1. Figure 1. Schematic of factors contributing to high ozone concentrations. Potential temperature profile (red line) with stable layer trapping ozone precursors (NO x and VOCs) within the Cold-Air Pool (CAP). Snow cover reflects solar radiation, increases photolysis rates, and leads to enhanced ozone (O 3 ) concentrations near the surface. Ice fogs are common in the CAP.  Z NO x Snow Cover VOCs O3O3

2. Figure 2. (a) WRF 12-, 4-, and 1.33-km domains with terrain contoured every 500 m. (b) Uintah Basin subdomain with terrain contoured every 250 m and major geographic features labeled. Black dots indicate locations of surface stations used for verification: Horsepool (HOR), Myton (MYT), Ouray (OUR), Red Wash (RED), Roosevelt (ROO), and Vernal (VER). Red line indicates position of vertical cross sections shown later. 1.33 km 4 km 12 km (a) 0 1000 2000 3000 3500 2500 1500 500 4000 4500 Uinta Mountains Wasatch Range Tavaputs Desolation Canyon Plateau WY CO UT (b) 1250 1500 1750 2250 2750 3250 3750 4000 3500 3000 2500 2000 ROO MYT OUR HOR RED VER

3. Figure 3. Snow depth (blue) and snow water equivalent (red) as a function of elevation for 0000 UTC 1 Feb 2013 for: prescribed snow applied to WRF simulations (black line); observations (O) from the Uintah Basin and surrounding mountains; and NAM analysis (X). NAM analysis data were extracted along a southeast to northwest transect from the center of the basin to the center of the Uinta Mountains.

4. Figure 4. Snow depth from (a) NAM analysis at 0000 UTC 01 Feb 2013, (b) “Full Snow” cases (BASE/FULL), (c) “No Western Snow” case (NW), and (d) “No Snow” case (NONE). (c) (b) (d) (a)

5. Figure 4. Snow depth from (a) NAM analysis at 0000 UTC 01 Feb 2013, (b) “Full Snow” cases (BASE/FULL), (c) “No Western Snow” case (NW), and (d) “No Snow” case (NONE). (c) (b) (d) (a)

6. Figure 4. Top row shows WRF surface albedo at 0100 UTC 1 February 2013 for (a) before and (b) after modifications to WRF snow albedo and VEGPARM.TBL. Bottom row shows Initialized snow depth (in m) at 0000 UTC 1 February 2013 for (a) “Full Snow” cases (BASE/FULL) and (b) “No Snow” case (NONE). Thin black lines indicate terrain contours every 500 m. (c) (b) (d) (a)

7. (c) (b) (d) (e)(f) (h) Figure 5. Observed and simulated vertical profiles at Roosevelt of (a, b) potential temperature, (c, d) relative humidity with respect to ice, (e, f) wind speed, and (g, h) wind direction for 1800 UTC 4 February 2013 (left column) and 1800 UTC 5 February 2013 (right column). (g)

8. (a) (b) Figure 6. SPoRT-derived VIIRS satellite images: (a) Snow-Cloud product at 1815 UTC 2 February 2013 and (b) Nighttime Microphysics RGB product at 0931 UTC 2 February.

9. Figure 7. (a) Ozone concentrations from 1-10 February 2013 for Roosevelt (black), Horsepool (blue), Vernal (red), and Ouray (green) with the 75 ppb (8-hour mean) NAAQS denoted by the dashed line. (b) Ceilometer backscatter (shaded) and estimated aerosol depth (black dots) at Roosevelt from 1 - 7 Feb 2013. Red, yellow, blue, and white shading denote fog and stratus clouds, high aerosol concentrations; low aerosol concentrations, and beam attenuation, respectively. 4 km (a) (b) Day of February

10. Figure 7. (a) Ozone concentrations from 1-10 February 2013 for Roosevelt (black), Horsepool (blue), Vernal (red), and Ouray (green) with the 75 ppb (8-hour mean) NAAQS denoted by the dashed line. (b) Ceilometer backscatter (shaded) and estimated aerosol depth (black dots) at Roosevelt from 1 - 7 Feb 2013. Red, yellow, blue, and white shading denote fog and stratus clouds, high aerosol concentrations; low aerosol concentrations, and beam attenuation, respectively. 4 km (a) (b) Day of February

11. Figure 8. Average 2-m temperature (in °C according to the scale below) for 1-6 February 2013 from (a) BASE, (b) FULL, (c) NW, (d) NONE simulations. (a) (c) (b) (d) -12 2-10 -80-2 -4-6

12. Figure 9. Average difference (BASE – FULL) for 1-6 February 2013 period in: (a) 2-m temperature (in °C according to the scale to the right) and (b) downwelling longwave radiation (in W m -2 according to the scale on the right) 0.5 1.5 1 0 2 -0.5 5 10 0 15 20 (a) (b) -5

13. Figure 10. Cloud characteristics from BASE (a,c,e) and FULL (b,d,f) simulations at 0600 UTC 5 Feb 2013. (a,b) Integrated cloud amount (in mm according to the scale on the right), (c) mean cloud water in bottom 15 model levels (in g kg -1 according to the scale on the right), (d) mean cloud ice in bottom 15 model levels (in g kg -1 according to the scale on the right), (e,f) net downwelling longwave radiation from clouds (in W m -2 according to the scale on the right). 0 0 0 0.2 0.05 0.15 0.25 0.1 0.2 0.05 0.15 0.25 0.1 0.20 0.30 0.1 0.20 0.30 0.1 80 20 60 100 40 120 80 20 60 100 40 120 (a) (c) (b) (d) (e)(f) 0 0 0

14. Figure 11. FULL simulation at 0600 UTC 4 February 2013 for (a) 2.3 km MSL wind speed (in m s -1 according to the scale on the right) and barbs (full barb 5 m s -1 ). (b) Vertical cross section of potential temperature (in K according to the scale on the right) along red line in Fig. 1.5b. (b) 280 288 300 304 296 284 276 292 278 286 298 302 294 282 290 STAHORROOOUR 3.0 2.0 1.5 2.5 Height (km) Distance (km) 50 100150 200 WE (a) 10 20 35 40 30 15 5 25 0

15. (a) -3 3 5 1 -5 -4 2 4 0 -2 2.6 3.0 2.2 1.8 2.4 2.8 2.0 1.6 Height (km) 1.4 2.6 3.0 2.2 1.8 2.4 2.8 2.0 1.6 Height (km) 1.4 (b) (c) (d) Figure 12. Average zonal wind in the vicinity of the cross-section in Fig. 1.5b for the 1-6 February 2013 period. Top row includes only daytime hours (0800 to 1700 MST), bottom row includes only nighttime hours (1800 to 0700 MST) for FULL (a and c), and NONE (b and d). Westerly (easterly) winds shaded in m s -1 according to the scale on the right in red (blue) with westerly (easterly) winds contoured every 2 m s -1 ( -0.5, -1, and -2 m s -1 only). Values are averaged over a ~26-km wide swath perpendicular to the cross section. Figure 12. Average zonal wind in the vicinity of the cross-section in Fig. 1.5b for the 1-6 February 2013 period. The FULL simulation results are shown on the top row, the NONE simulation on the bottom row for (a, c) daytime hours (0800 to 1700 MST) and (b, d) nighttime hours (1800 to 0700 MST). Westerly (easterly) winds shaded in m s -1 according to the scale on the right in red (blue) with westerly (easterly) winds contoured every 2 m s -1 ( -0.5, -1, and -2 m s -1 only). Values are averaged over a ~26-km wide swath perpendicular to the cross section.

16. Figures for AQ section

17. Figure 13. Mobile transect of ozone concentration from 1130 to 1500 MST 6 February 2013 as a function of: (a) geographic location and (b) time. (a) (b) Vernal Roosevelt Ouray

18. Figure 14. Average ozone concentration (ppb) during 1100-1700 MST 1-6 February 2013 on the lowest CMAQ model level (~17.5 m) from (a) FULL and (b) NONE simulations. The thin black line outlines regions where the ozone concentration exceeds 75 ppb while the reference terrain elevation of 1800 m is shown by the heavy black line. (a)(b) 2.6 3.0 2.2 1.8 2.4 2.8 2.0 1.6 Height (km) 1.4 1.2 2.6 3.0 2.2 1.8 2.4 2.8 2.0 1.6 Height (km) 1.4 1.2 80 100 60 40 70 90 50 30 (c)(d)

19. Figure 14. Average ozone concentration (ppb) during 1100-1700 MST 1-6 February 2013 on the lowest CMAQ model level (~17.5 m) from (a) FULL and (b) NONE simulations. The thin black line outlines regions where the ozone concentration exceeds 75 ppb while the reference terrain elevation of 1800 m is shown by the heavy black line. (a)(b) 2.6 3.0 2.2 1.8 2.4 2.8 2.0 1.6 Height (km) 1.4 1.2 80 100 60 40 70 90 50 30 (c)(d)

20. (a) (b) (c) 2.6 2.2 1.8 2.4 2.0 1.6 Height (km) 274282 294298 290278270286272 280292296 288 276284300 Figure 15. Time Series of ozone concentrations from (a) Roosevelt, and (b) Horsepool. Observations, CMAQ output from FULL and NONE simulations in blue, red, and black respectively. The NAAQS of 75 ppb is denoted by the thin black dashed line. (c) Time-Height of potential temperature (shaded according to scale on bottom and contoured in thin black) and ozone concentrations at Horsepool from FULL simulation. Ozone concentrations are contoured every 10 ppb, starting at 75 ppb and alternate between solid and dashed every 10 ppb.

21. (a) (b) (c) 2.6 2.2 1.8 2.4 2.0 1.6 Height (km) 280 290 270 285 275 295 300 Figure 15. Time Series of ozone concentrations from (a) Roosevelt, and (b) Horsepool. Observations, CMAQ output from FULL and NONE simulations in blue, red, and black respectively. The NAAQS of 75 ppb is denoted by the thin black dashed line. (c) Time-Height of potential temperature (shaded according to scale on bottom and contoured in thin black) and ozone concentrations at Horsepool from FULL simulation. Ozone concentrations are contoured every 10 ppb, starting at 75 ppb and alternate between solid and dashed every 10 ppb.

22. Tables

23. Table 1. Number of NAAQS exceedances for ozone at Ouray since 2009. Ouray is located in the center of the Uintah Basin in the region of lowest elevation and typically sees some of the highest ozone concentrations. Calendar Year Ouray Exceedances Uintah Basin Snow Cover in February 20091Partial 201040Full 201124Full 20121Partial 201339Full 20143Partial

24. Table. Summary of WRF setup and parameterizations ParameterChosen SetupReference Initial/Boundary ConditionsNAM Analysis Vertical Levels41 Domains3 one-way nests Resolution12 km, 4 km, 1.33 km Time Step45 s, 15 s, 5 s MicrophysicsThompsonThompson et al. 2008 Shortwave RadiationRRTMGIacono et al. 2008 Longwave RadiationRRTMGIacono et al. 2008 Boundary LayerMellor-Yamada-Janjic (MYJ)Janjic 1994 Surface LayerEta Similarity Land SurfaceNoahChen and Dudhia 2001 CumulusKain-Fritsch (12 km domain only)Kain 2004 Diffusion2nd order on coordinate surfaces

25. Table 3. Overview of WRF sensitivity studies Prescribed Snow Cover Cloud Ice Sedimentation Cloud Ice Auto- conversion to Snow Simulation Name Microphysics Sensitivity Simulations Full Snow in basinON BASE Full Snow in basinOFF FULL Snow Cover Sensitivity Simulations No Snow below 2100 m in Western 1/4 of basin OFF NW No Snow below 2000 m in basin OFF NONE

26. Table 3. Overview of WRF sensitivity studies Prescribed Snow Cover Vegetation Parameter Table Cloud Ice Sedimentation Cloud Ice Auto- conversion to Snow Simulation Name Microphysics Sensitivity Simulations Full Snow in basinModifiedOn BASE Full Snow in basinModifiedOff FULL Snow Cover Sensitivity Simulations No Snow below 2100 m in Western 1/4 of basin ModifiedOff NW No Snow below 2000 m in basin ModifiedOff NONE Alternate

27. Table 4. 2-m temperature errors from WRF simulations. Mean errors calculated from the six surface stations in Fig. 1.5b during the 1-6 February 2013 period. SimulationBias (C)Mean Abs Error (C)RMSE (C) BASE1.653.253.97 FULL0.112.442.98 NW0.252.442.99 NONE7.717.748.29

28. Table 5. Ozone concentration statistics from CMAQ model forced by FULL and NONE simulations during the 1-6 February 2013 period. FULLNONE Highest mean O 3 - Afternoon (ppb)97.281.2 Highest mean O 3 - Non afternoon (ppb)61.951.0 Maximum Hourly O 3 (ppb)134.4118.0 Area of mean afternoon O 3 > 75 ppb (km 2 )896144

29. Fig. 2.5: Snow depth from (a) NAM analysis at 0000 UTC 01 Feb 2013, (b) “Full Snow” cases (BASE/FULL), (c) “No Western Snow” case (NW), and (d) “No Snow” case (NONE). 0 0.1 0.2 0.4 0.3 0.5 0.6 0.7 0.9 0.8 1 (c) (b) (d) (a)