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. At this time, it can only be used with the GEOS-5 meteorology. 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

Updates on GEOS-5 PBL height & diurnal variation of aerosols

Sajeev Philip wrote:

GEOS-5(including MERRA)-driven simulations have a high nighttime bias for aerosols (for both full-mixing and non-local mixing schemes) compared with observations. For the full mixing scheme, the bias arises from the low nighttime PBLH. We correct this by restricting the PBLH from dropping below a minimum mechanical mixing depth, defined as a function of local friction velocity (Koracin and Bberkowicz, 1988; Lin and McElroy, 2010). Figure 1 shows the diurnal variation of PBLH compared with observations from the ARM-Southern Great Plains (black line) site. The nighttime shallow PBLH in GEOS-5 is evident (blue line), which is corrected by this method (red line).

Figure 2 shows the mean diurnal variation of PM2.5 over 115 sites across the continental U.S. (measurements by Beta Attenuation Mass monitors). The threshold on PBLH reduces the diurnal variation and better matches the diurnal variation in the observations. This approach with the full mixing scheme reduces annual mean PM2.5 and NO3- over North America by 15% and 30% respectively. It also reduces the annual mean bias versus observations for PM2.5 (slope changes from 1.47 to 1.09) and NO3- (slope changes from 2.23 to 1.52).

The non-local mixing scheme has an option to recalculate the PBLH online, and it fixes the bias in nighttime PBLH. However, aerosol concentrations are unaffected, partly due to the low sensitivity to PBLH in this scheme. At this time, we recommend use of full mixing scheme with our nocturnal-GEOS-5/MERRA-PBLH-correction.

Add the following correction for GEOS-5 PBLH to line 1452 in a3_read_mod.F (version 9-01-02):

     ! PBLH must be greater than a minimum mechanical mixing depth,
     ! defined as 700*friction velocity (Koracin and Berkowicz, 1988; Lin and McElroy, 2010)
#if   defined( GEOS_5 ) 
     DO J = 1, JJPAR
     DO I = 1, IIPAR
        PBL(I,J) = max( PBL(I,J), 700d0*USTAR(I,J) )
     ENDDO
     ENDDO
#endif

Add the following correction for MERRA PBLH to line 1081 in merra_a1_mod.F (version 9-01-02):

     ! PBLH must be greater than a minimum mechanical mixing depth,
     ! defined as 700*friction velocity (koracin and berkowicz, 1988; Lin and McElroy, 2010)
     DO J = 1, JJPAR
     DO I = 1, IIPAR
        PBLH(I,J) = max( PBLH(I,J), 700d0*USTAR(I,J) )
     ENDDO
     ENDDO

Koracin, D. and R. Berkowicz: 1988, Nocturnal Boundary Layer Height: Observations by Acoustic Sounders and Prediction in Terms of Surface Layer Parameters, Boundary-Layer Meteorol. 43, 65-83.

Lin, J. T. and McElroy, M. B.: Impacts of boundary layer mixing on pollutant vertical profiles in the lower troposphere: Implications to satellite remote sensing, Atmos. Environ., 44, 1726–1739, 2010.

-- Sajeev Philip (June 14, 2012)