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7 th Annual CMAS Conference October 6, 2008

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7 th Annual CMAS Conference October 6, 2008

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  1. Three-dimensional Model Studies of Exchange Processes in the Troposphere:Use of Potential Vorticity to Specify Aloft O3 in Regional ModelsRohit Mathur1, Hsin-Mu Lin2, Stuart McKeen3, Daiwen Kang2, David Wong11Atmospheric Modeling Division, NERL, U.S. EPA, RTP, NC2Science and Technology Corporation, RTP, NC3Chemical Sciences Division, ESRL, NOAA, Boulder, CO 7th Annual CMAS Conference October 6, 2008

  2. 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. LBC Specification Using Global CTMs: Mixed Success GFS-CMAQ Example FT over predictions primarily due to high bias in GFS O3 Also see presentation by Youhua Tang: Dust transport

  4. “Boundary Layer”: Surface – 3km “Free Troposphere”: 3km – Model top Understanding the Role of LBC Specification Relative contributions to background Modeled “background” O3 • Added diagnostic tracers to track impact of lateral boundary conditions: surface-3km (BL) and 3km-model top (FT) • Quantify modeled “background” O3 Average: July 1-August, 2006 • Significant spatial variability • Background could constitute a • sizeable fraction of the revised • O3 NAAQS LBC in FT determines surface background FT processes are important in regional modeling and deserve greater attention

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

  6. 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 Vd 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 CMAQ Configuration

  7. Bratts Lake (23) Kelowna (23) Sable Island Yarmouth Egbert (11) Paradox Walsingham Valparaiso Narragansett (22) Trinidad Head (23) Boulder (24) Beltsville Wallops Huntsville (24) Socorro (20) Table Mtn. (23) Holtville (11) Houston (12) Ron Brown (18) IONS Network Extensive ozonesonde data from IONS during summer 2006 provides a unique opportunity to assess model’s ability to simulate 3D O3 distributions over the CONUS Number of launches during study period in red

  8. Potential Vorticity and Ozone • Danielsen (1968) demonstrated the strong correlation between O3 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 O3 rich stratospheric air regionally and episodically • Modeled O3 specified by enforcing the condition of proportionality to PV: [O3] = c•PV

  9. 100 200 300 400 500 600 700 800 900 1000 • CONUS Domain • 12 km resolution • Vertical Discretization 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 O3 in upper troposphere

  10. 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 Estimating the O3-PV Proportionality Constant • Correlation varies by location • Slope: 20-39 ppb/PV unit; r2>0.7 • All data: • Slope: 30 ppb/PV unit; r2 = 0.76

  11. Numerical Experiments • Modeled O3 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 • O3 LBC also modulated at altitudes > 8.5km

  12. Can Use of Potential Vorticity Improve Simulated 3-D O3 Distribution?Case 1: Modeled O3 specified by enforcing proportionality to PV (altitude>8.5km); August 5-29, 2008Median 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 O3 • in regions of convective activity and • frontal passage

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

  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. Observed Max. 8-hr O3 (ppb) Comparison with Surface O3 MeasurementsDistributions of Max. 8-Hr. O3 Mean Bias

  16. Summary • Use of PV to specify O3 in the upper troposphere results in improvements in simulated 3-D O3 distributions • Improvements in representation of lower-mid tropospheric O3 mixing ratios in the southeastern U.S. (Huntsville, Houston profiles) • The impacts on simulated surface-level max. 8-hr O3 mixing ratios (relative to measurements) is minimal • Enhancements in surface-level O3 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 O3 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

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