Using global models and chemical observations to diagnose eddy diffusion

1. Using global models and chemical observations to diagnose eddy diffusion

2. Goal: determine the eddy diffusion rate in the upper mesosphere, including latitudinal and seasonal changes • Proposal: use a 3-D chemical model to determine which global measurements can best constrain the mean global diffusivity coefficient; use the measurements and model to narrow the range of diffusion rates • Why do we want/need to know diffusion? • theoretical (how much turbulence & diffusion is generated by gravity wave breaking?) • without knowing diffusive transport, we don’t know if/when our chemical simulations are correct • What do we know now? • current estimates from observations and numerical models differ widely • Why use chemicals? • different constituents are sensitive to diffusion over different altitude ranges • matching profiles for multiple constituents provides a stringent test of our estimates

3. WHAT MAKES A CHEMICAL USEFUL? • concentration large enough to be measurable • sensitivity to transport because of either • long lifetime & strong vertical gradient • short lifetime but equilibrium concentration depends on transported species • reactions and rate coefficients reasonably well known • consistent response (for example, increases monotonically with increasing diffusivity)

4. ROSE model • vertical range: tropopause to thermosphere • driven by meteorological observations at lower boundary • radiation and dynamics can be decoupled from chemistry (as in the present study) • time-dependent chemistry • easily changed for mechanistic studies • simulates • oxygen: O(1D), O, O2, O3 • hydrogen: H, OH, HO2, H2O, H2O2, H2, CH4 • nitrogen*: N, NO, NO2, NO3, HNO3, N2O5, N2O, HO2NO2 • chlorine: Cl, ClO, HCl, HOCl, ClONO2, CFCl3, CF2Cl2 • carbon: CO, CH2O, CO2 • *Note: thermospheric NO is specified based on SNOE empirical model (Marsh et al. 2004)

5. eddy diffusion coefficient Kzz in ROSE model chemical continuity eqn for mixing ratio c: Kzz is the eddy diffusion coefficient: calculated by the Hines gravity wave drag parameterization and includes effective Prandtl number m2/s

6. How the model is used • several model integrations with different levels of eddy diffusion in the chemical continuity eqn; otherwise identical • NOTE: large-scale dynamics is identical in all runs because • the dynamical Kzz does not change • these runs are uncoupled (climatological radiative gases) • comparison of averaged vertical profiles: global at all local times or day-only and night-only • subjective assessment of which provide the best constraints on diffusion assuming the availability of global measurements over all local times • actual application will depend on extent and accuracy of measurements

7. basic results • source gases and stratospheric species that are not useful because concentrations are too small in mesosphere • hydrogen family: H2O2 • nitrogen family: NO3, HNO3, N2O5, N2O, HO2NO2 • chlorine family: ClO, HOCl, ClONO2, CFCl3, CF2Cl2 • carbon family: CH2O • other species with low concentrations • oxygen family: O(1D) • species that are not useful because of weak vertical gradient • O2 and H2 • species that cannot be tested due to specified thermospheric NO in ROSE model • nitrogen: N, NO, NO2, • species considered below • oxygen family: O, O3 • hydrogen family: H, OH, HO2, H2O, CH4 • chlorine family: Cl, HCl • carbon family: CO, CO2

8. sample for interpretation of the model results • curves show global mean CO profiles from 4 model runs • CO increases with altitude due to a source in the thermosphere • higher diffusion leads to lower mesospheric CO due to upward transport of low-CO air • the differences among the 4 cases increase with altitude in the mesosphere • interpretation: CO could provide a good diagnostic of diffusion near the mesopause profile values range from zero to 2 x 10-4 vmr

9. CO and CO2 (transport) • With increasing diffusion, CO2 increases near the mesopause while CO decreases Kzz=0

10. Obs (SABER) & model (ROSE) of CO2 ROSE model SABER v 1.06

11. CO2: 3 model cases for January-February

12. Cl and HCl(transport) • With increasing diffusion, HCl increases near the mesopause while Cl decreases Kzz=0

13. long-lived hydrogen species • With increasing diffusion, CH4 and H2O increase in the lower and middle mesosphere and H increases in the upper mesosphere Kzz=0

14. atomic oxygen percentage change with diffusion is small the altitude of the rapid increase of O in the middle mesosphere moves down slightly with increased diffusion Kzz=0

15. ozone(photochemistry) Kzz=0 • both day & night ozone change with diffusion, but the signs are opposite • at night, lower O3 with higher Kzz is dominated by the impact of eddy diffusion on H • during day, diffusion increases O3 in the middle mesosphere through the increase in O • a valuable diagnostic since the response differs in day & night (easier to distinguish from other perturbations) Kzz=0

16. night ozone: 3 model cases for January-February high diffusion brings up water, which leads to ozone destruction

17. Obs & model of night ozone ROSE model SABER

18. OH and HO2 • daytime increase with increasing diffusion in the vicinity of the vmr maximum • nighttime differences not monotonic with changes in Kzz Kzz=0

19. Summary of useful chemicals: • Useful in middle mesosphere • CH4 • H2O • O3, day & night • Useful in upper mesosphere/mesopause • O3 night • CO • CO2

20. information about vertical structure of diffusion rate? high diffusion better above the mesopause? high diffusion worse at and below the mesopause?

21. Problems with this approach • molecular diffusion • these tracers are also sensitive to molecular diffusion • how best to treat the two together? • numerical formulation of diffusion • at present, models are being used to “validate” the upper mesosphere chemical observations from SABER