Mark Olsen Morgan State Univ./ NASA Goddard Space Flight Center Luke Oman

1. Using Reanalysis-Constrained Model Simulations to Evaluate and Investigate Stratosphere-Troposphere Exchange of Mass and Ozone Mark Olsen Morgan State Univ./ NASA Goddard Space Flight Center Luke Oman NASA Goddard Space Flight Center Junhua Liu USRA / NASA Goddard Space Flight Center SPARC DA Workshop, 2017

2. Primary Goals • Examine the air and ozone STE (and related quantities) in the MERRA-2 GMI Replay simulation compared to MERRA-2 reanalysis and the impact of horizontal resolution in the Replay simulations. • Examine and quantify the relationship between STE and tropospheric ozone.

3. Why is it important for models to accurately simulate stratosphere troposphere exchange (STE)? • STE is important to the chemistry of both regions and must be simulated well in models to attribute trends and impacts from anthropogenic sources. • Tropospheric ozone is a secondary pollutant (formed as a chemical product of primary pollutants), so knowing the input from the stratosphere is vital to determining the anthropogenic impact. • Ozone flux from the stratosphere is an important boundary condition to the amount of OH in the troposphere. • Upper-tropospheric/lower-stratospheric (UTLS) ozone is a significant trace gas in the radiative balance of the atmosphere.

4. How do we determine STE? • Unfortunately, we can’t directly measure the STE of mass and ozone in the atmosphere. • We have to rely on empirical methods or models to estimate the exchange between the stratosphere and troposphere. • Validation of the results then usually rely on comparisons to related measurables, such as the magnitude and distribution of tropospheric ozone. • Many methods have been devised to estimate STE using observations and/or models and analyses that have unique advantages and disadvantages.

5. How do we determine STE? • Approaches range from: • tracer-tracer correlations (e.g., McLinden et al., 2000) • combining trace gas observations with model or analysis circulation (e.g., Gettelman and Holton, 1997; Olsen et al., 2013) • trajectory analysis (e.g., Olsen et al., 2003; Skerlak et al., 2014) • mass balance (e.g., Appenzeller et al., 1996; Olsen et al., 2004). • Here, we use the lowermost stratosphere mass balance method to estimate the STE of mass and ozone (Olsen et al., 2004). • Hsu and Prather (2014) find that this method provides good estimates of STE compared to a rigorous tropospheric mass balance method applied at the model grid level.

6. Mass Balance Method 380 K F M Extratropical Tropopause = 3.5 PVU Tropopause F NP EQ Appenzeller et al., 1996 Ftrop=F380K+dM/dt Olsen et al., 2004 • Mass (M) balance of fluxes (F) into and out of lowermost stratosphere. • 380 K flux determined from model or analysis heating rate on surface. • Ozone mixing ratio is convolved with mass flux at 380 K to determine tropopause flux. • Note that annual flux is determined by 380 K flux.

7. The MERRA-2 GMI Replay Simulation • “MERRA-2 GMI” is a simulation by the GEOS-5 model run in Replay mode coupled with the GMI combined stratosphere troposphere chemistry mechanism. • Replay mode dynamically constrains the model with the MERRA-2 reanalysis winds, temperature, and pressure. The model reads in the fields every three hours and recomputes the analysis increments, which are applied as a forcing to the meteorology at every time step over the three hour replay interval. (Replay description and info from Orbe et al., 2017, JAMES)

8. The MERRA-2 GMI Replay Simulation • Advantages of Replay: • All of the subgrid-scale processes are recalculated online so that they are consistent with the large-scale analysis fields. • Replay provides a way to perform constrained meteorology simulations using the most recent version of the model (i.e. updated chemistry schemes and subgrid-scale parameterizations). • Ability to easily add tracers, etc. • MERRA-2 GMI was run on the same 72 level, 0.5° grid as the MERRA-2 reanalysis. • The simulation currently spans 1980 to 2016 (1980 to 2015 are used here). • Three additional complementary simulations were run: • 1° horizontal resolution • 2° horizontal resolution • 2° horizontal resolution with the simulation also “replaying” MERRA-2 water vapor.

9. MERRA-2 GMI StratO3 Tracer • A suite of various tracers were implemented. In this work, we use the StratO3 tracer. • At every time step, the tracer in the stratosphere is set equal to the ozone mixing ratio in that grid box. • In the troposphere, the tracer is subjected to a loss rate computed from the online chemical mechanism for every chemical time step. • Thus, the ratio of StratO3 to ozone provides an estimate of the amount of ozone in a grid box with stratospheric origin.

10. MERRA-2 GMI reproduces the fine-scale ozone structure in the UTLS Pressure Pressure

11. Air (or Mass) STE

12. Multi-Year Mean Seasonal 380 K Air (Mass) Flux NH SH • Similar shapes but all Replay simulations have greater flux across 380 K surface (above the extratropical lowermost stratosphere) throughout the year. • Increasing resolution brings the magnitude of air flux closer to MERRA-2. • Replaying to WV has large impact, particularly in the NH.

13. Age tracers suggest the Replay stratospheric age is slightly young compared to observations. • This implies a faster circulation ⟹ greater 380 K flux ⟹ greater STE Figure courtesy of Clara Orbe from Orbe et al. (2017, JAMES)

14. Multi-Year Mean Seasonal TropopauseAir (Mass) Flux NH SH • Relationships observed for 380 K flux remain similar for the flux across the tropopause. • For the flux across both the tropopause and 380 K surfaces, the greatest differences between Replay simulations tend to occur during summer/fall.

15. Annual Mean TropopauseAir Flux Time Series NH SH • Variability of annual air STE very similar among the replay simulations. • Increasing trend seen in all the NH replay simulations though it is not apparent in MERRA-2. (Nor is any trend consistent throughout the time period in the SH.)

16. Comparison of Annual Hemispheric STE Multi-year mean STE (Eg yr-1)

17. Ozone STE

18. Multi-Year Mean Seasonal 380 K Ozone Flux NH SH • Ozone flux largely mirrors the replay differences noted for the air flux. • The water vapor replay run shifts line lower than MERRA-2 results in NH late summer/fall. • Minimum in NH is broader and SH maximum is shifted compared to air flux results. This is due to seasonality of ozone at 380 K.

19. Multi-Year Mean Seasonal 380 K Mass (Air) Flux NH SH • Similar shapes to MERRA-2 but all Replay simulations have greater flux across 380 K surface throughout the year. • Increasing resolution brings the magnitude of air flux closer to MERRA-2. • Replaying to WV has greatest impact, particularly in the NH.

20. Multi-Year Mean Seasonal TropopauseOzone Flux NH SH • Again, the replay ozone differences are similar to air flux differences. • Large difference between Replay simulations and MERRA-2 in winter/spring due to greater air STE. • Seasonality of ozone at the tropopause significantly changes the seasonality of ozone STE compared to air STE.

21. Multi-Year Mean Seasonal TropopauseMass (Air) Flux NH SH • Relationships observed for 380 K flux remain similar for the flux across the tropopause. • For the flux across both the tropopause and 380 K surfaces, the greatest differences between simulations tend to occur during summer/fall.

22. Annual Mean TropopauseOzone Flux Time Series NH SH • Variability in ozone STE very similar in all simulations. • No identifiable trends in ozone STE.

23. Comparison of Annual Hemispheric Ozone STE Multi-year mean ozone STE (Tg yr-1)

24. Annual Mean Global Tropospheric Ozone Mass • Replay global tropospheric ozone mass much greater than MERRA-2. These values are more in line with multi-model assessment of Stevenson et al., 2006 and IPCC (336 ± 27 Tgand 300 Tg, respectively). • Increasing trend in replay simulations, but flattening after 2000. • Downward jump in MERRA-2 corresponds with switch to MLS and OMI.

25. Seasonal Mean Tropospheric Ozone Mass • Very similar seasonality in tropospheric ozone for all simulations. • Increasing resolution from 2° to 1° results in significant jump. Further increase in resolution results in little change. • Replaying to water vapor doesn’t significantly impact tropospheric ozone.

26. Annual Mean Tropopause and 380 K Pressure 380 K Tropopause • Differences are NOT due to different tropopause and/or 380 K pressures.

27. Meridional Perspective

28. Annual Zonal Mean Flux Across 380 K Air Ozone • All Replay simulations have increased upwelling in tropics and downwellingin extratropics with very similar zonal distribution. • “Turn-around” latitudes very similar among all simulations and MERRA-2.

29. Ozone Mixing Ratio at 380 K and Tropopause 3.5 PVU Tropopause 380 K • Although Replay simulations have greater midlatitude downwelling, MERRA-2 has greater ozone concentrations at midlatitude 380 K surface. • SH polar differences between simulations and reanalysis likely due to ozone hole chemistry differences and reduced available observations to constrain the profile in assimilation. • Those SH polar differences do not extend down to the tropopause.

30. Seasonal Ozone Mixing Ratio at the Tropopause(averaged over 30°-50°) MERRA-2 GMI • Large seasonal variability of ozone at the midlatitude dynamical tropopause. • STE results that use a chemically-defined tropopause (i.e., ozonopause), will exhibit a completely different seasonal cycle.

31. Zonal Mean Tropospheric Ozone Mass • Similar shape and location of peaks in the tropics and midlatitudes with the usual relative biases. • MERRA-2 tropospheric ozone profiles in the polar regions are known to be less consistent with sondes, etc. • No resolution differences in Replay simulations poleward of ~45°

32. StratO3 Tracer

33. Annual Zonal Mean Tropospheric Ozoneand StratO3 Mass • Similar shapes but shift in the location of the peaks (greater shift in NH). • Note that ozone distribution depends on same factors as StratO3 distribution, but it additionally depends on the tropospheric production processes.

34. Annual Zonal Mean StratO3 Massas Percent of Ozone • Percent of ozone that is stratospheric origin increases towards pole with no middle latitude maximum.

35. Annual Time Series StratO3 Massand NH Ozone STE • Bulk hemispheric STE magnitude has strong correlation with zonal mass of StratO3. • Results are very similar for mass integrated over the mid-latitudes.

36. Annual Time Series StratO3 Massand SH Ozone STE S • Correlation statistically significant but weaker in SH

37. Annual Time Series StratO3 Massand Ozone STE • Slopes of the two lines are nearly equal. 0.41 Gg/Tg in SH; 0.45 Gg/Tg in the NH

38. Annual Mean Ozone Mass and STE Net StratO3 and Ozone mass between 20° N and 60° N NH Air and Ozone STE 0.13 ± 0.02 (1.3% dec-1) 0.52 ± 0.17 (1.5% dec-1) 0.009 ± 0.04 -0.15 ± 0.33 • Trends of ozone STE and StratO3 mass are not significant. • The trend in tropospheric ozone mass is due to tropospheric production of ozone.

39. Strong correlation of lowermost stratospheric ozone with the ozone flux across 380 K surface

40. ...but not with the air flux across 380 K surface. NH Air Flux (Eg) • Air flux fit is not significantly different from zero, so lowermost stratosphere ozone mass does not significantly depend on the air flux. • By extension, ozone STE cannot be simply scaled from air STE.

41. Correlation of Ozone STE with Tropospheric StratO3 and Ozone Mass by Latitude Only statistically significant values are shown • Significant correlation of ozone STE with tropospheric StratO3 and ozone from middle latitudes to pole in NH, but only with StratO3 at middle latitudes in the SH.

42. Percent Change in Tropospheric Ozone Mass to 10% change in Ozone STE • The mass change with respect to a 10% change in ozone STE is estimatedusing a linear fit of ozone STE to tropospheric StratO3 and ozone. • The annual mean tropospheric ozone mass is highly buffered with 1%-1.5% change in mass to a 10% change in ozone STE.

43. A Few of the Major Points • All Replay simulations exhibit greater STE than MERRA-2, consistent with age-of-air being slightly young compared to observations. • The 1° resolution simulation is very similar to the 0.5° simulation in most comparisons. The 2° simulation shows large differences. • Replaying to water vapor greatly improves the 2° simulation for the flux calculations. It doesn’t have large effect on ozone chemistry. • Between 15% to 50% of annual mean tropospheric ozone is of stratospheric origin depending on latitude. • Increasing trend in NH tropospheric ozone is primarily tropospheric in origin, not from increasing STE. • Only a 1%-1.5% change in NH annual tropospheric ozone mass is expected to occur with a 10% change in NH ozone STE. In SH, the change is not statistically significant.