Persistent Cold-Air Pool Research Articles

Impact of climate change on persistent cold-air pools in an alpine valley during the 21st century

Abstract. When anticyclonic conditions persist over mountainous regions in winter, cold-air pools (i.e. thermal inversions) develop in valleys and persist from a few days to a few weeks. During these persistent cold-air pool (PCAP) episodes the atmosphere inside the valley is stable and vertical mixing is prevented, promoting the accumulation of pollutants close to the valley bottom and worsening air quality. The purpose of this paper is to address the impact of climate change on PCAPs until the end of this century for the alpine Grenoble valleys. The long-term projections produced with the general circulation model MPI (from the Max Planck Institute) downscaled over the Alps with the regional climate model MAR (Modèle Atmosphérique Régional) are used to perform a statistical study of PCAPs over the period 1981–2100. The trends of the main characteristics of PCAPs, namely their intensity, duration, and frequency, are investigated for two future scenarios, SSP2–4.5 and SSP5–8.5. We find that the intensity of PCAPs displays a statistically significant decreasing trend for the SSP5–8.5 scenario only. This decay is explained by the fact that air temperature over the century increases more at 2 m above the valley bottom than in the free air at mid-altitudes in the valley; this might be due to the increase of specific humidity near the ground. The vertical structure of two PCAPs, one in the past and one around 2050, is next investigated in detail. For this purpose, the WRF (Weather Research and Forecasting) model, forced by MAR for the worst-case scenario (SSP5–8.5), is used at a high resolution (111 m). The PCAP episodes are carefully selected from the MAR data so that a meaningful comparison can be performed. The future episode is warmer at all altitudes than the past episode (by at least 4 ∘C) and displays a similar inversion height, which are very likely generic features of future PCAPs. The selected episodes also have similar along-valley wind but different stability, with the future episode being more stable than the past episode. Overall, this study shows that the atmosphere in the Grenoble valleys during PCAP episodes tends to be slightly less stable in the future under the SSP5–8.5 scenario, and statistically unchanged under the SSP2–4.5 scenario, but that very stable PCAPs can still form.

Evaluation of a cloudy cold-air pool in the Columbia River basin in different versions of the High-Resolution Rapid Refresh (HRRR) model

Abstract. The accurate forecast of persistent orographic cold-air pools in numerical weather prediction models is essential for the optimal integration of wind energy into the electrical grid during these events. Model development efforts during the second Wind Forecast Improvement Project (WFIP2) aimed to address the challenges related to this. We evaluated three versions of the National Oceanic and Atmospheric Administration (NOAA) High-Resolution Rapid Refresh model with two different horizontal grid spacings against in situ and remote sensing observations to investigate how developments in physical parameterizations and numerical methods targeted during WFIP2 impacted the simulation of a persistent cold-air pool in the Columbia River basin. Differences amongst model versions were most apparent in simulated temperature and low-level cloud fields during the persistent phase of the cold-air pool. The model developments led to an enhanced low-level cloud cover, resulting in better agreement with the observations. This removed a diurnal cycle in the near-surface temperature bias at stations throughout the basin by reducing a cold bias during the night and a warm bias during the day. However, low-level clouds did not clear sufficiently during daytime in the newest model version, which leaves room for further model developments. The model developments also led to a better representation of the decay of the cold-air pool by slowing down its erosion.

Confronting Uncertainties of Simulated Air Pollution Concentrations during Persistent Cold Air Pool Events in the Salt Lake Valley, Utah.

Air pollutant accumulations during wintertime persistent cold air pool (PCAP) events in mountain valleys are of great concern for public health worldwide. Uncertainties associated with the simulated meteorology under stable conditions over complex terrain hinder realistic simulations of air quality using chemical transport models. We use the Community Multiscale Air Quality (CMAQ) model to simulate the gaseous and particulate species for 1 month in January 2011 during the Persistent Cold Air Pool Study (PCAPS) in the Salt Lake Valley (SLV), Utah (USA). Results indicate that the temporal variability associated with the elevated NOx and PM2.5 concentrations during PCAP events was captured by the model (r = 0.20 for NOx and r = 0.49 for PM2.5). However, concentrations were not at the correct magnitude (NMB = -35/12% for PM2.5 during PCAPs/non-PCAPs), where PM2.5 was underestimated during PCAP events and overestimated during non-PCAP periods. The underestimated PCAP strength is represented by valley heat deficit, which contributed to the underestimated PM2.5 concentrations compared with observations due to the model simulating more vertical mixing and less stable stratification than what was observed. Based on the observations, the dominant PM2.5 species were ammonium and nitrate. We provide a discussion that aims to investigate the emissions and chemistry model uncertainties using the nitrogen ratio method and the thermodynamic ammonium nitrate regime method.

Local identification of persistent cold air pool conditions in the Great Hungarian Plain

Cold air pool (CAP) is a winter-time, anticyclonic weather event: a cold air layer confined by the topography and warm air aloft. If its duration is more than one day, then it is called persistent cold air pool (PCAP). CAPs are mainly examined in small basins and valleys. Fewer studies pay attention to PCAPs in much larger basins (with an area of more than 50 000 km2), and it is not evident how effective the existing numerical definitions are in cases of extensive PCAP events. A possible method of identifying PCAPs in a large basin is to identify PCAP weather conditions at different measuring sites across the basin. If there are PCAP weather conditions at most of the sites, then it is likely to be an extensive PCAP. In this work, we examine which of the documented CAP definitions can be used for reliable local detection of CAP conditions. Daily weather reports and meteorological data from two locations in the 52 000 km2 sized Great Hungarian Plain have been used to obtain a reference set of days with PCAP weather conditions during two consecutive winter months. Several numerical CAP definitions were compared for their performance in recognizing the presence of PCAP weather conditions using radiosonde measurements and reanalysis data. The lowest error was produced by using the heat deficit (HD) method. So this is considered the most suitable method for local identification of PCAPs in the Great Hungarian Plain.

Community-Based Measurements Reveal Unseen Differences during Air Pollution Episodes.

Short-term exposure to fine particulate matter (PM2.5) pollution is linked to numerous adverse health effects. Pollution episodes, such as wildfires, can lead to substantial increases in PM2.5 levels. However, sparse regulatory measurements provide an incomplete understanding of pollution gradients. Here, we demonstrate an infrastructure that integrates community-based measurements from a network of low-cost PM2.5 sensors with rigorous calibration and a Gaussian process model to understand neighborhood-scale PM2.5 concentrations during three pollution episodes (July 4, 2018, fireworks; July 5 and 6, 2018, wildfire; Jan 3-7, 2019, persistent cold air pool, PCAP). The firework/wildfire events included 118 sensors in 84 locations, while the PCAP event included 218 sensors in 138 locations. The model results accurately predict reference measurements during the fireworks (n: 16, hourly root-mean-square error, RMSE, 12.3-21.5 μg/m3, n(normalized)RMSE: 14.9-24%), the wildfire (n: 46, RMSE: 2.6-4.0 μg/m3; nRMSE: 13.1-22.9%), and the PCAP (n: 96, RMSE: 4.9-5.7 μg/m3; nRMSE: 20.2-21.3%). They also revealed dramatic geospatial differences in PM2.5 concentrations that are not apparent when only considering government measurements or viewing the US Environmental Protection Agency's AirNow visualizations. Complementing the PM2.5 estimates and visualizations are highly resolved uncertainty maps. Together, these results illustrate the potential for low-cost sensor networks that combined with a data-fusion algorithm and appropriate calibration and training can dynamically and with improved accuracy estimate PM2.5 concentrations during pollution episodes. These highly resolved uncertainty estimates can provide a much-needed strategy to communicate uncertainty to end users.

Surface Turbulent Fluxes during Persistent Cold-Air Pool Events in the Salt Lake Valley, Utah. Part II: Simulations

AbstractRealistically representing the land–atmosphere interactions during persistent cold-air pools (PCAPs) is critical in simulating the strength of PCAPs, where uncertainties in simulating the PCAP strength will impact the ability to model the poor air quality. To quantify the model performance for land–atmosphere exchange, measurements of surface turbulent and radiative energy fluxes during two PCAPs, one weak and one strong, in Utah were compared with simulations from the Weather Research and Forecasting (WRF) Model. The results show that the WRF Model simulated the surface energy fluxes well in the weak PCAP case and that the performance degraded in the strong PCAP case. The significantly overestimated surface sensible heat flux H and latent heat flux (LE) in the strong PCAP were related, in part, to the overestimated net radiation and soil moisture and unsuitable turbulence parameterizations. The simulation using the Mellor–Yamada–Nakanishi–Niino planetary boundary layer scheme produced the least bias in both net radiation and surface turbulent fluxes for the strong PCAP case, which is expected because of the local higher-order (2.5) turbulence closure scheme. The surface exchange coefficient (CH), a crucial variable used to calculate H, was overall overestimated by the WRF Model. The underestimation of the nondimensional vertical temperature gradient in the Monin–Obukhov stability function was responsible for the overestimated CH, where the stability functions deviate significantly from expected values from observations for the stable atmospheric boundary layer. Our study highlights the need to improve the flux–profile parameterizations under stable conditions over complex terrain by including impacts due to mountainous terrain, such as surface radiative flux divergence and the diurnal mountain wind system.

Patterns of distributive environmental inequity under different PM2.5 air pollution scenarios for Salt Lake County public schools

Previous studies have cataloged social disparities in air pollution exposure in US public schools with respect to race/ethnicity and socioeconomic status. These studies rely upon chronic, averaged measures of air pollution, which fosters a static conception of exposure disparities. This paper examines PM2.5 exposure disparities in Salt Lake County (SLC), Utah public schools under three different PM2.5 scenarios—relatively clean air, a moderate winter persistent cold air pool (PCAP), and a major winter PCAP—with respect to race/ethnicity, economic deprivation, student age, and school type. We pair demographic data for SLC schools (n = 174) with modelled PM2.5 values, obtained from a distributed network of sensors placed through a community-university partnership. Results from generalized estimating equations controlling for school district clustering and other covariates reveal that patterns of social inequality vary under different PM2.5 pollution scenarios. Charter schools and schools serving economically deprived students experienced disproportionate exposure during relatively clean air and moderate PM2.5 PCAP conditions, but those inequalities attenuated under major PCAP conditions. Schools with higher proportions of racial/ethnic minority students were unequally exposed under all PM2.5 pollution scenarios, reflecting the robustness of racial/ethnic disparities in exposure. The findings speak to the need for policy changes to protect school-aged children from environmental harm in SLC and elsewhere.

Local and non‐local controls on a persistent cold‐air pool in the Arve River Valley

AbstractA numerical model is used to simulate a persistent cold‐air pool (PCAP) event that occurred in the section of the Arve River Valley around Passy in the French Alps. During this period, an upper‐level ridge from the Atlantic moved over Europe, allowing a PCAP to form and persist over time. The impact of the upper‐level ridge on the PCAP and on the dynamics within the valley section is quantified by examining the mass and heat budgets of the valley atmosphere. During the persistent stage, the magnitude of the flow through the tributary valleys is enhanced by the large‐scale flow. Also, the direction of the flow through one of the tributaries is found to be determined by the height of the PCAP with respect to that of the tributary above the valley floor. The tributary flows, together with subsiding motions at the valley top, control by and large the night‐time valley‐scale circulation and the thermal structure of the upper part of the PCAP, whereas thermally driven valley flows control its lower part. When the upper‐level ridge passes over the Arve River Valley, warm air advection through the tributaries continuously erodes the upper part of the PCAP during night‐time, thereby reducing its depth, while down‐valley flows export the air mass out of the valley. As the ridge moves away from the valley, the near‐surface air is found to be trapped within the valley. This trapping results from the advection of warm air in the upper part of the PCAP by the large‐scale flow channelled through one of the tributaries. This reduces the thermally induced pressure difference in the down‐valley direction, thereby suppressing the near‐surface down‐valley flow. The study therefore highlights the interplay between the large‐scale flow, the tributary flows and the thermal structure of the PCAP.

Surface Turbulent Fluxes during Persistent Cold-Air Pool Events in the Salt Lake Valley, Utah. Part I: Observations

AbstractThe land surface is coupled to the atmospheric boundary layer through surface turbulent fluxes. Persistent cold-air pools (PCAPs) form in topographic depressions where cold, dense air fills the valley basin and in the presence of air pollution is accompanied by poor air quality. For the first time, the surface turbulence dataset from seven monitors during the Persistent Cold-Air Pool Study conducted in Salt Lake Valley, Utah (December 2010–February 2011), are analyzed. We found that the surface sensible (H) and latent (LE) heat fluxes were lower during strong PCAP events compared with non-PCAPs. The higher ratio of heat flux to net radiation (H/Rn and LE/Rn) for strong PCAPs compared with weak PCAPs is suspected to be related to the presence of boundary layer clouds, which could enhance the turbulent mixing through cloud top–down mixing. The daily average ground heat flux (G) was a similar order of magnitude toHand LE during wintertime. The highest surface turbulent fluxes and energy balance closure occurred in the stability range of −0.05 <ξ≤ −0.02, or under slightly unstable conditions, near the neutral stability range. The median surface exchange coefficient (Ch), a crucial parameter to determine surface turbulent fluxes in land surface models, was slightly higher at the bare land site (BL) than the short vegetation sites (PH and CR) in wintertime, suggesting the importance of dynamic land-use information in numerical models.

Investigating fine particulate matter sources in Salt Lake City during persistent cold air pool events

During wintertime, Salt Lake City experiences periods of high-pressure atmospheric stagnation that leads to prolonged periods with stable atmospheric boundary layers (ABL) due to the unique orography of the region. These atmospheric conditions typically lead to persistent cold air pool (PCAP) events, where the stable ABL remains over several days, degrading air quality in the Salt Lake Valley. In this work, we applied novel source apportionment methods to identify sources that were the greatest contributors to elevated secondary PM2.5 concentrations during PCAP events in winter 2007. A source-oriented modeling approach that relies on emissions inventory data (i.e., source-specific data) was utilized for source characterization. A correction was applied to chemical transport model (CTM) outputs to improve the estimates of source contributions to secondary PM2.5 components (organic carbon, nitrate, sulfate, and ammonium). As a result, gasoline vehicles (25%), natural gas combustion (21%), and other sources (29%) were identified as the largest sources of nitrate, which was the dominant aerosol component during the PCAP periods ([NO3−]max = 37.5 μg m−3). Further, receptor-oriented source apportionment modeling was conducted using the chemical mass balance method with gas constraints (CMB-GC) with location-specific, mathematically optimized PM2.5 source profiles to examine the multi-year trend in PCAP pollution. These optimized profiles were enhanced by integrating the initial guess profiles with CTM results and observations. CMB-GC was applied for the period 2007–2015 at an urban monitor, and the use of seasonally varying optimized source profiles improved results by resolving more mass than when using static source profiles, increasing the predicted-observed PM2.5 mass ratio from 0.91 to 1.03. The averaged biomass burning impacts exhibited the most significant change, increasing from an average of 0.85 to 1.87 μg m−3 when using the static profiles and the optimized profiles, respectively. The static and optimized CMB-GC results were similar for secondary components but differed slightly for mobile impacts. Stratification of PCAP vs. non-PCAP CMB-GC source impacts revealed that the dominant sources of PM2.5 varied under the two meteorological conditions.

Monitoring of greenhouse gases and pollutants across an urban area using a light-rail public transit platform

Anthropogenic emissions within urban environments are characterized by spatial heterogeneity and temporal variability that present challenges for measuring urban greenhouse gases and air pollutants. To address these challenges, we mounted instruments on public transit light-rail train cars that traverse the metropolitan Salt Lake Valley (SLV) in Utah, USA to observe the temporal and spatial variability of atmospheric species including carbon dioxide (CO2), methane (CH4), ozone (O3), fine particulate matter (PM2.5), and nitrogen dioxide (NO2). Utilizing electrified light-rail public transit as an observational platform enables real-time measurements with low operating costs while avoiding self-contamination from vehicle exhaust. We examine temporal averages and case studies of each species that reveal gradients, intermittent point sources, seasonal and diel changes, and complex relationships resulting from emissions, atmospheric chemistry, and meteorological conditions. CO2 and NO2 are related through the combustion of fossil fuel and we observed a broad spatial gradient across the city as well distinct plumes at traffic intersections and, for NO2, a large plume adjacent to a locomotive rail yard. Distributions of O3 were strongly correlated with NO2 due to atmospheric photochemical and titration processes. Episodes of high PM2.5 had distinct spatial patterns depending on meteorological conditions during wintertime persistent cold-air pool episodes. The spatial pattern of CH4 was characterized by distinct plumes associated with industrial and commercial facilities, some of which followed temporal patterns indicative of daytime working hours; other plumes were persistent throughout the whole day, suggestive of leak-related fugitive emissions. The ongoing multi-year record of spatial and temporal air quality observations provides a valuable data set for future air quality exposure studies. Our results suggest pollution and greenhouse gas emission monitoring and exposure assessment could be greatly enhanced by deploying instruments on public transit systems in urban centers worldwide.

Coupling between Chemical and Meteorological Processes under Persistent Cold-Air Pool Conditions: Evolution of Wintertime PM2.5 Pollution Events and N2O5 Observations in Utah's Salt Lake Valley.

The Salt Lake Valley experiences severe fine particulate matter pollution episodes in winter during persistent cold-air pools (PCAPs). We employ measurements throughout an entire winter from different elevations to examine the chemical and dynamical processes driving these episodes. Whereas primary pollutants such as NOx and CO were enhanced twofold during PCAPs, O3 concentrations were approximately threefold lower. Atmospheric composition varies strongly with altitude within a PCAP at night with lower NOx and higher oxidants (O3) and oxidized reactive nitrogen (N2O5) aloft. We present observations of N2O5 during PCAPs that provide evidence for its role in cold-pool nitrate formation. Our observations suggest that nighttime and early morning chemistry in the upper levels of a PCAP plays an important role in aerosol nitrate formation. Subsequent daytime mixing enhances surface PM2.5 by dispersing the aerosol throughout the PCAP. As pollutants accumulate and deplete oxidants, nitrate chemistry becomes less active during the later stages of the pollution episodes. This leads to distinct stages of PM2.5 pollution episodes, starting with a period of PM2.5 buildup and followed by a period with plateauing concentrations. We discuss the implications of these findings for mitigation strategies.

Simulations of a Cold-Air Pool in Utah’s Salt Lake Valley: Sensitivity to Land Use and Snow Cover

Obtaining realistic land-surface states for initial and boundary conditions is important for the numerical weather prediction of many atmospheric phenomena. Here we investigate model sensitivity to land use and snow cover for a persistent wintertime cold-air pool in northern Utah during 1–8 January 2011. A Weather Research and Forecast model simulation using the 1993 United States Geological Survey land-use and North American Mesoscale model reanalysis snow-cover datasets is compared to an improved configuration using the modified 2011 National Land Cover Database and a more realistic representation of snow cover. The improved surface specification results in an increase (decrease) in urban land cover (Great Salt Lake surface area), and changes to the snow-cover initialization, depth, extent, and albedo. The results obtained from the model simulations are compared to observations collected during the Persistent Cold-Air Pool Study. The changes in land use and snow cover and the resulting impacts on the surface albedo and surface heat fluxes contributed to near-surface air temperature increases of 1– $$2\,^{\circ }\hbox {C}$$ in urban areas and decreases of 2– $$4\,^{\circ }\hbox {C}$$ in areas surrounding the Great Salt Lake. Although wind speeds in the boundary layer were overestimated in both simulations, shallow thermally-driven and terrain-forced flows were generally lessened in intensity and breadth in response to the decreased areal extent of the Great Salt Lake and increases in the urban footprint.

Laser Ceilometer Investigation of Persistent Wintertime Cold-Air Pools in Utah’s Salt Lake Valley

AbstractAs part of the winter 2010/11 Persistent Cold-Air Pool Study in Utah’s Salt Lake Valley, a laser ceilometer was used to continuously measure aerosol-layer characteristics in support of an investigation of the meteorological processes producing the cold-air pools. A surface-based aerosol layer was present during much of the winter. Comparisons were made between ceilometer-measured and visual characteristics of the aerosol layers. A 3–4 January 2011 case study illustrated the meteorological value of time–height backscatter cross sections when used as a base map for meteorological analyses. A variety of meteorological mixing processes were illustrated using ceilometer backscatter data. The mean altitude of the top of the aerosol layer during undisturbed subperiods of the 1 December–7 February experimental period was 1811 m MSL, with a standard deviation of 185 m. The mean aerosol depth was ~500 m AGL in the 1200-m-deep valley. There was surprisingly little variation in the wintertime aerosol layer depth despite large variations in bulk atmospheric stability and ground-based fine particulate matter concentrations.

Dynamically Induced Displacements of a Persistent Cold-Air Pool

We examine the influence of a passing weather system on a persistent cold-air pool (CAP) during the Persistent Cold-Air Pool Study in the Salt Lake Valley, Utah, USA. The CAP experiences a sequence of along-valley displacements that temporarily and partially remove the cold air in response to increasing along-valley winds aloft. The displacements are due to the formation of a mountain wave over the upstream topography as well as adjustments to the regional horizontal pressure gradient and wind-stress divergence acting on the CAP. These processes appear to help establish a balance wherein the depth of the CAP increases to the north. When that balance is disrupted, the CAP tilt collapses, which sends a gravity current of cold air travelling upstream and thereby restores CAP conditions throughout the valley. Intra-valley mixing of momentum, heat, and pollution within the CAP by Kelvin–Helmholtz waves and seiching is also examined.

A numerical study of a persistent cold air pool episode in the Salt Lake Valley, Utah

The Weather Research and Forecasting (WRF) model is used to simulate a persistent cold air pool (CAP) episode observed in December 2010 during the Persistent Cold Air Pool Study (PCAPS) field campaign in Salt Lake Valley, Utah. The availability of intensive observations from PCAPS, especially the added vertical profiles, allows for an in-depth analysis of CAP structure and evolution and model evaluation beyond what was done in similar work before. Comparisons of the WRF simulation with surface observations and upper air soundings indicate that WRF is capable of simulating the observed spatial and temporal variation of the valley atmosphere during the CAP episode. The model also successfully captures the formation and removal of the persistent temperature inversion. Process analysis helps quantify the contribution of various physical processes to the buildup and breakup of the inversion. The inversion developed due primarily to a rapid warming above the valley by synoptic subsidence and warm advection and to strong radiative cooling within the valley under clear sky conditions. The inversion was lifted when strong southerly winds entered the valley from the gap mixing out the cold air in the valley. Synoptic-scale cold advection above the valley contributed to the inversion removal by weakening the inversion from aloft. Sensitivity experiments suggested that the WRF simulation of the CAP episode is more sensitive to large-scale initialization fields with North American Mesoscale Model outperforming North American Regional Reanalysis and National Centers for Environmental Prediction (NCEP) Eta analysis than to the choice of boundary layer parameterizations and land surface models.

The Persistent Cold-Air Pool Study

The Persistent Cold-Air Pool Study (PCAPS) was conducted in Utah's Salt Lake valley from 1 December 2010 to 7 February 2011. The field campaign's primary goal was to improve understanding of the physical processes governing the evolution of multiday cold-air pools (CAPs) that are common in mountain basins during the winter. Meteorological instrumentation deployed throughout the Salt Lake valley provided observations of the processes contributing to the formation, maintenance, and destruction of 10 persistent CAP episodes. The close proximity of PCAPS field sites to residences and the University of Utah campus allowed many undergraduate and graduate students to participate in the study. Ongoing research, supported by the National Science Foundation, is using the PCAPS dataset to examine CAP evolution. Preliminary analyses reveal that variations in CAP thermodynamic structure are attributable to a multitude of physical processes affecting local static stability: for example, synoptic-scale processes impact changes in temperatures and cloudiness aloft while variations in boundary layer forcing modulate the lower levels of CAPs. During episodes of strong winds, complex interactions between the synoptic and mesoscale f lows, local thermodynamic structure, and terrain lead to both partial and complete removal of CAPs. In addition, the strength and duration of CAP events affect the local concentrations of pollutants such as PM2.5.

Maintenance of a Mountain Valley Cold Pool: A Numerical Study

Abstract A persistent cold-air pool in the Yampa Valley of northwestern Colorado was simulated with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The observed cold-air pool, which was identified by temperature measurements along a line of surface stations ascending the eastern side of the valley, remained in place throughout the day of 10 January 2004. The baseline simulation with horizontal resolution of 1 km, which is close to the resolution of operational regional mesoscale model forecasts, neither matched the strength of the observed cold-air pool nor retained the cold pool throughout the day. Varying the PBL parameterization, increasing the vertical resolution, and increasing the model spinup time did not significantly improve the results. However, the inclusion of snow cover, increased horizontal resolution, and an improved treatment of horizontal diffusion did have a sizable effect on the forecast quality. The snow cover in the baseline simulation was essential for preventing the diurnal heating from eroding the cold pool, but was only sufficient to produce a nearly isothermal temperature structure within the valley, largely because of an increased reflection of solar radiation. The increase of horizontal resolution to 333 and 111 m resulted in a stronger cold-air pool and its retention throughout the day. In addition to improving the resolution of flow features in steep terrain, resulting in, for example, less drainage out of the valley, the increase in horizontal resolution led to a better forecast because of a reduced magnitude of horizontal diffusion calculated along the terrain-following model surfaces. Calculating horizontal diffusion along the constant height levels had a beneficial impact on the quality of the simulations, producing effects similar to those achieved by increasing the horizontal resolution, but at a fraction of the computational cost.