Boundary layer mixing

This page describes the planetary boundary layer mixing options in GEOS-Chem.

Overview

At present there are two PBL mixing schemes in GEOS-Chem:

1. TURBDAY (Dale Allen, U. Maryland): Full PBL mixing. This is contained in source code file pbl_mix_mod.f.
2. VDIFF (Jintai Lin and Michael McElroy): Non-local PBL mixing scheme. This is contained in the source code files vdiff_pre_mod.f and vdiff_mod.f90.

TURBDAY

The TURBDAY mixing scheme is the default PBL mixing scheme in GEOS-Chem. It can be used with all of the versions of the GEOS meteorology (e.g. GEOS-3, GEOS-4, GEOS-5) and with the GISS/GCAP meteorology.

To use TURBDAY, make sure you have the following settings in your input.geos file:

Turn on PBL Mixing?     : T
 => Use non-local PBL?  : F

Description

The TURBDAY algorithm is fairly simple, as described by Wu et al, 2007:

The treatment of boundary layer turbulence is also different in GEOS and GISS. The mixing depth in GEOS is estimated from the bulk Richardson number with surface friction [Holtslag and Boville, 1993] and in the GISS GCM it is estimated on the basis of the vertical profile of turbulent kinetic energy [Canuto, 1994; Canuto et al., 2001]. In either case, GEOS-Chem assumes instantaneous vertical mixing from the surface through the mixing depth [Bey et al., 2001].

Tracer is inputted to the routine in units of mixing ratio [v/v]. The PBL depth and related pressure quantities are taken (or derived) from the met field archive.

At each surface grid box (I,J) location:

1. The total air mass (AA) within the PBL is computed.
2. The total mass of each tracer N (CC(N)) underneath the PBL top pressure is computed.
3. The mean mixing ratio of tracer underneath the PBL top is computed. This is just the quotient CC(N)/AA.
4. The updated concentrations for each tracer N at grid boxes (I,J,L) underneath the PBL top are computed as:

   TRACER(I,J,L,N),new  = TRACER(I,J,L,N),old + ( DTC(I,J,L,N) / AD(I,J,L) )

   where

   DTC(I,J,L,N) = [ ALPHA * (mean mixing ratio below PBL) * AD(I,J,L) ] 
                - [ ALPHA * TRACER(I,J,L,N),old           * AD(I,J,L) ]
 
   AD(I,J,L)    = Air mass at grid box (I,J,L)

   ALPHA        = Day/night Mixing coefficients.  
                  These are always 1, for full mixing at all times of day.

   DTC          = is the change in mass (kg) due to BL mixing, therefore:
   DTC/AD       = is the change in (v/v) mixing ratio units. 

--Bob Y. 12:13, 18 February 2010 (EST)

Validation

See Bey et al [2001] and Wu et al [2007] below.

VDIFF

The VDIFF scheme is an option in GEOS-Chem. To use VDIFF, make sure you have the following settings in your input.geos file:

Turn on PBL Mixing?     : T
 => Use non-local PBL?  : T

Description

Jintai Lin implemented a 'new' PBL mixing scheme into GEOS-Chem. It is a non-local scheme formulated by Holtslag and Boville (1993). Unlike the full mixing assumption where emissions, dry depositions and concentrations of individual species are evenly distributed in the PBL (the depth of which being taken from meteorological datasets such as GEOS-5), the non-local scheme considers different states of mixing within the PBL as determined by the static instability. In the case of a stable PBL (e.g., in the night), the scheme shrinks to the well known local scheme developed based on the K-theory, and the derived mixing is weak -- much weaker than full-mixing. In the case of an unstable PBL (e.g., in a typical hot summer afternoon), a 'non-local' term is introduced to account for the PBL-wide mixing triggered by large eddies. In a extremely unstable PBL, the magnitude of mixing is close to full-mixing. The non-local scheme has been shown to be able to simulate relatively well mixing of meteorological parameters and chemical tracers under various conditions of PBL, and is more realistic than the assumption of a fully mixed PBL. Analysis of the two schemes is conducted by Lin et al. (2008, AE) and Lin et al. (2010, AE, accepted).

How the non-local scheme works: It first calculates the PBL depth, then eddy diffusivity (K) for tracers. K is used later to derive the mixing of tracers. In current GEOS-Chem setup, however, the PBL height is taken from the meteorological datasets rather than being derived with the scheme in order to enhance the consistency with the meteorological datasets. Nonetheless, the user has the choice of turning on the online calculation of PBL height, which option is provided in the code (vdiff_mod.f).

Validation

See Lin & McElroy, 2010 below.

References

1. Bey I., D. J. Jacob, R. M. Yantosca, J. A. Logan, B. Field, A. M. Fiore, Q. Li, H. Liu, L. J. Mickley, and M. Schultz, Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation, J. Geophys. Res., 106, 23,073-23,096, 2001. PDF
2. Canuto, V.M., Large-eddy simulation of turbulence: A subgrid scale model including shear, vorticity, rotation and buoyancy, Astrophys. J., 428, 729–752, 1994.
3. Canuto, V. M., A. Howard, Y. Cheng, and M. S. Dubovikov, Ocean turbulence, part I: One-point closure model—Momentum and heat vertical diffusivities, J. Phys. Ocean., 31, 1413–1426, 2001.
4. Holtslag, A.A.M., and B. Boville, Local versus nonlocal boundary layer diffusion in a global climate model, J. Clim.>, 6, 1825–1842, 1993.
5. Lin, J.-T., D. Youn, X.-Z. Liang, and D. J. Wuebbles: Global model simulation of summertime U.S. ozone diurnal cycle and its sensitivity to PBL mixing, spatial resolution, and emissions, Atmos. Environ., doi:10.1016/j.atmosenv.2008.08.012, August 2008. Link
6. Lin, J.-T., and M. McElroy, Impacts of boundary layer mixing on pollutant vertical profiles in the lower troposphere: Implications to satellite remote sensing, Atmospheric Environment, doi:10.1016/j.atmosenv.2010.02.009, in press, 2010. PDF

--Bob Y. 14:47, 12 March 2014 (EDT)

Difference between PBLH and mixing depth

The meteorological fields from GEOS provide the depth of the mixed layer (mixing depth), not the planetary boundary layer height (PBLH), even though the variable provided in the files is PBLH. The planetary boundary layer (PBL), is the layer of the atmosphere that interacts with the surface on a time scale of a day or less. The free troposphere has a general slow sinking motion, balancing the few locations where deep convection or frontal lifting injects PBL air to high altitudes. The compressional heating from this sinking air produces a semi-permanent subsidence inversion that caps the PBL and sharply restricts mixing between the PBL and the free troposphere. After sunrise, surface heating erodes the stable residual layer from below, producing an unstable mixed layer that grows over the morning hours to eventually reach the full depth of the PBL. See Brasseur and Jacob (2016) for this discussion and illustrations. When we discuss the PBL height in terms of mixing, what we actually mean is the mixed layer, and that is what is calculated by the online calculation of PBLH (mixed layer) in Holtslag and Boville (1993) or in the GEOS model.