UCX chemistry mechanism
On this page, we provide information about the Universal tropospheric-stratospheric Chemistry eXtension (UCX) mechanism in GEOS-Chem. UCX combines both tropospheric and stratospheric reactions into a single chemistry mechanism. It was developed by Sebastian Eastham, Debra Weisenstein, and Steven Barrett at the Laboratory for Aviation and the Environment at the Massachusetts Institute of Technology.
NOTE: The UCX chemistry mechanism is included in the full-chemistry mechanism (all options).
Overview
The UCX adds online stratospheric chemistry to GEOS-Chem. Switching on the UCX results in the following changes to GEOS-Chem:
- Photolysis and UV transfer is calculated using the Fast-JX v7.0a model in 18 wavelength bins, covering 177-850 nm. Switching off the UCX removes 6 of the high-frequency bins for the sake of computational speed
- In-model column ozone can now be used to estimate ozone optical depth for photolysis calculations, allowing photochemistry to couple to modeled ozone depletion or production
- Long-lived species such as N2O, CH4, OCS, CFCs, HCFCs and halons are emitted, advected and lost as part of the chemistry mechanism, instead of ignored or set to fixed mixing ratios
- Stratospheric water is treated as an advected tracer, and can be enhanced by changes in tropospheric meteorology and upwelling CH4
- Stratospheric aerosols (LBS, STS, NAT and ice) are formed according to thermodynamics equilibrium. These aerosols are advected, interact with radiation, can settle (based on a log-normal approximation) and take part in heterogeneous chemistry as described in Kirner et al 2011
- A mesospheric steady-state approximation is applied to NOy and SOx to prevent formation of an unrealistic high-altitude reservoir while conserving total nitrogen and sulfur
All other mesospheric chemistry is approximated using the linearized GMI mechanism. In total, the UCX decreases run speed by about 50% in a 4x5 simulation. For more information on the UCX please see the following documents, which describe the UCX mechanism in more detail:
- Eastham, S.D., Weisenstein, D.K., Barrett, S.R.H., Development and evaluation of the unified tropospheric–stratospheric chemistry extension (UCX) for the global chemistry-transport model GEOS-Chem, Atmos. Env., June 2014. (Article)
- Technical Note: The GEOS-Chem Unified Tropospheric-Stratospheric Chemistry Extension (UCX): Interim Distribution, September 2013 (Article)
Previous issues that are now resolved
Fix for UCX mesospheric NOy routine
This update was included in v11-02f (approved 17 May 2018).
Sebastian Eastham wrote:
- This is just a small fix for the UCX mesospheric NOy routine. UCX currently uses the N, NO, NO2, and NO3 entries in the State_Chm%Species array to determine total NOx, using the species database molar masses to convert between molec/cm3 and kg. However the entry for N isn’t necessarily meaningful, as KPP never acts on mesospheric boxes (yet!), and furthermore the molar mass for N is not necessarily meaningful either – if I remember rightly it is -1 because N is not a tracer. This patch works around that by assuming that “NO” = NO + N, and ignoring whatever is in Species(I,J,L,id_N). This is by no means urgent but should slightly improve the mesospheric simulation.
--Bob Yantosca (talk) 15:57, 17 May 2018 (UTC)
UCX stratospheric water boundary condition update
This update was included in v11-02e (approved 24 Mar 2018).
Sebastian Eastham wrote:
- Chris Holmes wrote a minor update to the UCX stratospheric water boundary condition (attached), which I think would constitute a useful improvement for GEOS-Chem v11-02. This fix just ensures that the boundary condition for stratospheric water vapor is set at or above the tropical cold point, rather than at the meteorological tropopause. Since the met tropopause can sometimes be below the cold point, the pre-fix UCX would sometimes allow excessive water vapor to enter the stratosphere.
--Lizzie Lundgren (talk) 20:09, 5 January 2017 (UTC)
Fix for mesospheric NOx chemistry
This update was validated with the 1-month benchmark simulation v11-01j and approved on 03 Dec 2016
Sebastian Eastham wrote:
- The first fix is to the call to UCX_NOX itself. In the current version of GEOS-Chem, the call to UCX_NOX in the FlexChem module occurs while the species array is in molec/cm3, but UCX_NOX expects kg. Moving the call to be after the conversion fixes this issue.
- The second fix is to re-establish day-time steady-state for NOx in the mesosphere. This code was already present, but was not functioning correctly, as only NO was being used to estimate total NOx. Partitioning of NOx between N, NO, NO2 and NO3 is now calculated directly in the mesosphere. This assumes that the species will be locally in steady state as long as there is daylight, and that this steady state occurs on a much faster timescale than loss by the reactions N + NO -> N2 + O or N + NO2 -> N2O + O. O(1D) and O(3P) concentrations are taken from archived 2D data, along with J-rates for NO, NO2, NO3 and N2O. The O3 concentration is taken from the species array. Loss rates for NOx and N2O (in addition to the rate of NOx production from N2O) are also estimated, based on the aforementioned loss reactions and destruction of N2O by O(1D) and photolysis.
- Finally, I’ve updated the aforementioned chemistry scheme to improve accuracy. The first change is that the J-rates are no longer taken from 2D data. Instead, the J-rate calculated at the top of the stratosphere is applied vertically throughout the column. This mirrors the approach used to calculate mesospheric H2SO4 photolysis. The second change is that a heavily simplified NOx chemistry mechanism is applied during mesospheric night. Previously, it was assumed that there would be no mesospheric NOx chemistry at night. However, this led to an overestimate of N and NO, and an underestimate of NO2 and NO3 (especially at high latitudes). It is now assumed that when there is no sunlight there is negligible N, O(1D) and O(3P). This reduces the mesospheric NOx mechanism to a 3-reactant chain. All local N is converted immediately to NO, and then only the two following reactions occur: NO + O3 -> NO2 + O2 and NO2 + O3 -> NO3 + O2. The evolution of the partitioning between these species is too slow to be well-represented by steady state, but the simplicity of the mechanism allows for an analytical solution. The extra overhead should be effectively zero, as it requires the evaluation of only three exponentials in each mesospheric night-time grid cell.
--Melissa Sulprizio (talk) 16:27, 21 November 2016 (UTC)
Fix for black carbon in ucx_mod.F
This fix was validated with 1-month benchmark simulation v11-01b and 1-year benchmark simulation v11-01b-Run0. This version was approved on 19 Aug 2015.
Sebastian Eastham wrote:
- After one hour of simulation with the standard initial restart file, a layer of black carbon appears at the stratopause, and the mesopause empties of black carbon entirely (although I’m not sure why there is so much up there to begin with). This layer is due to an error in the sedimentation calculation. On line 2699 of ucx_mod.F, the line
RWET(IBC) = RAA(IND999,29)
Should be changed to
RWET(IBC) = RAA(IND999,29) * 1.0e-6_fp
This was just a unit conversion error, and has probably been there since the UCX was first developed. I only noticed it because there seems to be a lot more high altitude black carbon in the model now than there was in v9-01-03, when the UCX was developed. I’m still looking into why this is the case.
--Melissa Sulprizio (talk) 16:06, 13 July 2015 (UTC)
References
- Eastham, S.D., Weisenstein, D.K., Barrett, S.R.H., Development and evaluation of the unified tropospheric–stratospheric chemistry extension (UCX) for the global chemistry-transport model GEOS-Chem, Atmos. Env., June 2014. (Article)
- Technical Note: The GEOS-Chem Unified Tropospheric-Stratospheric Chemistry Extension (UCX): Interim Distribution, September 2013 (Article)