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