Dynamic tropopause

On this page we describe the implementation of the dynamic tropopause in GEOS-Chem.

Overview

GEOS-Chem is primarily a tropospheric model of atmospheric chemistry and composition. Its NOx-Ox-HC-aerosol (aka "full chemistry") simulation employs a detailed chemistry mechanism is used in the troposphere, while a much simpler mechanism (based on simple loss rates by OH and photolysis) is invoked in the stratosphere. Therefore, a robust definition of the tropopause is required.

The location of the tropopause is computed at each dynamic timestep of a GEOS-Chem simulation, according to the following criteria (in routine GeosCore/calc_met_mod.F90

      !==============================================================
      ! Define the various query fields of State_Met
      !
      ! NOTE: For convenience, we set State_Met%InPbl in routine
      ! COMPUTE_PBL_HEIGHT (in module GeosCore/pbl_mix_mod.F).
      !
      ! NOTE: For certain queries we test against level numbers,
      ! (e.g. LLSTRAT, LLCHEM), but should really test level
      ! pressure edges, so that this algorithm will be robust if
      ! we switch to different met fields or interface with
      ! different ESMs.  Add this at a later time. (bmy, 1/8/18)
      !==============================================================

      ! Is this grid box within the troposphere?
      State_Met%InTroposphere(I,J,L) = &
           ( State_Met%PEDGE(I,J,L) > State_Met%TROPP(I,J) )

      ! Is this grid box within the stratosphere or mesosphere?
      State_Met%InStratMeso(I,J,L) = &
           ( .not. State_Met%InTroposphere(I,J,L) )

      ! Is this grid box within the stratosphere (but not mesosphere)?
      State_Met%InStratosphere(I,J,L) = &
           ( L <= State_Grid%MaxStratLev .and. State_Met%InStratMeso(I,J,L) )

      ! Is grid box (I,J,L) within the chemistry grid?
      IF ( L > State_Grid%MaxChemLev ) THEN

         ! Chemistry is not done higher than the mesopause
         State_Met%InChemGrid(I,J,L) = .FALSE.

      ELSE

         ! Chemistry grid goes up to stratopause
         State_Met%InChemGrid(I,J,L) = ( L <= State_Grid%MaxChemLev )

      ENDIF

without regard for the tropopause location.

--Bob Y. 13:56, 13 September 2010 (EDT)

Polar cap

Jennifer Logan (see correspondence below) suggested that we should cap the variable tropopause at 200hPa in near-polar regions (90-60S and 60-90N), to avoid the problem with anomalously high tropopause heights at high latitudes. This fix was standardized in GEOS-Chem v7-04-13.

Jennifer Logan wrote:

After looking at the two papers I sent, I think we should restrict the tropopause at latitudes > 60 deg. to pressures greater than 200 mb (about 11 km). From Fig. 3 in Seidel and Randel, there are tropopause (TP) heights as high as 13.5 km in the Antarctic (median height is ~9.8 km, 250 mb), but I don't think we want to be doing trop. chem there. The median TP pressure at ~80 N is ~300 mb, compared to ~250 mb at 70-85 S. The extratropical TP heights are higher (lower pressure) in the SH than in the NH according to Fig. 3.

This approach is also very easy to explain in a paper.

Jennifer