We present an unprecedented set of high-resolution climate simulations, consisting of a 500-year pre-industrial control simulation and a 250-year historical and future climate simulation from 1850 to 2100. A high-resolution configuration of the Community Earth System Model version 1.3 (CESM1.3) is used for the simulations with a nominal horizontal resolution of 0.25°for the atmosphere and land models and 0.1°for the ocean and sea-ice models. At these resolutions, the model permits tropical cyclones and ocean mesoscale eddies, allowing interactions between these synoptic and mesoscale phenomena with large-scale circulations. An overview of the results from these simulations is provided with a focus on model drift, mean climate, internal modes of variability, representation of the historical and future climates, and extreme events. Comparisons are made to solutions from an identical set of simulations using the standard resolution (nominal 1°) CESM1.3 and to available observations for the historical period to address some key scientific questions concerning the impact and benefit of increasing model horizontal resolution in climate simulations. An emerging prominent feature of the high-resolution pre-industrial simulation is the intermittent occurrence of polynyas in the Weddell Sea and its interaction with an Interdecadal Pacific Oscillation. Overall, high-resolution simulations show significant improvements in representing global mean temperature changes, seasonal cycle of sea-surface temperature and mixed layer depth, extreme events and in relationships between extreme events and climate modes. Plain Language Summary Although the current generation of climate models has demonstrated high fidelity in simulating and projecting global temperature change, these models show large uncertainties when it comes to questions concerning how rising global temperatures will impact local weather conditions. This is because the resolution (~100 km) at which the majority of climate models simulate the climate is not fine enough to resolve these small-scale regional features. Conducting long-term (multi-centuries) high-resolution (~10 km) climate simulations has been a great challenge for the research community due to the extremely high computational demands. Through international

[1] The presence of regional pollution in cities, while required theoretically, has only been recognized recently. This pollution forms a background that can limit efforts to reduce air pollution in cities. Its concentrations are difficult to quantify, however, even with transport models. This paper describes a novel technique for quantifying regional aerosol solely from a series of fast-response measurements of total aerosol at a single point within a city. It uses Beijing as an example and utilizes the strong asymmetric ''sawtooth cycles'' of aerosol there, which contain a smoothly increasing baseline, dominated by secondary inorganic aerosol, with daily cycles superimposed. The sawtooths have average durations of 5 days and are controlled by synoptic cycles, specifically the passage of cold fronts. During a sawtooth, the concentration of aerosol increases rapidly while back trajectories rotate from the northwest to the west and south, the air becomes more humid, and the particle size of the aerosol decreases. Ancillary measurements around Beijing show that the baselines represent regional aerosol, while the daily peaks represent local aerosol. For Beijing, the regional component averages about 50% and can range from 10%-20% during northwesterly flow to 70% or so during southerly flow. A preliminary error analysis shows that the uncertainties of the concentrations of regional aerosol can be up to 50% for a single day but <10% when totaled over a sawtooth. This technique should be applicable to a wide range of locales because sawtooths have also been observed in other places in northeast China and over much of eastern North America. Sawtooths should be a general feature of populated midlatitude areas crossed regularly by the polar front. They are also seen for trace gases and should yield analogous local and regional components.

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The Weather Research and Forecasting (WRF) model is evaluated by conducting various sensitivity experiments over central California including the San Francisco Bay Area (SFBA), with the goal of establishing a WRF model configuration to be used by the Bay Area Air Quality Management District (BAAQMD) for its air quality applications. For the two selected cases, a winter particulate matter case and a summer ozone case, WRF solutions are evaluated both quantitatively by comparing the error statistics and qualitatively by analyzing the model-simulated mesoscale features. Model evaluation is also performed for the SFBA, Sacramento Valley, and San Joaquin Valley subregions. The recommended WRF configuration includes use of the Rapid Radiative Transfer Model/Dudhia (or RRTMG) radiation schemes and the Pleim-Xiu land surface physics, combined with a multiscale four-dimensional data assimilation strategy throughout the simulation period to assimilate the available observations, including standard observations from the World Meteorological Organization and local special observations. With the recommended model configuration, WRF is able to simulate the meteorological variables with reasonable error, with the added value, although relatively small, of assimilating the additional BAAQMD local special observations. Mesoscale features, simulated reasonably well for both cases, include the upslope and downslope flows that occur along the mountains that surround the Central Valley of California, as well as the mesoscale eddies that develop within the valley.

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