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- Author or Editor: J. M. Brown x
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Abstract
In an attempt to determine the relative contribution of the direct incorporation of cold air (detrainment from overshooting convective cloud tops) to the production of mesohighs in the vicinity of the tropopause, two numerical simulations were performed using a 20 km horizontal resolution, 20-level primitive equation model. One simulation included direct cooling and the other did not. The results showed that including the cooling increased the high-level pressure and wind perturbations by approximately 30 and 40%; respectively. The simulation results also showed that in spite of the omission of the direct cloud cooling, a high-level cold pool was still generated. The cooling was accomplished by adiabatic expansion in response to the lifting by the convectively driven mesoscale vertical circulation. Thus, it appears that the mesoscale adiabatic expansion is the dominant effect in elevated-mesohigh production and the detrainment of overshooting air is an important modifying factor.
Abstract
In an attempt to determine the relative contribution of the direct incorporation of cold air (detrainment from overshooting convective cloud tops) to the production of mesohighs in the vicinity of the tropopause, two numerical simulations were performed using a 20 km horizontal resolution, 20-level primitive equation model. One simulation included direct cooling and the other did not. The results showed that including the cooling increased the high-level pressure and wind perturbations by approximately 30 and 40%; respectively. The simulation results also showed that in spite of the omission of the direct cloud cooling, a high-level cold pool was still generated. The cooling was accomplished by adiabatic expansion in response to the lifting by the convectively driven mesoscale vertical circulation. Thus, it appears that the mesoscale adiabatic expansion is the dominant effect in elevated-mesohigh production and the detrainment of overshooting air is an important modifying factor.
Abstract
A hybrid density function is given for describing wind-speed distributions having nonzero probability of “calm.” A Weibull probability graph paper designed specifically for plotting wind-speed distributions is used to determine distribution parameters to within a few percent of values obtained by the maximum likelihood technique. Data from the National Weather Service are used to demonstrate the use of the hybrid density function and the Weibull graph paper.
Abstract
A hybrid density function is given for describing wind-speed distributions having nonzero probability of “calm.” A Weibull probability graph paper designed specifically for plotting wind-speed distributions is used to determine distribution parameters to within a few percent of values obtained by the maximum likelihood technique. Data from the National Weather Service are used to demonstrate the use of the hybrid density function and the Weibull graph paper.
Abstract
The temporal variation of specific humidity during morning hours was evaluated by analytic and numerical model scaling as well as by observational means. The scaling quantified (i) the gradual increase in the shelter increase humidity as the surface temperatures inversion is eroded during the morning hours; (ii) the sharp decrease in the shelter specific humidity when the newly developed boundary layer merges with the previous day's elevated neutral layer. The relation of these patterns to the early-morning thermal stratification and the Bowen ratio was estimated. Observational data supported the general features suggested by the scaling evaluations. The applied significance of the presented specific-humidity patterns is outlined.
Abstract
The temporal variation of specific humidity during morning hours was evaluated by analytic and numerical model scaling as well as by observational means. The scaling quantified (i) the gradual increase in the shelter increase humidity as the surface temperatures inversion is eroded during the morning hours; (ii) the sharp decrease in the shelter specific humidity when the newly developed boundary layer merges with the previous day's elevated neutral layer. The relation of these patterns to the early-morning thermal stratification and the Bowen ratio was estimated. Observational data supported the general features suggested by the scaling evaluations. The applied significance of the presented specific-humidity patterns is outlined.
Abstract
A unique and promising method is outlined for determining the complex part of the index of refraction of atmospheric dust by utilizing measurements of diffuse and direct solar radiation. The solution requires comparison of measurements of the ratio of diffuse to direct solar radiation with theoretical calculations of the same values. A knowledge of the ground albedo is necessary for the solution. The ground albedo may, in fact, also be determined from the measured ratios, but the solution in this case tends to he more sensitive to measurement than when the ground albedo is known.
Abstract
A unique and promising method is outlined for determining the complex part of the index of refraction of atmospheric dust by utilizing measurements of diffuse and direct solar radiation. The solution requires comparison of measurements of the ratio of diffuse to direct solar radiation with theoretical calculations of the same values. A knowledge of the ground albedo is necessary for the solution. The ground albedo may, in fact, also be determined from the measured ratios, but the solution in this case tends to he more sensitive to measurement than when the ground albedo is known.
Abstract
On the afternoon of 3 June 1981 a severe thunderstorm spawned two tornadoes which moved across a portion of metropolitan Denver. The tornadoes were classified as strong F2 intensity, and caused damage totaling over $1 5 million. The synoptic-scale setting for this event was similar to that associated with many other occurrences of severe convection in eastern Colorado, with post-frontal moist southeasterly upslope flow at low levels and southwesterly flow aloft in advance of an approaching trough.
We chose to study this event in part because of its occurrence within the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork. Emphasis is placed on mesoscale evolution culminating in the formation of the tornadic storm. A zone of surface convergence and cyclonic vorticity developed during the early daylight hours over and north of Denver between southeasterly flow over the plains and a region of lighter, generally northerly flow just east of the foothills. The tornadic storm formed from a complex interaction of older thunderstorm cells, and subsequently intensified at the southern end of the convergence-vorticity zone and moved to the north-northeast. Other severe convection also occurred along this zone later in the afternoon.
The development of this convergence-vorticity zone under conditions of ambient southeasterly flow appears to be topographically forced by a ridge of higher terrain which extends eastward from south of Denver, but the specific processes involved are unclear. The zone occurs frequently and, for the years 1981 and 1982 for which the PROFS mesonet data were available, was associated with a disproportionate number of severe weather events, especially tornadoes.
Abstract
On the afternoon of 3 June 1981 a severe thunderstorm spawned two tornadoes which moved across a portion of metropolitan Denver. The tornadoes were classified as strong F2 intensity, and caused damage totaling over $1 5 million. The synoptic-scale setting for this event was similar to that associated with many other occurrences of severe convection in eastern Colorado, with post-frontal moist southeasterly upslope flow at low levels and southwesterly flow aloft in advance of an approaching trough.
We chose to study this event in part because of its occurrence within the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork. Emphasis is placed on mesoscale evolution culminating in the formation of the tornadic storm. A zone of surface convergence and cyclonic vorticity developed during the early daylight hours over and north of Denver between southeasterly flow over the plains and a region of lighter, generally northerly flow just east of the foothills. The tornadic storm formed from a complex interaction of older thunderstorm cells, and subsequently intensified at the southern end of the convergence-vorticity zone and moved to the north-northeast. Other severe convection also occurred along this zone later in the afternoon.
The development of this convergence-vorticity zone under conditions of ambient southeasterly flow appears to be topographically forced by a ridge of higher terrain which extends eastward from south of Denver, but the specific processes involved are unclear. The zone occurs frequently and, for the years 1981 and 1982 for which the PROFS mesonet data were available, was associated with a disproportionate number of severe weather events, especially tornadoes.
Abstract
Velocity data were obtained within Park Avenue in Oklahoma City, Oklahoma, using three-dimensional sonic anemometers under unstable atmospheric conditions. These data are used to produce velocity spectra, cospectra, and weighted joint probability density functions at various heights and horizontal locations in the street canyon. This analysis has helped to describe a number of physically interesting urban flow phenomena. Previous research has shown that the ratio of Reynolds shear stresses to normal stresses is typically much smaller deep within the canopy than those ratios found at the top of canopy and in the roughness sublayer. The turbulence in this region exhibits significant contributions to all four quadrants of a weighted joint-probability density function of horizontal and vertical velocity fluctuations, yielding the characteristic small Reynolds shear stresses in the flow. The velocity cospectra measured at the base of the canopy show evidence of discrete frequency bands of both positive and negative correlation that yield a small correlation, as indicated by the Reynolds shear stresses. Two major peaks were often observed in the spectra and cospectra: a low-frequency peak that appears to be associated with vortex shedding off the buildings and a midfrequency peak generally associated with canyon geometry. The low-frequency peak was found to produce a countergradient contribution to the along-wind vertical velocity covariance. Standard spectral tests for local isotropy indicate that isotropic conditions occur at different frequencies depending on spatial location, demonstrating the need to be thorough when testing for local isotropy with the urban canopy.
Abstract
Velocity data were obtained within Park Avenue in Oklahoma City, Oklahoma, using three-dimensional sonic anemometers under unstable atmospheric conditions. These data are used to produce velocity spectra, cospectra, and weighted joint probability density functions at various heights and horizontal locations in the street canyon. This analysis has helped to describe a number of physically interesting urban flow phenomena. Previous research has shown that the ratio of Reynolds shear stresses to normal stresses is typically much smaller deep within the canopy than those ratios found at the top of canopy and in the roughness sublayer. The turbulence in this region exhibits significant contributions to all four quadrants of a weighted joint-probability density function of horizontal and vertical velocity fluctuations, yielding the characteristic small Reynolds shear stresses in the flow. The velocity cospectra measured at the base of the canopy show evidence of discrete frequency bands of both positive and negative correlation that yield a small correlation, as indicated by the Reynolds shear stresses. Two major peaks were often observed in the spectra and cospectra: a low-frequency peak that appears to be associated with vortex shedding off the buildings and a midfrequency peak generally associated with canyon geometry. The low-frequency peak was found to produce a countergradient contribution to the along-wind vertical velocity covariance. Standard spectral tests for local isotropy indicate that isotropic conditions occur at different frequencies depending on spatial location, demonstrating the need to be thorough when testing for local isotropy with the urban canopy.
Abstract
Velocity data were obtained from sonic anemometer measurements within an east–west-running street canyon located in the urban core of Oklahoma City, Oklahoma, during the Joint Urban 2003 field campaign. These data were used to explore the directional dependence of the mean flow and turbulence within a real-world street canyon. The along-canyon vortex that is a key characteristic of idealized street canyon studies was not evident in the mean wind data, although the sensor placement was not optimized for the detection of such structures. Instead, surface wind measurements imply that regions of horizontal convergence and divergence exist within the canopy, which are likely caused by taller buildings diverting the winds aloft down into the canopy. The details of these processes appear to be dependent on relatively small perturbations in the prevailing wind direction. Turbulence intensities within the canyon interior appeared to have more dependence on prevailing wind direction than they did in the intersections. Turbulence in the intersections tended to be higher than was observed in the canyon interior. This behavior implies that there are some fundamental differences between the flow structure found in North American–style cities where building heights are typically heterogeneous and that found in European-style cities, which generally have more homogeneous building heights. It is hypothesized that the greater three-dimensionality caused by the heterogeneous building heights increases the ventilation of the urban canopy through mean advective transport as well as enhanced turbulence.
Abstract
Velocity data were obtained from sonic anemometer measurements within an east–west-running street canyon located in the urban core of Oklahoma City, Oklahoma, during the Joint Urban 2003 field campaign. These data were used to explore the directional dependence of the mean flow and turbulence within a real-world street canyon. The along-canyon vortex that is a key characteristic of idealized street canyon studies was not evident in the mean wind data, although the sensor placement was not optimized for the detection of such structures. Instead, surface wind measurements imply that regions of horizontal convergence and divergence exist within the canopy, which are likely caused by taller buildings diverting the winds aloft down into the canopy. The details of these processes appear to be dependent on relatively small perturbations in the prevailing wind direction. Turbulence intensities within the canyon interior appeared to have more dependence on prevailing wind direction than they did in the intersections. Turbulence in the intersections tended to be higher than was observed in the canyon interior. This behavior implies that there are some fundamental differences between the flow structure found in North American–style cities where building heights are typically heterogeneous and that found in European-style cities, which generally have more homogeneous building heights. It is hypothesized that the greater three-dimensionality caused by the heterogeneous building heights increases the ventilation of the urban canopy through mean advective transport as well as enhanced turbulence.
Abstract
The effect of surface characteristics on the daytime change in the potential for development of deep convection resulting from surface flux of heat and moisture is evaluated by conceptual, scaling, and numerical modeling approaches. It is shown that deep convection depends significantly on the Bowen ratio; for smaller Bowen ratio, the thermodynamic potential for deep convection increases. The elevation and the intensity of the capping stable layer have an opposing impact on deep convection: increasing moisture accumulation through evapotranspiration was supportive but was counteracted by the enhancement of dry entrainment. Based on an approximate treatment of the effect of cloudiness on solar irradiance, it was found that development of fair weather cumulus has a secondary effect on deep convection potential. Observational and operational aspects of the influence of surface conditions on evapotranspiration and development of deep convection are presented.
Abstract
The effect of surface characteristics on the daytime change in the potential for development of deep convection resulting from surface flux of heat and moisture is evaluated by conceptual, scaling, and numerical modeling approaches. It is shown that deep convection depends significantly on the Bowen ratio; for smaller Bowen ratio, the thermodynamic potential for deep convection increases. The elevation and the intensity of the capping stable layer have an opposing impact on deep convection: increasing moisture accumulation through evapotranspiration was supportive but was counteracted by the enhancement of dry entrainment. Based on an approximate treatment of the effect of cloudiness on solar irradiance, it was found that development of fair weather cumulus has a secondary effect on deep convection potential. Observational and operational aspects of the influence of surface conditions on evapotranspiration and development of deep convection are presented.
Abstract
Theoretical computations of the intensity and polarization of diffusively transmitted sunlight are presented for two wavelengths, λ = 4290 Å and λ = 5000 Å. The computations are for atmospheres containing various distributions of aerosols, as well as normal molecular constituents, and allow for all significant orders of scattering. The theoretical computations are compared with observations, and it is shown that inclusion of aerosols in the theoretical models results in considerably better agreement between observation and theory than can be achieved by assuming a pure molecular atmosphere for the theoretical computations.
Abstract
Theoretical computations of the intensity and polarization of diffusively transmitted sunlight are presented for two wavelengths, λ = 4290 Å and λ = 5000 Å. The computations are for atmospheres containing various distributions of aerosols, as well as normal molecular constituents, and allow for all significant orders of scattering. The theoretical computations are compared with observations, and it is shown that inclusion of aerosols in the theoretical models results in considerably better agreement between observation and theory than can be achieved by assuming a pure molecular atmosphere for the theoretical computations.
Abstract
Large eddy simulations sometimes use monotone advection schemes. Such schemes are dissipative, and the effective subgrid model then becomes the combined effect of the intended model and of the numerical dissipation. The impacts on simulation reliability are examined for the cases of dry convective and neutral planetary boundary layers. In general it is found that the results in the well-resolved flow interior are insensitive to the details of the advection scheme. However, unsatisfactory results may be obtained if numerical dissipation dominates where the flow becomes less well resolved as the surface is approached.
Abstract
Large eddy simulations sometimes use monotone advection schemes. Such schemes are dissipative, and the effective subgrid model then becomes the combined effect of the intended model and of the numerical dissipation. The impacts on simulation reliability are examined for the cases of dry convective and neutral planetary boundary layers. In general it is found that the results in the well-resolved flow interior are insensitive to the details of the advection scheme. However, unsatisfactory results may be obtained if numerical dissipation dominates where the flow becomes less well resolved as the surface is approached.
