3979 in 8065f6a

	# Check if deep convection scheme used. If not set use_gw_convect_dp=.false.
	if ($deep_scheme =~ /off/) {
	$use_gw_convect_dp = '.false.';
	}

I don't think we want to put similar logic in build-namelist for use_gw_movmtn_pbl as convective heating is an optional future enhancement; the default version doesn't need convective heating.

[jimmielin]

Thanks for the correction @adamrher, I didn't know about that bit in build-namelist. I think this is ready to go in as-is. I will merge it in together with the ZM fix since both are now expected to be bit-for-bit.