[Question&Discussion] Below cloud mixing in convection

[yuanjianz]

Your name

Yuanjian Zhang

Your affiliation

WashU

Background

In the current version of geos-chem, we have a complete mixing below cloud base in convection transport (If the convective mass flux below cloud base is non-negligible, then average out all mixing ratios below the cloud). This algorithom was first brought into geos-chem from Lin S.J. 1996 and updated by Amos et al., 2012 during the period of MERRA.

In Lin's original code, the cloud base is fixed at layer 3, while in Amos's update, cloud base is dynamically determined by searching through layer and finding the first level where convective precipitation formation rate (DQRCU) is larger than zero.

What's the problem?

When I am evaluating GEOS-IT, there was a bug that DQRCU changes definition to net preicipitation rate (see #2469 (comment)). Then I got the chance to compare between two distintly different cloud bases. Below are average cloud base level for 2019 for GEOS-IT using original DQRCU and fixed DQRCU, with MERRA2 being a reference.

merra2_ras is default MERRA2 implementation; geosit_ras is GEOS-IT using original DQRCU; geosit_gf is GEOS-IT adding reevaporation back to DQRCU. M is the globally average value; R is its pearson correlation to merra2

My following fullchem simulation seems to show aerosols' strong sensitivities to cloud base height:

201907 fullchem simulation; geosit_gf - geosit_ras (low cloud base - high cloud base)

Discussion

Below cloud mixing seems to strongly influence surface aerosol concentration in my experiments. Last update to this was version 9 by Amos with respect to MERRA. So it might be worth discussing its implementation further now. There are several points that I am specifically interested in:

1. What's GEOS's diagnostic for cloud base and why we are using DQRCU as cloud base?
   a. Before GEOS-5.2.0, cloud base is diagnosed to be the lowest two model level (Rienecker et al., 2008)
   b. After transition from MERRA to MERRA2, cloud base is set as the PBL level (Molod et al., 2015).
   c. What's the current cloud base in GEOS-IT or GEOS-FP?
2. Given the cloud base, what should we treat our below-cloud mixing?
   a. In GEOS-CTM, tracer mixing ratio is treated as well-mixed only below PBL, should we switch to use PBL level as the cloud base and mix below it? (If so, could it duplicate the PBL mixing we already have?)

Reference

[1] Lin, SJ. "Description of the parameterization of cumulus transport in the 3D Goddard Chemistry Transport Model, NASA/GSFC, 1996.
[2] Amos, Helen Marie, Daniel J. Jacob, C. D. Holmes, Jenny A. Fisher, Qiaoqiao Wang, Robert M. Yantosca, Elizabeth Sturges Corbitt et al. "Gas-particle partitioning of atmospheric Hg (II) and its effect on global mercury deposition." Atmospheric Chemistry and Physics 12, no. 1 (2012): 591-603.
[3] Rienecker, Michele M., M. J. Suarez, R. Todling, J. Bacmeister, L. Takacs, H-C. Liu, W. Gu et al. The GEOS-5 Data Assimilation System-Documentation of Versions 5.0. 1, 5.1. 0, and 5.2. 0. No. NASA/TM-2008-104606-VOL-27. 2008.
[4] Molod, Andrea, Lawrence Takacs, Max Suarez, and Julio Bacmeister. "Development of the GEOS-5 atmospheric general circulation model: Evolution from MERRA to MERRA2." Geoscientific Model Development 8, no. 5 (2015): 1339-1356.

@yantosca, can you help me add a tag for Transport WG attention?
Tagging @djxjacob @viral211 @aschuh for interest in GEOS-IT

[yantosca]

Tagging the Transport WG co-chairs: @keknowland @aschuh @ktravis213