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- Author or Editor: D. Stauffer x
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Abstract
The homogeneous nucleation theory of liquid droplets in supersaturated vapors is reviewed. Taking into consideration the microscopic surface tension and extrapolating from the triple point to the critical point (T = Te ) of the liquid-gas phase transition, we reexamine homogeneous nucleation theory. A calculation of the growth rate for microscopic clusters due to the incorporation of much smaller clusters (instead of single molecules) is given and an appropriate variable, scaled supersaturation, is presented to study the mechanism of homogeneous nucleation. For a given nucleation rate, the scaled supersaturation is expected to be nearly independent of temperature below Te .; this is confirmed by experimental data. A generalized form for the droplet model is proposed. Previous theories (“classical,” Lothe-Pound, Reiss, Katz, Cohen) are shown to be special cases of this generalized form and all are shown to be invalid near the critical point. A quantitative theory is made by extrapolating Fisher's droplet model from the critical region to the triple point. For the free energy of embryo formation we include the contributions due to internal vibrations and self-avoiding walks. The microscopic surface tension for the droplet is estimated from the measured coexistence curve; it is found to agree with the bulk macroscopic surface tension near the triple and critical points and to be smaller in the intermediate region. With these considerations we have calculated the scaled supersaturation for water vapor as a function of temperature for given measured nucleation rates. The proposed theory is in good agreement with experiment.
Abstract
The homogeneous nucleation theory of liquid droplets in supersaturated vapors is reviewed. Taking into consideration the microscopic surface tension and extrapolating from the triple point to the critical point (T = Te ) of the liquid-gas phase transition, we reexamine homogeneous nucleation theory. A calculation of the growth rate for microscopic clusters due to the incorporation of much smaller clusters (instead of single molecules) is given and an appropriate variable, scaled supersaturation, is presented to study the mechanism of homogeneous nucleation. For a given nucleation rate, the scaled supersaturation is expected to be nearly independent of temperature below Te .; this is confirmed by experimental data. A generalized form for the droplet model is proposed. Previous theories (“classical,” Lothe-Pound, Reiss, Katz, Cohen) are shown to be special cases of this generalized form and all are shown to be invalid near the critical point. A quantitative theory is made by extrapolating Fisher's droplet model from the critical region to the triple point. For the free energy of embryo formation we include the contributions due to internal vibrations and self-avoiding walks. The microscopic surface tension for the droplet is estimated from the measured coexistence curve; it is found to agree with the bulk macroscopic surface tension near the triple and critical points and to be smaller in the intermediate region. With these considerations we have calculated the scaled supersaturation for water vapor as a function of temperature for given measured nucleation rates. The proposed theory is in good agreement with experiment.
Abstract
To better understand the physical processes of the stable boundary layer and to quantify “submeso motions” in moderately complex terrain, exploratory case-study analyses were performed using observational field data supplemented by gridded North American Regional Reanalysis data and Pennsylvania State University real-time Weather Research and Forecasting Model output. Submeso motions are nominally defined as all motions between the largest turbulent scales and the smallest mesoscales. Seven nighttime cases from August and September of 2011 are chosen from a central Pennsylvania [“Rock Springs” (RS)] network of eight ground-based towers and two sound detection and ranging (sodar) systems . The observation network is located near Tussey Ridge, ~15 km southeast of the Allegheny Mountains. The seven cases are classified by the dominant synoptic-flow direction and proximity to terrain to assess the influence of synoptic conditions on the local submeso and mesogamma motions. It is found that synoptic winds with a large crossing angle over nearby Tussey Ridge can generate mesogamma wave motions and larger-magnitude submeso temperature and wind fluctuations in the RS network than do winds from the direction of the more distant Allegheny Mountains. Cases with synoptic winds that are nearly parallel to the topographic contours or are generally weak exhibit the smallest fluctuations. Changes in the magnitude of near-surface submeso temperature and wind fluctuations in response to local indicator variables are also analyzed. The observed submeso wind and temperature fluctuations are generally larger when the low-level wind speed and thermal stratification, respectively, are greater, but the synoptic flow and its relation to the terrain also play an important role.
Abstract
To better understand the physical processes of the stable boundary layer and to quantify “submeso motions” in moderately complex terrain, exploratory case-study analyses were performed using observational field data supplemented by gridded North American Regional Reanalysis data and Pennsylvania State University real-time Weather Research and Forecasting Model output. Submeso motions are nominally defined as all motions between the largest turbulent scales and the smallest mesoscales. Seven nighttime cases from August and September of 2011 are chosen from a central Pennsylvania [“Rock Springs” (RS)] network of eight ground-based towers and two sound detection and ranging (sodar) systems . The observation network is located near Tussey Ridge, ~15 km southeast of the Allegheny Mountains. The seven cases are classified by the dominant synoptic-flow direction and proximity to terrain to assess the influence of synoptic conditions on the local submeso and mesogamma motions. It is found that synoptic winds with a large crossing angle over nearby Tussey Ridge can generate mesogamma wave motions and larger-magnitude submeso temperature and wind fluctuations in the RS network than do winds from the direction of the more distant Allegheny Mountains. Cases with synoptic winds that are nearly parallel to the topographic contours or are generally weak exhibit the smallest fluctuations. Changes in the magnitude of near-surface submeso temperature and wind fluctuations in response to local indicator variables are also analyzed. The observed submeso wind and temperature fluctuations are generally larger when the low-level wind speed and thermal stratification, respectively, are greater, but the synoptic flow and its relation to the terrain also play an important role.
Abstract
Coastal regions have historically represented a significant challenge for air quality investigations because of water–land boundary transition characteristics and a paucity of measurements available over water. Prior studies have identified the formation of high levels of ozone over water bodies, such as the Chesapeake Bay, that can potentially recirculate back over land to significantly impact populated areas. Earth-observing satellites and forecast models face challenges in capturing the coastal transition zone where small-scale meteorological dynamics are complex and large changes in pollutants can occur on very short spatial and temporal scales. An observation strategy is presented to synchronously measure pollutants “over land” and “over water” to provide a more complete picture of chemical gradients across coastal boundaries for both the needs of state and local environmental management and new remote sensing platforms. Intensive vertical profile information from ozone lidar systems and ozonesondes, obtained at two main sites, one over land and the other over water, are complemented by remote sensing and in situ observations of air quality from ground-based, airborne (both personned and unpersonned), and shipborne platforms. These observations, coupled with reliable chemical transport simulations, such as the National Oceanic and Atmospheric Administration (NOAA) National Air Quality Forecast Capability (NAQFC), are expected to lead to a more fully characterized and complete land–water interaction observing system that can be used to assess future geostationary air quality instruments, such as the National Aeronautics and Space Administration (NASA) Tropospheric Emissions: Monitoring of Pollution (TEMPO), and current low-Earth-orbiting satellites, such as the European Space Agency’s Sentinel-5 Precursor (S5-P) with its Tropospheric Monitoring Instrument (TROPOMI).
Abstract
Coastal regions have historically represented a significant challenge for air quality investigations because of water–land boundary transition characteristics and a paucity of measurements available over water. Prior studies have identified the formation of high levels of ozone over water bodies, such as the Chesapeake Bay, that can potentially recirculate back over land to significantly impact populated areas. Earth-observing satellites and forecast models face challenges in capturing the coastal transition zone where small-scale meteorological dynamics are complex and large changes in pollutants can occur on very short spatial and temporal scales. An observation strategy is presented to synchronously measure pollutants “over land” and “over water” to provide a more complete picture of chemical gradients across coastal boundaries for both the needs of state and local environmental management and new remote sensing platforms. Intensive vertical profile information from ozone lidar systems and ozonesondes, obtained at two main sites, one over land and the other over water, are complemented by remote sensing and in situ observations of air quality from ground-based, airborne (both personned and unpersonned), and shipborne platforms. These observations, coupled with reliable chemical transport simulations, such as the National Oceanic and Atmospheric Administration (NOAA) National Air Quality Forecast Capability (NAQFC), are expected to lead to a more fully characterized and complete land–water interaction observing system that can be used to assess future geostationary air quality instruments, such as the National Aeronautics and Space Administration (NASA) Tropospheric Emissions: Monitoring of Pollution (TEMPO), and current low-Earth-orbiting satellites, such as the European Space Agency’s Sentinel-5 Precursor (S5-P) with its Tropospheric Monitoring Instrument (TROPOMI).
