1. Atmospheric Modeling Division National Exposure Research Laboratory Three-dimensional Model Studies of Exchange Processes in the Troposphere: Use of Potential Vorticity to Specify Aloft O 3 in Regional Models Rohit Mathur 1, Hsin-Mu Lin 2, Stuart McKeen 3, Daiwen Kang 2, David Wong 1 1 Atmospheric Modeling Division, NERL, U.S. EPA, RTP, NC 2 Science and Technology Corporation, RTP, NC 3 Chemical Sciences Division, ESRL, NOAA, Boulder, CO 7 th Annual CMAS Conference October 6, 2008

2. Atmospheric Modeling Division National Exposure Research Laboratory 1 Why Study 3-D Pollutant Distributions? Our models are 3-D, even though the focus is primarily on pollution within the PBL Expanding domains (Continental and larger) and longer simulation periods (seasonal to annual) makes examination of trace species simulation in the Free-troposphere imperative – Sufficient opportunities for exchange between the BL and FT – Mass of summertime upper troposphere overturns in 5-10 days due to convection [Bertram et al.,2007] Implying an equivalent mass flux from the FT to the BL, thereby impacting “background” levels In most regional models simulated variability in the FT is largely dictated by specification of lateral boundary conditions due to efficient transport by fast winds –Static LBCs can not adequately represent the variability –Global model biases can propagate and influence regional calculations

3. Atmospheric Modeling Division National Exposure Research Laboratory 2 FT over predictions primarily due to high bias in GFS O 3 LBC Specification Using Global CTMs: Mixed Success GFS-CMAQ Example Also see presentation by Youhua Tang: Dust transport

4. Added diagnostic tracers to track impact of lateral boundary conditions: surface-3km (BL) and 3km-model top (FT) –Quantify modeled “background” O 3 “Boundary Layer”: Surface – 3km “Free Troposphere”: 3km – Model top Average: July 1-August, 2006 Modeled “background” O 3 Relative contributions to background LBC in FT determines surface background FT processes are important in regional modeling and deserve greater attention Understanding the Role of LBC Specification Significant spatial variability Background could constitute a sizeable fraction of the revised O 3 NAAQS

5. Verification Tools CMAQ WRF Post PRDGEN AQF Post Vertical interpolation Horizontal interpolation to Lambert grid CMAQ-ready meteorology and emissions Gridded product files for users Chemistry/Transport/Deposition model NAM Meteorology model Performance feedback for users/developers PREMAQ Meteorological Observations Emission Inventory Data WRF-NMM Air Quality Observations WRF-NMM-CMAQ AQF Modeling System

6. Atmospheric Modeling Division National Exposure Research Laboratory 5 CMAQ Configuration Advection –Horizontal: Piecewise Parabolic Method –Vertical: Upstream with re-diagnosed vertical velocity to satisfy mass conservation Turbulent Mixing –Asymmetric Convective Model (ACM2) Cloud Processes –Mixing and aqueous chemistry –Scavenging and wet deposition –“In-cloud” mixing based on the Asymmetric Convective Mixing Dry Deposition –M3dry modified to use NAM land surface parameters –Estimated V d sensitive to NAM LSM changes Gas-Phase Chemistry –ODE Solver: EBI solver –CBM-IV in Operational model –Below cloud attenuation based on ratio of NAM radiation reaching the surface to its clear-sky value

7. Atmospheric Modeling Division National Exposure Research Laboratory 6 Kelowna (23) Bratts Lake (23) Trinidad Head (23) Table Mtn. (23) Socorro (20) Boulder (24) Houston (12) Ron Brown (18) Huntsville (24) Valparaiso Beltsville Wallops Holtville (11) Egbert (11) Walsingham Narragansett (22) Yarmouth Sable Island Paradox IONS Network Extensive ozonesonde data from IONS during summer 2006 provides a unique opportunity to assess model’s ability to simulate 3D O 3 distributions over the CONUS Number of launches during study period in red

8. Atmospheric Modeling Division National Exposure Research Laboratory 7 Potential Vorticity and Ozone Danielsen (1968) demonstrated the strong correlation between O 3 mixing ratios and potential vorticity –Both have high values in the stratosphere and low in the troposphere Several modeling studies have used this correlation to examine stratosphere-troposphere exchange – Air mass flux into the troposphere mainly induced by deformational flow – Leading to intensified downward fluxes of O 3 rich stratospheric air regionally and episodically Modeled O 3 specified by enforcing the condition of proportionality to PV: [O 3 ] = cPV

9. Atmospheric Modeling Division National Exposure Research Laboratory 8 Experiment Design Study Period –August 5-29, 2008 2 layer configurations –22 layers (used in CMAQ AQF) –56 layers (exactly matching NAM’s layer structure; surface to ~100mb) For each layer configuration –Base Run using default LBC profile; horizontally invariant –Incorporation of Potential Vorticity (from NAM fields to specify O 3 in upper troposphere 100 200 300 400 500 600 700 800 900 1000 CONUS Domain 12 km resolution Vertical Discretization

10. Atmospheric Modeling Division National Exposure Research Laboratory 9 Estimating the O 3 -PV Proportionality Constant Little information available how proportionality varies with height and latitude or episodically Previous studies – 20-35 ppb/PV unit (Carmichael et al., 1998; Asia) –50 ppb/PV unit (McCaffrey et al, 2004; East US) –100 ppb/PV unit (Ebel et al., 1991; Europe) To calibrate relationship: PVs computed from NAM data regressed with ozonesonde data Correlation varies by location Slope: 20-39 ppb/PV unit; r 2 >0.7 All data: Slope: 30 ppb/PV unit; r 2 = 0.76

11. Atmospheric Modeling Division National Exposure Research Laboratory 10 Numerical Experiments Modeled O 3 specified by enforcing proportionality to PV Case 1 – c=20 ppb/PV unit – Top 3 model layers 22 (>8.5km) and 56 (>12km) layer configurations Case 2 – c=30 ppb/PV unit – Altitudes > 8.5km 22 and 56 layer configurations – O 3 LBC also modulated at altitudes > 8.5km

12. Can Use of Potential Vorticity Improve Simulated 3-D O 3 Distribution? Case 1: Modeled O 3 specified by enforcing proportionality to PV (altitude>8.5km); August 5-29, 2008 Median values of modeled and observations paired in space and time Improvements in upper and mid-troposphere at all IONS sites Improved representation of lower-mid tropospheric values in SE U.S. Enhancements in surface-level O 3 in regions of convective activity and frontal passage

13. Case 1: Comparison of 22 and 56 layer configurations Similar at lower altitudes Vertical resolution + PV results in even greater improvement in upper and mid-troposphere

14. Impact of Proportionality Constant Case 1 Case 2 Case 2: consistent over-estimation ~8-14 km Optimal Configuration: between case 1 and 2?

15. Atmospheric Modeling Division National Exposure Research Laboratory 14 Comparison with Surface O 3 Measurements Distributions of Max. 8-Hr. O 3 Mean Bias Observed Max. 8-hr O 3 (ppb)

16. Atmospheric Modeling Division National Exposure Research Laboratory 15 Summary Use of PV to specify O 3 in the upper troposphere results in improvements in simulated 3-D O 3 distributions –Improvements in representation of lower-mid tropospheric O 3 mixing ratios in the southeastern U.S. (Huntsville, Houston profiles) The impacts on simulated surface-level max. 8-hr O 3 mixing ratios (relative to measurements) is minimal –Enhancements in surface-level O 3 do occur in regions with frontal passage and convective activity –Impacts could be larger during spring and winter when STE events are more frequent Improving the vertical resolution of the model, 56L vs. 22L, results in better agreement between simulated and observed O 3 profiles in the middle and upper troposphere Method is sensitive to the choice of constant of proportionality as well as the altitudes at which it is applied