Planetary boundary layer mixing

There are two planetary boundary layer (PBL) mixing schemes in GEOS-Chem:

Scheme	Author	Description	Source code module
TURBDAY	Dale Allen [1]	Full PBL mixing	`pbl_mix_mod.F90`
VDIFF	Jintai Lin [2] [3] Michael McElroy [2]	Non-local PBL mixing scheme	`vdiff_mod.F90`

Affiliation

VDIFF (non-local PBL mixing)

The VDIFF scheme is the default PBL mixing scheme in GEOS-Chem. To use VDIFF, make sure you have the following settings in your geoschem_config.yml file.

pbl_mixing:
  activate: true
  use_non_local_pbl: true

Description

Jintai Lin implemented a non-local 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-FP), 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., at night), the scheme reduces to the well-known local scheme based on K-theory, and the derived mixing is weak — much weaker than full-mixing.
In the case of an unstable PBL (e.g., on a typical hot summer afternoon), a non-local term is introduced to account for PBL-wide mixing triggered by large eddies. In an extremely unstable PBL, the magnitude of mixing approaches full-mixing.

The non-local scheme has been shown to simulate mixing of meteorological parameters and chemical tracers reasonably well under various PBL conditions, and is more realistic than the assumption of a fully mixed PBL. Analysis of the two schemes is conducted by Lin et al. [2008] and Lin and McElroy [2010].

How the non-local scheme works: it first calculates the PBL depth, then the eddy diffusivity (\(K\)) for tracers. \(K\) is used to derive tracer mixing. In the current GEOS-Chem setup, however, the PBL height is taken from the meteorological datasets rather than being derived within the scheme, in order to maintain consistency with those datasets. Nonetheless, the user may enable the online calculation of PBL height; this option is available in the code GeosCore/vdiff_mod.F90.

Validation

See Lin and McElroy [2010].

TURBDAY (full PBL mixing)

The TURBDAY mixing scheme was formerly the default mixing scheme in GEOS-Chem but is now optional. It can be used with all of the different types of meteorological fields (GEOS-FP, MERRA-2, GEOS-IT, GISS/GCAP2).

To use TURBDAY, make sure you have the following settings in your geoschem_config.yml file.

pbl_mixing:
  activate: true
  use_non_local_pbl: false

The updated concentrations for each tracer N at grid boxes (I,J,L) underneath the PBL top are computed as:

SPECIES(I,J,L,N),new  = Species(N)%(I,J,L),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 *    (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          = Change in mass (kg) due to BL mixing, therefore:

DTC/AD       = Change in (v/v) mixing ratio units.

Validation

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

Difference Between PBLH and Mixing Depth

The GEOS meteorological fields from NASA GMAO provide the depth of the mixed layer (mixing depth), not the planetary boundary layer height (PBLH), even though the variable in the files is labelled PBLH. The planetary boundary layer (PBL) is the layer of the atmosphere that interacts with the surface on a timescale 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 Jacob and Brasseur [2016] for this discussion and illustrations. When discussing PBL height in terms of mixing, what is actually meant is the mixed layer— and that is what is calculated by the online PBLH (mixed layer) computation in Holtslag and Boville [1993] or in the NASA GEOS model.