Introduction

Background• Elevated PM2.5 in Salt Lake Valley Large population, high vehicle emissions • Elevated O3 in Uintah Basin Significant industrial fossil fuel extraction activities,

shallow cold pool and highly reflective snow surface

Contact: erik.crosman@utah.edu

Meteorological Processes Impacting Cold-Air Pools

Future Work • Simulating both clear and cloudy, calm and

disturbed CAPs• Testing ice fog and aerosol-aware Thompson

schemes (Kim et al. 2014; Thompson & Eidhammer 2014)

• Testing several other new PBL, surface layer, and cloud microphysics schemes

• Additional research regarding use of targeted large-eddy simulations, albedo/snow treatment, land use, and initialization

Left images: NASA SPoRT satellite images: (a) Snow-Cloud product at 1115 MST 2 February 2013 and (b) Nighttime Microphysics RGB product at 0331 MST 2 February 2013. Right images: Difference between ice fog and stratus cloud WRF simulations during 31 December-7 February 2013. (c) 2-m temperature (°C) and (d) downwelling longwave radiation (W m-2)

Uintah Basin

Great Salt Lake Basin

Utah

• High wintertime ozone concentrations in rural areas associated with oil and gas development and high particulate concentrations in urban areas are topics of concern in Western US basins

• The physical processes that contribute to the formation, maintenance, and decay of persistent wintertime cold-air pools (CAPs) are only partially understood

• Weather Research and Forecasting (WRF) model simulations used in concert with observations from the Persistent Cold Air Pool and Uintah Basin Ozone Studies

• Ongoing efforts to improve model capability to simulate cold pool conditions

Boundary-Layer CloudsThe CAPs are sensitive to clouds. Clear skies and fresh snowpack results in colder nighttime temperatures. Stratus clouds are more optically thick to infrared radiation than ice fogs and result in more surface warming. Cloud-topped CAPs are generally deeper with less polluted surface layers, and may be less susceptible to erosion by weak weather systems

Warming and Cooling Aloft and Large-Scale Flow Persistent wintertime CAPs are largely driven by changes in mid-level weather patterns. High pressure moving over the intermountain west oftenresults in rapid warming at mountaintop level, while complex topography blocks warm advection at the lower levels and entraps cold air in the basins. A strong cold front with rapid cooling aloft is often required to destroy the CAP. Weaker weather systems may bring partial mix-outs to upwind portions of basins (Lareau and Horel 2014)

A multitude of atmospheric processes contribute to CAPs (Lareau et al. 2013). Forecasting CAP intensity, vertical structure, cloudiness, and decay remains difficult (Lareau and Horel 2015)

Weather Research and Forecasting (WRF) Model • Run as mesoscale model (ΔX

~1.33 km) and as large-eddy simulation (ΔX ~0.250 km)

• 2-3 nested grids • Large-eddy simulations (LES)

without PBL scheme, 1.5 order TKE subgrid-scale turbulence closure

• Mesoscale simulation uses YSU PBL scheme

• Thompson microphysics (with and without Neemann et al. 2014 modifications)

• No meteorology nudging used• Idealized modifications to snow

cover and albedo applied

WRF LES

WRF Mesoscale

Great Salt Lake TemperatureNumerical studies show that Salt Lake Valley CAPs are sensitive to GSL temperature. A colder lake results in greater advection of high-stability cold air inland in the afternoon associated with the lake breeze front. A colder lake also results in more widespread and persistent low clouds which cool the daytime boundary layer

Average simulated 2-m temperature (in °C) between 31 January and 6 February 2013 for (a) snow and (b) no snow

Plan view of the Salt Lake Valley. MODIS true color images on (a) 2040 UTC 12 December 2010 and (b) 1900 UTC 6 January 2011. (c) 2-m Temperature difference (°C) between SNOW and NO SNOW WRF simulation for 27-31 January 2011 CAP

Simulated 2-m temperature (°C, left) and wind speed (m s-1, right) during a partial CAP mix-out event

References Kim, C. K., and Coauthors, 2014: Numerical modelling of ice fog in

interior Alaska using the weather research and forecasting model, Pure Appl. Geophys., 1–20.

Lareau, N.P., and Coauthors, 2013: The persistent cold-air pool study. Bulletin of the American Meteorological Society, 94, 51-63

Lareau, N.P., J.D. Horel, 2014: Dynamically Induced Displacements of a Persistent Cold-Air Pool, Boundary-Layer Meteorology

Lareau, N.P. , and J. Horel, 2014: Turbulent Erosion Of Persistent Cold-Air Pools: Numerical Simulations. Journal of Atmospheric Sciences, accepted

Neemann, E., E. Crosman, J. Horel, and L. Avey, 2014: Simulations of a cold-air pool associated with elevated wintertime ozone in the Uintah Basin, Utah. ACPD, 14, 1-48

Thompson, G., and T. Eidhammer, 2014: A study of aerosol impacts on clouds and precipitation development in a large winter cyclone. Journal of the Atmospheric Sciences, 71, 3636-3658

Snow CoverSnow cover has a significant impact on CAPs through nocturnal radiative cooling and daytime reflection of incoming solar insolation. The presence of snow cover decreases simulated 2-m temperatures by 2-12 ᵒC. The SLV is less sensitive to snow cover variations than the Uintah Basin due to the increased urban land use and adjacent Great Salt Lake (GSL)

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Time-height of potential temperature (K) 27-31 January 2011 for LES simulations with (a) full snow cover in SLV, (b) GSL -3 ᵒC cool lake temperature anomaly, and (c) +3 ᵒC warm lake temperature anomaly

Average 2-m daytime temperature difference (°C) for 27-31 January 2011 between +3 ᵒC warm lake temperature anomaly (left) and -3 ᵒC cool lake temperature anomaly (right)

Vertical Mixing and Winds

Ɵ PBL: YSUΔX 1335 m

Ɵ PBL: noneΔX 250 m

PCAPS Ɵ observations

Time-height of potential temperature 27-31 January 2011. Top: WRF Mesoscale simulation; Middle: WRF LES simulation; Bottom: PCAPS observations

Large-eddy simulation (LES) less dispersive, allow cold pool to be deeper and persist longer. Elevated cloud layers are able to persists in LES simulation

Snow Depth and AlbedoImproving Numerical Weather Prediction of Cold Air Pools

WRF surface albedo at 18:00MST 31 January 2013 for (a) before and (b) after modifications to WRF snow albedo and vegetation parameter table. See Neemann et al. 2014

Land Use and Initialization Time

Cloud Occurrence and Type

GreatSaltLake Salt

Lake Valley

Recent simulations have shown CAP sensitivity to variations in land use (USGS vs MODIS vs NLCD 2006 options) as well as initialization time. Starting a simulation during an ongoing CAP results in poor CAP simulation

Modifying the snow depth and albedo in the Uintah Basin resulted in improved simulations of 1-7 February 2013 CAP (Neemann et al. 2014)

CAPs are very sensitive to differences between liquid and ice clouds as well as clear versus cloudy. Simple modifications were employed to produce ice fog in the Uintah Basin with beneficial results in Neemann et al. 2014. The WSM3 microphysics scheme was used to remove unwanted spurious stratus

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