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- Author or Editor: B. Stankov x
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Abstract
We calculated integral scales for horizontal and vertical velocity components, temperature, humidity and ozone concentration, as well as for their variances and covariances from aircraft measurements in the convective atmospheric boundary layer over both ocean and land surfaces. We found that the integral scales of the second-order moment quantities are 0.67± 0.09 that of the variables themselves. Consequently, only the second-order moment integral scales are presented here. These results are used to calculate the averaging lengths necessary to measure second-order moment quantities to a given accuracy. We found that a measurement length of 10 to 100 times the boundary-layer height is required to measure variances to 10% accuracy, while scalar fluxes require a measurement length of 102 to 104 and stress a measurement length of 103 to 105 times the boundary layer height. We also show that the ratio of the wavelength of the spectral peak to the integral scale can be used to estimate the sharpness of the spectral peak.
Abstract
We calculated integral scales for horizontal and vertical velocity components, temperature, humidity and ozone concentration, as well as for their variances and covariances from aircraft measurements in the convective atmospheric boundary layer over both ocean and land surfaces. We found that the integral scales of the second-order moment quantities are 0.67± 0.09 that of the variables themselves. Consequently, only the second-order moment integral scales are presented here. These results are used to calculate the averaging lengths necessary to measure second-order moment quantities to a given accuracy. We found that a measurement length of 10 to 100 times the boundary-layer height is required to measure variances to 10% accuracy, while scalar fluxes require a measurement length of 102 to 104 and stress a measurement length of 103 to 105 times the boundary layer height. We also show that the ratio of the wavelength of the spectral peak to the integral scale can be used to estimate the sharpness of the spectral peak.
Abstract
Even slight terrain inhomogeneities can cause large horizontal variations in the clear, stably stratified, nocturnal boundary layer largely through cold air drainage. By early morning the valleys and depressions can be several degrees cooler than the adjacent slopes and plateaus. As surface heating begins in the morning, these horizontal variations can lead to abrupt changes in temperature and wind speed at valley observation sites, as the boundary layer warms and becomes unstably stratified. Temperature and wind speed changes of 12 K and 6 m s−1 respectively, within a 30 min period are observed even in valleys as shallow as 50 m with slopes of only 0.007. These changes are too large to be accounted for by vertical convergence of turbulent beat flux. Rather, it appears that a well-mixed boundary layer is advected into the valley from the upstream slopes or plateaus. Data from the National Hail Research Experiment (NHRE) 1976 surface mesonet are used to show that, statistically, this abrupt change is a frequent occurrence, throughout the summer, even in broad shallow valleys, but almost never occurs on plateau observation sites.
A case study from the Haswell, Colorado, experiment of 1975 shows in detail, through a variety of observations, the sequence of events that occurs during this rapid morning transition. As surface heating begins, the valley air, which is about 4 K colder than the air over the upstream slope and plateau, becomes less stably stratified and increasingly turbulent. Eventually, the shear stress at the top of the boundary layer becomes large enough to pull the cold air out of the valley. The valley air is then replaced by warmer upstream air that is already well mixed. The criteria necessary for this transition to occur are evaluated and generalized for application to other situations. These criteria are then applied to several previous observational studies of the dissipation of cold air pools formed in valleys through nighttime radiational cooling.
The observed transition in temperature typically precedes the velocity transition by 20–40 min. This lag appears to be due to both the adverse pressure gradient developed during the temperature transition, and the difference in the shear and temperature gradient production terms in the equations for shear stress and heat flux.
Abstract
Even slight terrain inhomogeneities can cause large horizontal variations in the clear, stably stratified, nocturnal boundary layer largely through cold air drainage. By early morning the valleys and depressions can be several degrees cooler than the adjacent slopes and plateaus. As surface heating begins in the morning, these horizontal variations can lead to abrupt changes in temperature and wind speed at valley observation sites, as the boundary layer warms and becomes unstably stratified. Temperature and wind speed changes of 12 K and 6 m s−1 respectively, within a 30 min period are observed even in valleys as shallow as 50 m with slopes of only 0.007. These changes are too large to be accounted for by vertical convergence of turbulent beat flux. Rather, it appears that a well-mixed boundary layer is advected into the valley from the upstream slopes or plateaus. Data from the National Hail Research Experiment (NHRE) 1976 surface mesonet are used to show that, statistically, this abrupt change is a frequent occurrence, throughout the summer, even in broad shallow valleys, but almost never occurs on plateau observation sites.
A case study from the Haswell, Colorado, experiment of 1975 shows in detail, through a variety of observations, the sequence of events that occurs during this rapid morning transition. As surface heating begins, the valley air, which is about 4 K colder than the air over the upstream slope and plateau, becomes less stably stratified and increasingly turbulent. Eventually, the shear stress at the top of the boundary layer becomes large enough to pull the cold air out of the valley. The valley air is then replaced by warmer upstream air that is already well mixed. The criteria necessary for this transition to occur are evaluated and generalized for application to other situations. These criteria are then applied to several previous observational studies of the dissipation of cold air pools formed in valleys through nighttime radiational cooling.
The observed transition in temperature typically precedes the velocity transition by 20–40 min. This lag appears to be due to both the adverse pressure gradient developed during the temperature transition, and the difference in the shear and temperature gradient production terms in the equations for shear stress and heat flux.
Abstract
Data from the Boulder Atmospheric Observatory (BAO) are used to investigate the wave and turbulence structure of the convective atmospheric mixed layer and the overlying inversion. Three cases are discussed, one in considerable detail, in which the depth of the mixed layer is below the top of the 300 m tower at the BAO and is nearly steady state for several hours. Velocity and temperature variances and spectra, coherences between vertical velocity and temperature, and vertical velocities at different levels on the tower are used to show that although the mixed-layer behavior is for the most part similar to that found in previous studies, there are some significant differences due mainly to the relatively large shear term in the turbulence energy equation compared with buoyancy, both within the mixed layer and in the capping inversion. For example, the wavelength of the spectral maximum for vertical velocity in the upper half of the mixed layer is about three times the boundary-layer height, which is about twice that estimated in a previous experiment. The wavelength is up to 5.5 times the mixed-layer height above the top of the mixed layer. Within the mixed layer, terms in the turbulence kinetic energy equation are similar to previous studies. Above the mixed layer, shear production becomes large, and is approximately balanced by the sum of the buoyancy, dissipation and transport terms. The temperature variance and flux budgets also have large terms and significant residuals in the overlying inversion.
Abstract
Data from the Boulder Atmospheric Observatory (BAO) are used to investigate the wave and turbulence structure of the convective atmospheric mixed layer and the overlying inversion. Three cases are discussed, one in considerable detail, in which the depth of the mixed layer is below the top of the 300 m tower at the BAO and is nearly steady state for several hours. Velocity and temperature variances and spectra, coherences between vertical velocity and temperature, and vertical velocities at different levels on the tower are used to show that although the mixed-layer behavior is for the most part similar to that found in previous studies, there are some significant differences due mainly to the relatively large shear term in the turbulence energy equation compared with buoyancy, both within the mixed layer and in the capping inversion. For example, the wavelength of the spectral maximum for vertical velocity in the upper half of the mixed layer is about three times the boundary-layer height, which is about twice that estimated in a previous experiment. The wavelength is up to 5.5 times the mixed-layer height above the top of the mixed layer. Within the mixed layer, terms in the turbulence kinetic energy equation are similar to previous studies. Above the mixed layer, shear production becomes large, and is approximately balanced by the sum of the buoyancy, dissipation and transport terms. The temperature variance and flux budgets also have large terms and significant residuals in the overlying inversion.
Abstract
The paper describes a convective boundary layer experiment conducted in April 1978 at the Boulder Atmospheric Observatory, and examines the spectral behavior of wind velocity and temperature from the Observatory's 300 m tower, from aircraft flights alongside the tower and from a surface network of anemometers, for evidence of terrain influence on turbulence structure. The gently rolling terrain at the site does not seem to affect the turbulence spectra from the tower in any perceptible manner, except for minor shifts in the vertical velocity and temperature spectral peaks. The aircraft vertical velocity spectra showed different shapes for alongwind and crosswind sampling directions, as in earlier measurements over ocean surfaces, and their peaks are displaced to higher wavenumbers compared with the tower spectra. Long-term spectra of horizontal wind components from surface stations around the tower exhibit no particular sensitivity to site selection. Under near-stationary conditions the peak of the spectrum of the streamwise component tends to reflect more closely the predominant boundary layer. convective scales than does the peak of the lateral wind component. The problem of identifying those scales in the presence of large shifts in wind direction is discussed.
Abstract
The paper describes a convective boundary layer experiment conducted in April 1978 at the Boulder Atmospheric Observatory, and examines the spectral behavior of wind velocity and temperature from the Observatory's 300 m tower, from aircraft flights alongside the tower and from a surface network of anemometers, for evidence of terrain influence on turbulence structure. The gently rolling terrain at the site does not seem to affect the turbulence spectra from the tower in any perceptible manner, except for minor shifts in the vertical velocity and temperature spectral peaks. The aircraft vertical velocity spectra showed different shapes for alongwind and crosswind sampling directions, as in earlier measurements over ocean surfaces, and their peaks are displaced to higher wavenumbers compared with the tower spectra. Long-term spectra of horizontal wind components from surface stations around the tower exhibit no particular sensitivity to site selection. Under near-stationary conditions the peak of the spectrum of the streamwise component tends to reflect more closely the predominant boundary layer. convective scales than does the peak of the lateral wind component. The problem of identifying those scales in the presence of large shifts in wind direction is discussed.
