Search Results
Abstract
Observations show that two phenomena that are generally considered mutually exclusive occur both night and day during spring thaw over flat cultivated prairies: extremely large potential temperature lapse rates and low levels of atmospheric turbulence. Magnitudes of the large lapse rates routinely exceeded 20°C (100 m)−1, whereas levels of turbulence during night and day were at values usually associated with moderately stable and near neutral conditions, respectively. Coexistence of these phenomena occurs during the time of year when a significantly large portion of the prairie is characterized by water and/or ice while the remainder comprises bare soil surfaces. Whereas freezing water is a sensible heat source during the night and melting ice a heat sink during the day, the bare soil acts in a reverse manner being a heat sink during the night and a source during the day. Air moving over the prairie surface is, therefore, subject to, both day and night, an alternating succession of warm and cool surfaces.
Observed boundary layer depths characterized by the very large lapse rates were about 10 m in depth. Calculations based on the first law of thermodynamics show that these depths are consistent with sensible heat fluxes from the prairie surface of about 2 W m−2. Assessments of these small heat fluxes through applications of the energy balance equation show that they are in agreement with the known behavior of melting ice and freezing water during spring thaw.
Extremely large potential temperature lapse rates and low levels of turbulence seem, therefore, to occur because small heat fluxes are being introduced into the air on an interruptible basis. Very large lapse rates persist because the lack of a sustained heat flux does not allow for development of the vigorous turbulence needed for their eradication.
Abstract
Observations show that two phenomena that are generally considered mutually exclusive occur both night and day during spring thaw over flat cultivated prairies: extremely large potential temperature lapse rates and low levels of atmospheric turbulence. Magnitudes of the large lapse rates routinely exceeded 20°C (100 m)−1, whereas levels of turbulence during night and day were at values usually associated with moderately stable and near neutral conditions, respectively. Coexistence of these phenomena occurs during the time of year when a significantly large portion of the prairie is characterized by water and/or ice while the remainder comprises bare soil surfaces. Whereas freezing water is a sensible heat source during the night and melting ice a heat sink during the day, the bare soil acts in a reverse manner being a heat sink during the night and a source during the day. Air moving over the prairie surface is, therefore, subject to, both day and night, an alternating succession of warm and cool surfaces.
Observed boundary layer depths characterized by the very large lapse rates were about 10 m in depth. Calculations based on the first law of thermodynamics show that these depths are consistent with sensible heat fluxes from the prairie surface of about 2 W m−2. Assessments of these small heat fluxes through applications of the energy balance equation show that they are in agreement with the known behavior of melting ice and freezing water during spring thaw.
Extremely large potential temperature lapse rates and low levels of turbulence seem, therefore, to occur because small heat fluxes are being introduced into the air on an interruptible basis. Very large lapse rates persist because the lack of a sustained heat flux does not allow for development of the vigorous turbulence needed for their eradication.
Abstract
Turbulence data collected at the 10-m level during convective conditions at a site located amid flat terrain in Alberta have been analyzed with respect to wind speed U and normalized static stability S′ n . These two parameters have been assumed to respectively represent mechanical and thermal forces that engender atmospheric turbulence at ground level.
Observations of atmospheric turbulence show wide scatter in the value of the standard deviations of transverse, longitudinal and vertical wind fluctuations (σ v , σ u , σ w ) for the same apparent conditions of mechanical and thermal forces (i.e., wind speed and static stability). It has been assumed that the large scatter is attributable to random localized effects such as those caused by the breaking of internal gravity waves. For this reason the present analysis has been restricted to median values of σ v , σ u , and σ w in an effort to discern a pattern of behavior that may be explained in terms of U and S′ n . Equations have been empirically developed for median standard deviations of wind fluctuations in terms of wind speed and static stability. One-to-one correlation coefficients between predicted and observed data were typically in excess of 0.90.
Results of this study complement findings of a previous study done with the same database except for stable atmospheric situations. Information from the two studies allows for the estimation of parameters (σ v /U, σ u /U, σ w /U) for use in plume dispersion models under a wide range of wind and stability conditions. Estimation procedures depend upon easily measured meteorological variables (U, S′ n ). Illustrations of the dependencies have been provided.
Abstract
Turbulence data collected at the 10-m level during convective conditions at a site located amid flat terrain in Alberta have been analyzed with respect to wind speed U and normalized static stability S′ n . These two parameters have been assumed to respectively represent mechanical and thermal forces that engender atmospheric turbulence at ground level.
Observations of atmospheric turbulence show wide scatter in the value of the standard deviations of transverse, longitudinal and vertical wind fluctuations (σ v , σ u , σ w ) for the same apparent conditions of mechanical and thermal forces (i.e., wind speed and static stability). It has been assumed that the large scatter is attributable to random localized effects such as those caused by the breaking of internal gravity waves. For this reason the present analysis has been restricted to median values of σ v , σ u , and σ w in an effort to discern a pattern of behavior that may be explained in terms of U and S′ n . Equations have been empirically developed for median standard deviations of wind fluctuations in terms of wind speed and static stability. One-to-one correlation coefficients between predicted and observed data were typically in excess of 0.90.
Results of this study complement findings of a previous study done with the same database except for stable atmospheric situations. Information from the two studies allows for the estimation of parameters (σ v /U, σ u /U, σ w /U) for use in plume dispersion models under a wide range of wind and stability conditions. Estimation procedures depend upon easily measured meteorological variables (U, S′ n ). Illustrations of the dependencies have been provided.
Abstract
This study investigates the behavior of wind fluctuations observed at the 10-m level over a flat terrain site located some 100 km east of the Rocky Mountains. The purposes were to assess residual fluctuations in order to ascertain effects attributable to the nonhomogenous, nonstationary character of turbulence and to evaluate influences of gravity waves. Residual wind fluctuations were defined for purposes of this study as the differences between observed half-hourly average standard deviations of wind fluctuations (σ v , σ u , σ w ) and those that are expected to occur in association with simultaneous wind speeds and static stabilities. These latter fluctuations were estimated from equations developed by Leahey, Hansen, and Schroeder (LHS).
Results of the analyses showed, as expected, that residual distributions for nonwesterly wind conditions were nearly Gaussian. Standard deviations for residuals of horizontal fluctuations, attributable to the nonhomogenous, nonstationary nature of turbulence, were 0.165 and 0.210 m s−1 for stable and unstable situations, respectively. For residuals associated with vertical fluctuations they were, respectively, 0.065 and 0.075 m s−1.
Residuals for horizontal and vertical wind fluctuations observed when winds were from the mountains showed a greater tendency for the positive bias associated with gravity waves. This tendency was most evident under unstable conditions when gravity wave influences on horizontal fluctuations were apparent about 25% of the time. These influences are explained as being associated with mountain lee waves occurring at the planetary boundary layer's capping inversion. They are evidenced at the 10-m level because atmospheric mixing processes occurring in thermally unstable atmospheric situations bring momentum generated from these waves downward to the ground.
Nonstationary and nonhomogenous atmospheric turbulence effects result in wind fluctuations whose half-hourly average standard deviations differ from those predicted by the LHS equations. Differences under stable atmospheres and low to moderate wind speeds are typically less than 50% of predicted values. They decrease as a percentage of predicted values with increasing wind speed and decreasing stability.
Abstract
This study investigates the behavior of wind fluctuations observed at the 10-m level over a flat terrain site located some 100 km east of the Rocky Mountains. The purposes were to assess residual fluctuations in order to ascertain effects attributable to the nonhomogenous, nonstationary character of turbulence and to evaluate influences of gravity waves. Residual wind fluctuations were defined for purposes of this study as the differences between observed half-hourly average standard deviations of wind fluctuations (σ v , σ u , σ w ) and those that are expected to occur in association with simultaneous wind speeds and static stabilities. These latter fluctuations were estimated from equations developed by Leahey, Hansen, and Schroeder (LHS).
Results of the analyses showed, as expected, that residual distributions for nonwesterly wind conditions were nearly Gaussian. Standard deviations for residuals of horizontal fluctuations, attributable to the nonhomogenous, nonstationary nature of turbulence, were 0.165 and 0.210 m s−1 for stable and unstable situations, respectively. For residuals associated with vertical fluctuations they were, respectively, 0.065 and 0.075 m s−1.
Residuals for horizontal and vertical wind fluctuations observed when winds were from the mountains showed a greater tendency for the positive bias associated with gravity waves. This tendency was most evident under unstable conditions when gravity wave influences on horizontal fluctuations were apparent about 25% of the time. These influences are explained as being associated with mountain lee waves occurring at the planetary boundary layer's capping inversion. They are evidenced at the 10-m level because atmospheric mixing processes occurring in thermally unstable atmospheric situations bring momentum generated from these waves downward to the ground.
Nonstationary and nonhomogenous atmospheric turbulence effects result in wind fluctuations whose half-hourly average standard deviations differ from those predicted by the LHS equations. Differences under stable atmospheres and low to moderate wind speeds are typically less than 50% of predicted values. They decrease as a percentage of predicted values with increasing wind speed and decreasing stability.
Abstract
Turbulence data were collected with the use of a sonic anemometer from October 1988 to September 1989. The study site was situated amid flat terrain near Calgary, Alberta. The data have been analyzed with respect to wind speed and stability. Simple empirical equations have been established that relate median standard deviations of transverse, longitudinal, and vertical wind fluctuations (σ v , σ u , σ w ) to wind speed and static stability. One-to-one correlation coefficients between predicted and observed data were typically in excess o.90.
Dispersion models utilize the ratios of turbulence parameters to wind speed (i.e., σ v /U, σ u /U, σ w /U). These ratios, referred to as standard deviations of the wind angles, have been derived as functions of wind speed and potential temperature gradients. Values of the standard deviation of the transverse wind angle σθ are shown to be independent of stability. Standard deviations of the longitudinal and vertical wind angles (σϕ, σϕ) have the same exponential dependency on stability at moderate to high wind speeds. Relations between σθ, σϕ, and σϕ and meteorological parameters of wind speed and stability are presented in graphical form.
Abstract
Turbulence data were collected with the use of a sonic anemometer from October 1988 to September 1989. The study site was situated amid flat terrain near Calgary, Alberta. The data have been analyzed with respect to wind speed and stability. Simple empirical equations have been established that relate median standard deviations of transverse, longitudinal, and vertical wind fluctuations (σ v , σ u , σ w ) to wind speed and static stability. One-to-one correlation coefficients between predicted and observed data were typically in excess o.90.
Dispersion models utilize the ratios of turbulence parameters to wind speed (i.e., σ v /U, σ u /U, σ w /U). These ratios, referred to as standard deviations of the wind angles, have been derived as functions of wind speed and potential temperature gradients. Values of the standard deviation of the transverse wind angle σθ are shown to be independent of stability. Standard deviations of the longitudinal and vertical wind angles (σϕ, σϕ) have the same exponential dependency on stability at moderate to high wind speeds. Relations between σθ, σϕ, and σϕ and meteorological parameters of wind speed and stability are presented in graphical form.
Abstract
Wind fluctuation data collected under stable atmospheric conditions at two prairie sites and a site located near the Rocky Mountain foothills have been analyzed. Results of the analysis show a marked tendency for horizontal fluctuation angles to vary inversely with wind speed. In contrast, vertical fluctuation angles tended to be invariant with wind speed.
Atmospheric turbulence was much greater at the foothills site than at the prairie sites. This was mainly due to the fact that standard donations of vertical wind angles were almost twice as great. Standard deviations of horizontal fluctuation angles were only about 20% greater.
Abstract
Wind fluctuation data collected under stable atmospheric conditions at two prairie sites and a site located near the Rocky Mountain foothills have been analyzed. Results of the analysis show a marked tendency for horizontal fluctuation angles to vary inversely with wind speed. In contrast, vertical fluctuation angles tended to be invariant with wind speed.
Atmospheric turbulence was much greater at the foothills site than at the prairie sites. This was mainly due to the fact that standard donations of vertical wind angles were almost twice as great. Standard deviations of horizontal fluctuation angles were only about 20% greater.
Abstract
It is important to assess the representativeness of mesoscale wind data because most short range pollution models assume that wind velocity will remain constant over distances in the order of 10 km. Previous observational studies have shown that average hourly mesoscale differences in wind directions and speeds might be typically about 25 degrees and 1 m s−1.
Initial results of this study using all available data, tended to agree with the above findings. Further analyses, however, were performed for periods to which most pollution models are restricted. These periods are usually characterized by the absence of mesoscale wind phenomena and terrain effects associated with katabatic winds. Hourly wind direction differences for these periods were found to be typically only about 10 degrees regardless of atmospheric stability. Wind speed differences were still typically about 1 m s−1.
Differences of both wind speed and direction were normally distributed, suggesting that horizontal mesoscale wind velocity differences occur randomly. For this reason it may be impractical to attempt the development of short-range plume dispersion models that physically account for horizontal inhomogeneities.
Abstract
It is important to assess the representativeness of mesoscale wind data because most short range pollution models assume that wind velocity will remain constant over distances in the order of 10 km. Previous observational studies have shown that average hourly mesoscale differences in wind directions and speeds might be typically about 25 degrees and 1 m s−1.
Initial results of this study using all available data, tended to agree with the above findings. Further analyses, however, were performed for periods to which most pollution models are restricted. These periods are usually characterized by the absence of mesoscale wind phenomena and terrain effects associated with katabatic winds. Hourly wind direction differences for these periods were found to be typically only about 10 degrees regardless of atmospheric stability. Wind speed differences were still typically about 1 m s−1.
Differences of both wind speed and direction were normally distributed, suggesting that horizontal mesoscale wind velocity differences occur randomly. For this reason it may be impractical to attempt the development of short-range plume dispersion models that physically account for horizontal inhomogeneities.
