Fix convection issues when meteorology is Grell Freitas scheme

[yuanjianz]

Name and Institution (Required)

Name: Yuanjian Zhang
Institution: WashU

Describe the update

See discussion #1409

GEOS-IT and GEOS-FP after Jun 2020 switch to Grell Freitas convection from RAS. Some meterology field definitions are changed. This update accommodates these fields to geos-chem convection.

Reported by @viral211 first:

There was an error in archiving the convective precipitation fields (PFLCU and PFICU) in our GEOS-IT and GEOS-FP data after the switch to Grell-Freitas (the fields are just set to zero). These fields are used in GEOS-Chem to calculate convective washout and re-evaporation

@yuanjianz:

Since DQRCU by definition is changed to net precipitation (MERRA-2 definition DQRCU minus layer-wise re-evaporation), do you think we should add layer-wise REEVAP back to match the DQRCU for determining cloud base height? It may lower the cloud base height somehow, given it seems some gas tracer is substantially below observation constraint?

Expected changes

Lower cloud base and restored washout in geos-chem convection driven by GEOS-IT and GEOS-FP after Jun 2020.

Related Github Issue

#2469

[viral211]

Shouldn't this be: PDOWN(K) = PDOWN(K+1) + DQRCU_MET(K) * DELP(K) * G0_100 * 100e+0_fp

[yuanjianz]

I did this to align with the pre-existing configuration:

geos-chem/GeosCore/convection_mod.F90

Lines 569 to 575 in bef56c6

	! PFICU and PFLCU are on level edges
	PFICU => State_Met%PFICU (I,J,2:State_Grid%NZ+1) ! Dwnwd flx of conv
	! ice precip
	! [kg/m2/s]
	PFLCU => State_Met%PFLCU (I,J,2:State_Grid%NZ+1) ! Dwnwd flux of conv
	! liquid precip
	! [kg/m2/s]

But if we look at where PDOWN is used, it seems to imply that PDOWN(K) to be the flux at the bottom of level K. So I think you are right, and we need to fix both RAS and GF's PDOWN.

Also, I have a question about:

geos-chem/GeosCore/convection_mod.F90

Lines 1101 to 1107 in bef56c6

	! Check if...
	! (1) there is precip coming into box (I,J,K) from (I,J,K+1)
	! (2) there is re-evaporation happening in grid box (I,J,K)
	! (3) there is species to re-evaporate
	IF ( PDOWN(K+1) > 0 .and. &
	REEVAPCN(K) > 0 .and. &
	T0_SUM > 0 ) THEN

Why is essential to have re-evaporation to have wash-out? Couldn't it be washout event without re-evaporation?

[viral211]

You are right. Re-evaporation should not be a condition for washout, but I guess there will always be re-evaporation when the precip falls through drier air below the cloud base. Could you remove the re-evaporation condition and see how much difference it makes?
Let's fix PDOWN in RAS too. Thanks!

[yuanjianz]

Hi Viral, please check the latest commit.

[viral211]

Hi @yuanjianz, this looks great! One thing I could not tell for certain is whether we go through the entire routine even if there is no convection taking place in a column at the time step, or if there is an early exit somewhere if, say, the convective mass flux is zero throughout the column.