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  • 1
    Publication Date: 2017-03-03
    Description: Smartphone pressure observations have the potential to greatly increase surface observation density on convection-resolving scales. Currently available smartphone pressure observations are tested through assimilation in a mesoscale ensemble for a 3-day, convectively active period in the eastern United States. Both raw pressure (altimeter) observations and 1-h pressure (altimeter) tendency observations are considered. The available observation density closely follows population density, but observations are also available in rural areas. The smartphone observations are found to contain significant noise, which can limit their effectiveness. The assimilated smartphone observations contribute to small improvements in 1-h forecasts of surface pressure and 10-m wind, but produce larger errors in 2-m temperature forecasts. Short-term (0–4 h) precipitation forecasts are improved when smartphone pressure and pressure tendency observations are assimilated as compared with an ensemble that assimilates no observations. However, these improvements are limited to broad, mesoscale features with minimal skill provided at convective scales using the current smartphone observation density. A specific mesoscale convective system (MCS) is examined in detail, and smartphone pressure observations captured the expected dynamic structures associated with this feature. Possibilities for further development of smartphone observations are discussed.
    Print ISSN: 0882-8156
    Electronic ISSN: 1520-0434
    Topics: Geography , Physics
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  • 2
    Publication Date: 2018-03-01
    Description: Over half a billion smartphones worldwide are now capable of measuring atmospheric pressure, providing a pressure network of unprecedented density and coverage. This paper describes novel approaches for the collection, quality control, and bias correction of such smartphone pressures. An Android app was developed and distributed to several thousand users, serving as a test bed for onboard pressure collection and quality-control strategies. New methods of pressure collection were evaluated, with a focus on reducing and quantifying sources of observation error and uncertainty. Using a machine learning approach, complex relationships between pressure bias and ancillary sensor data were used to predict and correct future pressure biases over a 4-week period from 10 November to 5 December 2016. This approach, in combination with simple quality-control checks, produced an 82% reduction in the average smartphone pressure bias, substantially improving the quality of smartphone pressures and facilitating their use in numerical weather prediction.
    Print ISSN: 0739-0572
    Electronic ISSN: 1520-0426
    Topics: Geography , Geosciences , Physics
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  • 3
    Publication Date: 2016-08-26
    Description: A general lightning data assimilation technique is developed and tested with observations from the World Wide Lightning Location Network (WWLLN). The technique nudges the water vapor mixing ratio toward saturation within 10 km of a lightning observation and is more general than other approaches that require specific model microphysics or flash rates. This approach is applied to both deterministic and ensemble forecasts of the 29 June 2012 derecho event over the eastern United States and a deterministic forecast of the 17 November 2013 convective event over the Midwest using the Weather Research and Forecasting (WRF) Model run at a convection-permitting resolution. Lightning data are assimilated over the first three hours of the forecasts, and the subsequent impact on forecast quality is evaluated. For both events, the deterministic simulations with lightning-based nudging produce more realistic predicted composite reflectivity fields. For the forecasts of the 29 June 2012 event using ensemble data assimilation, forecast improvements from lightning assimilation were more modest than for the deterministic forecasts, suggesting that lightning assimilation may produce greater improvements in convective forecasts where conventional observations (e.g., aircraft, surface, radiosonde, satellite) are less dense or unavailable.
    Print ISSN: 0739-0572
    Electronic ISSN: 1520-0426
    Topics: Geography , Geosciences , Physics
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  • 4
    Publication Date: 2016-08-22
    Description: A number of studies have suggested that heat waves will increase in frequency, intensity, and duration under anthropogenic global warming. However, changes to heat extremes and temperature distributions in general are less understood in regions of complex terrain and substantial land–water contrasts. Such surface variations are important in the northwest United States, where synoptic- and mesoscale circulations interact with local topography and land–water interfaces to produce heat waves that have substantial impacts on fire weather, air quality, wind energy, and the population in general. Thus it is crucial to identify how the synoptic- and mesoscale circulations that drive such local extremes will change in a warming world. This paper analyzes changes to the conditions associated with heat extremes over the northwest United States by utilizing global and regional climate models. A maximum in 700-hPa warming is projected over the northwest United States and southwest Canada, with low-level warming attenuated near the coast. Soil moisture declines are projected over the region, which further enhances future extremes. It is found that low-level zonal wind distributions over the northwest United States become narrower, leading to fewer days with strong offshore flow. This is important since extreme warming events are associated with the strongest offshore/downslope flow. Historical and future regional temperature distributions are described and it is shown that although CMIP5 models predict increases in heat extremes for west Oregon and Washington, these increases are less than for inland areas.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
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  • 5
    Publication Date: 2016-08-04
    Description: Large-scale synoptic circulations have a profound effect on western U.S. summer weather and climate. Heat waves, water availability, the distribution of monsoonal moisture, fire-weather conditions, and other phenomena are impacted by the position and amplitude of large-scale synoptic circulations. Furthermore, regional weather is modulated by the interactions of the large-scale flow with terrain and land–water contrasts. It is therefore crucial to understand projected changes in large-scale circulations and their variability under anthropogenic global warming. Although recent research has examined changes in the jet stream, storm tracks, and synoptic disturbances over the Northern Hemisphere under global warming, most papers have focused on the cold season. In contrast, this work analyzes the projected trends in the spatial distribution and amplitude of large-scale synoptic disturbances over the western United States and eastern Pacific during July and August. It is shown that CMIP5 models project weaker mean midtropospheric gradients in geopotential height as well as attenuated temporal variability in geopotential height, temperature, vorticity, vertical motion, and sea level pressure over this region. Most models suggest reduced frequency of troughs and increased frequency of ridges over the western United States. These changes in the variability of synoptic disturbances have substantial implications for future regional weather and climate.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
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  • 6
    Publication Date: 2017-07-17
    Print ISSN: 0027-0644
    Electronic ISSN: 1520-0493
    Topics: Geography , Geosciences , Physics
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  • 7
    Publication Date: 2017-07-18
    Description: This paper describes changes in the climatology, structure, and seasonality of cool-season atmospheric rivers influencing the U.S. West Coast by examining the climate simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) that are forced by the representative concentration pathway (RCP) 8.5 scenario. There are only slight changes in atmospheric river (AR) frequency and seasonality between historical (1970–99) and future (2070–99) periods considering the most extreme days (99th percentile) in integrated water vapor transport (IVT) along the U.S. West Coast. Changes in the 99th percentile of precipitation are only significant over the southern portion of the coast. In contrast, using the number of future days exceeding the historical 99th percentile IVT threshold produces statistically significant increases in the frequency of extreme IVT events for all winter months. The peak in future AR days appears to occur approximately one month earlier. The 10-model mean historical and end-of-century composites of extreme IVT days reflect canonical AR conditions, with a plume of high IVT extending from the coast to the southwest. The similar structure and evolution associated with ARs in the historical and future periods suggest little change in large-scale structure of such events during the upcoming century. Increases in extreme IVT intensity are primarily associated with integrated water vapor increases accompanying a warming climate. Along the southern portion of the U.S. West Coast there is less model agreement regarding the structure and intensity of ARs than along the northern portions of the coast.
    Print ISSN: 1525-755X
    Electronic ISSN: 1525-7541
    Topics: Geography , Geosciences , Physics
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  • 8
    Publication Date: 2017-10-01
    Description: The Olympic Mountains Experiment (OLYMPEX) took place during the 2015/16 fall–winter season in the vicinity of the mountainous Olympic Peninsula of Washington State. The goals of OLYMPEX were to provide physical and hydrologic ground validation for the U.S.–Japan Global Precipitation Measurement (GPM) satellite mission and, more specifically, to study how precipitation in Pacific frontal systems is modified by passage over coastal mountains. Four transportable scanning dual-polarization Doppler radars of various wavelengths were installed. Surface stations were placed at various altitudes to measure precipitation rates, particle size distributions, and fall velocities. Autonomous recording cameras monitored and recorded snow accumulation. Four research aircraft supplied by NASA investigated precipitation processes and snow cover, and supplemental rawinsondes and dropsondes were deployed during precipitation events. Numerous Pacific frontal systems were sampled, including several reaching “atmospheric river” status, warm- and cold-frontal systems, and postfrontal convection.
    Print ISSN: 0003-0007
    Electronic ISSN: 1520-0477
    Topics: Geography , Physics
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  • 9
    Publication Date: 2019-02-01
    Description: A series of large wildfires began over the terrain north of San Francisco, California, during the evening of 8 October 2017 and spread across nearly 250,000 acres, including areas near the towns of Santa Rosa and Napa. These “Wine Country” wildfires were the most destructive in California history, with 44 deaths; the loss of 9,000 buildings; damage to approximately 21,000 structures; $10 billion of insured losses; and substantially greater total economic loss. This paper describes the synoptic and mesoscale conditions that were associated with the wildfires, with strong, easterly “Diablo” winds playing a central role in both initiating and supporting the fires. The climatological conditions preceding the fires are reviewed, including near-normal precipitation and above-normal temperatures during the summer, as well as much above-normal precipitation the previous winter, which led to abundant dry grass that provided fuel for the wind-driven fires. High-resolution meteorological modeling realistically simulated the strong winds associated with this event. Importantly, operational mesoscale forecast models provided excellent forecasts of the high winds several days in advance. It appears that a vulnerable power system, urbanization of fire-prone areas, flammable invasive species, and poor communication of dangerous conditions contributed to this catastrophic event. The potential for mitigating or preventing such destructive wildfires using skillful weather prediction is examined, as well as the possible role of global warming.
    Print ISSN: 0003-0007
    Electronic ISSN: 1520-0477
    Topics: Geography , Physics
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  • 10
    Publication Date: 2018-10-01
    Description: Over a half-billion smartphones are now capable of measuring atmospheric pressure, potentially providing a global surface observing network of unprecedented density and coverage. An earlier study by the authors described an Android app, uWx, that served as a test bed for advanced quality control and bias correction strategies. To evaluate the utility and quality of the resulting smartphone pressure observations, ensemble data assimilation experiments were performed for two case studies over the Pacific Northwest. In both case studies, smartphone pressures improved the analyses and forecasts of assimilated and nonassimilated variables. In case I, which considered the passage of a front across the region, cycled smartphone pressure assimilation consistently improved 1-h forecasts of the altimeter setting, 2-m temperature, and 2-m dewpoint. During a postfrontal period, cycled smartphone pressure assimilation improved mesoscale forecasts of hourly precipitation accumulation. In case II, which considered a major coastal windstorm, cycling experiments assimilating smartphone pressures improved 10-m wind forecasts as well as the predicted track and intensity. For both cases, free-forecast experiments initialized with smartphone data produced forecast improvements extending several hours, suggesting the utility of crowdsourced smartphone pressures for short-term numerical weather prediction.
    Print ISSN: 0882-8156
    Electronic ISSN: 1520-0434
    Topics: Geography , Physics
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