Boundary layer mixing

NOTE: Page under construction!

Overview

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

1. TURBDAY (Dale Allen, U. Maryland): Full PBL mixing
2. VDIFF (Jintai Lin and Michael McElroy): Non-local PBL mixing

TURBDAY

Description

The TURBDAY algorithm is fairly simple. 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 tracer mass (CC) underneath the PBL top pressure is computed.

3. The mean mixing ratio of tracer underneath the PBL top (CC_AA) is computed. This is just the quotient CC/AA.

4. The updated tracer at each level L underneath the PBL top is computed as:

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

 where

 DTC(L,N) = [ ALPHA * (mean mixing ratio below PBL) * Airmass at (L) ] 
          - [ ALPHA * TRACER(L,N),old               * Airmass at (L) ]
 
 DTC is the change in mass (kg) due to BL mixing, 
 so DTC/AD is the change in (v/v) mixing ratio units.  

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

--Bob Y. 11:34, 18 February 2010 (EST)

Validation

Text to be added.

VDIFF

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).

References

1. 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

2. 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, accepted. 2010. PDF

Known issues

None at this time.

--Bob Y. 16:57, 17 February 2010 (EST)