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Abstract
A two-dimensional, non-steady model of the flow over an infinitely wide, warm, rough city is presented. The model consists of two layers, a lower analytical constant-flux layer, and an upper finite-difference transition layer, in which the vorticity and heat conduction equations are solved. The atmosphere is assumed to be Boussinesq, hydrostatic and slab symmetric, while all motions are assumed to be adiabatic. Finite-difference solutions are obtained over a variable, interlaced grid, with the use of a time-splitting technique, in conjunction with the donor cell method of differencing the advection terms.
Simulations were carried out reproducing the daytime flow of a neutral atmosphere over a rough city, and the nighttime flow of stable atmosphere over a rough, warm city. Comparisons are presented to show that the model is capable of reproducing many of the observed characteristics of the urban boundary layer.
Abstract
A two-dimensional, non-steady model of the flow over an infinitely wide, warm, rough city is presented. The model consists of two layers, a lower analytical constant-flux layer, and an upper finite-difference transition layer, in which the vorticity and heat conduction equations are solved. The atmosphere is assumed to be Boussinesq, hydrostatic and slab symmetric, while all motions are assumed to be adiabatic. Finite-difference solutions are obtained over a variable, interlaced grid, with the use of a time-splitting technique, in conjunction with the donor cell method of differencing the advection terms.
Simulations were carried out reproducing the daytime flow of a neutral atmosphere over a rough city, and the nighttime flow of stable atmosphere over a rough, warm city. Comparisons are presented to show that the model is capable of reproducing many of the observed characteristics of the urban boundary layer.
Abstract
Differences in the temperature fields through the lowest 700 m of the atmosphere in and around New York City during the hours near sunrise are analyzed. Data were obtained by an instrumented helicopter on 42 predetermined test mornings from July 1964 to December 1966.
Results show urban surface temperature inversions to be less intense, and far less frequent, than those in the surrounding non-urban regions. A high frequency of weak elevated inversion layers at an average height of 310 m was observed over the city.
The average intensity of the urban heat island, i.e., urban temperature excess, was a maximum near the surface and decreased to zero at 300 m. On mornings with relatively strong urban elevated inversion layers the heat island extended to well over 500 m. For more than two-thirds of the test mornings there existed an elevated “cross-over layer” in which rural temperatures were higher than urban temperatures. The magnitude of the cross-over effect was less than that of the heat island effect.
Abstract
Differences in the temperature fields through the lowest 700 m of the atmosphere in and around New York City during the hours near sunrise are analyzed. Data were obtained by an instrumented helicopter on 42 predetermined test mornings from July 1964 to December 1966.
Results show urban surface temperature inversions to be less intense, and far less frequent, than those in the surrounding non-urban regions. A high frequency of weak elevated inversion layers at an average height of 310 m was observed over the city.
The average intensity of the urban heat island, i.e., urban temperature excess, was a maximum near the surface and decreased to zero at 300 m. On mornings with relatively strong urban elevated inversion layers the heat island extended to well over 500 m. For more than two-thirds of the test mornings there existed an elevated “cross-over layer” in which rural temperatures were higher than urban temperatures. The magnitude of the cross-over effect was less than that of the heat island effect.
Abstract
Data obtained from the extensive mesoscale anemometer network established during the New York University Urban Air Polution Dynamics Program were used to study the effects of New York City on frontal movement. Frontal position and movement were determined using sequential hourly streamline and isotach analyses of the flow through the New York City metropolitan area.
Results showed that frontal movement during nonheat-island periods was retarded significantly over the entire central urban area. The retardation was probably due to the increased surface frictional drag exerted on the front by the increased surface roughness of the city as compared to that of its surrounding environs. During periods with well-developed urban heat islands, the results showed a retardation in frontal speed over the upwind half of the city, followed by a significant acceleration of the front over its downwind half.
Abstract
Data obtained from the extensive mesoscale anemometer network established during the New York University Urban Air Polution Dynamics Program were used to study the effects of New York City on frontal movement. Frontal position and movement were determined using sequential hourly streamline and isotach analyses of the flow through the New York City metropolitan area.
Results showed that frontal movement during nonheat-island periods was retarded significantly over the entire central urban area. The retardation was probably due to the increased surface frictional drag exerted on the front by the increased surface roughness of the city as compared to that of its surrounding environs. During periods with well-developed urban heat islands, the results showed a retardation in frontal speed over the upwind half of the city, followed by a significant acceleration of the front over its downwind half.
Abstract
Month-to-month variations in the early morning surface-based and near-noon elevated inversions at San José, Calif., were determined from slow rise radiosondes launched during a four-year period. A high frequency of shallow, radiative, surface-based inversions were found in winter during the early morning hours, while during the same period in summer, a low frequency of deeper based inversions arose from a combination of radiative and subsidence processes. The frequency of elevated inversions in the hours near noon was lowest during fall and spring, while inversion bases were highest and thicknesses least during these periods.
Abstract
Month-to-month variations in the early morning surface-based and near-noon elevated inversions at San José, Calif., were determined from slow rise radiosondes launched during a four-year period. A high frequency of shallow, radiative, surface-based inversions were found in winter during the early morning hours, while during the same period in summer, a low frequency of deeper based inversions arose from a combination of radiative and subsidence processes. The frequency of elevated inversions in the hours near noon was lowest during fall and spring, while inversion bases were highest and thicknesses least during these periods.
Abstract
The two–dimensional, non-steady URBMET urban boundary layer model was integrated on a variable and staggered grid, with time–splitting and a donor cell advection scheme. Simulations for periods of up to 14 h of meteorological time were initially carried out using a constant time step for both the advection and diffusion processes, and using an eddy diffusivity which varies in space and time. The simulators were then repeated using a variable, but equal, time step for both
By trial and error it was found that the minimum value of the time step normally associated with the diffusion term in the case of a constant eddy mixing coefficient in a constant, non-staggered grid could be exceeded by 50% without causing computational instability. The number of required time steps in the second series of runs, and thus the required computer time, was reduced by a factor of two from that needed for the first series. Results showed no significant changes from those resulting from the fixed time step simulations.
Another series of simulations was carried out using unequal and variable time steps, with the advection process evaluated using a time step which was up to 20 times larger than that for the diffusion process. This change reduced the required computer time by an additional factor of three. Results showed an apparent reduction in the amount of numerically induced diffusion, and a corresponding enhancement in the strength of the circulation cells associated with the urban heat island.
A final series of simulations demonstrated that anthropogenic moisture produces a significant buoyancy of the air above an urban area. This moisture enhanced the buoyancy produced by the urban heat island, and thus increased the intensity of the urban breeze effect.
Abstract
The two–dimensional, non-steady URBMET urban boundary layer model was integrated on a variable and staggered grid, with time–splitting and a donor cell advection scheme. Simulations for periods of up to 14 h of meteorological time were initially carried out using a constant time step for both the advection and diffusion processes, and using an eddy diffusivity which varies in space and time. The simulators were then repeated using a variable, but equal, time step for both
By trial and error it was found that the minimum value of the time step normally associated with the diffusion term in the case of a constant eddy mixing coefficient in a constant, non-staggered grid could be exceeded by 50% without causing computational instability. The number of required time steps in the second series of runs, and thus the required computer time, was reduced by a factor of two from that needed for the first series. Results showed no significant changes from those resulting from the fixed time step simulations.
Another series of simulations was carried out using unequal and variable time steps, with the advection process evaluated using a time step which was up to 20 times larger than that for the diffusion process. This change reduced the required computer time by an additional factor of three. Results showed an apparent reduction in the amount of numerically induced diffusion, and a corresponding enhancement in the strength of the circulation cells associated with the urban heat island.
A final series of simulations demonstrated that anthropogenic moisture produces a significant buoyancy of the air above an urban area. This moisture enhanced the buoyancy produced by the urban heat island, and thus increased the intensity of the urban breeze effect.
Abstract
Temporal changes in the spatial distribution of sulfur dioxide concentrations in New York City resulting from the passage of sea breeze and synoptic fronts were studied using data from the New York University/New York City Urban Air Pollution Data Set. Results show that upwind portions of New York City experience decreasing concentrations with the passage of sea breeze fronts, while downwind portions experience increasing concentrations. Synoptic fronts produce increasing concentrations in the less urbanized areas to the east and west of Manhattan and decreasing concentrations in Manhattan. The one synoptic front which moved extremely slowly showed extreme frictional retardation and produced the opposite effects on the concentration field.
Abstract
Temporal changes in the spatial distribution of sulfur dioxide concentrations in New York City resulting from the passage of sea breeze and synoptic fronts were studied using data from the New York University/New York City Urban Air Pollution Data Set. Results show that upwind portions of New York City experience decreasing concentrations with the passage of sea breeze fronts, while downwind portions experience increasing concentrations. Synoptic fronts produce increasing concentrations in the less urbanized areas to the east and west of Manhattan and decreasing concentrations in Manhattan. The one synoptic front which moved extremely slowly showed extreme frictional retardation and produced the opposite effects on the concentration field.
workshop summary
Workshop on Modeling the Urban Boundary Layer, Las Vegas, Nevada, 5 May 1975
Abstract
Meteorological and air-quality data, as well as surface tracer concentration values, were collected during 1990 to assess the impacts of Navajo Generating Station (NGS) emissions on Grand Canyon National Park (GCNP) air quality. These data have been used in the present investigation to determine between direct and indirect transport routes taken by the NGS plume to produce measured high-tracer concentration events at GCNP.
The meteorological data were used as input into a three-dimensional mass-consistent wind model, whose output was used as input into a horizontal forward-trajectory model. Calculated polluted air locations were compared with observed surface-tracer concentration values.
Results show that complex-terrain features affect local wind-flow patterns during winter in the Grand Canyon area. Local channeling, decoupled canyon winds, and slope and valley flows dominate in the region when synoptic systems are weak. Direct NGS plume transport to GCNP occurs with northeasterly plume-height winds, while indirect transport to the park is caused by wind direction shifts associated with passing synoptic systems. Calculated polluted airmass positions along the modeled streak lines match measured surface-tracer observations in both space and time.
Abstract
Meteorological and air-quality data, as well as surface tracer concentration values, were collected during 1990 to assess the impacts of Navajo Generating Station (NGS) emissions on Grand Canyon National Park (GCNP) air quality. These data have been used in the present investigation to determine between direct and indirect transport routes taken by the NGS plume to produce measured high-tracer concentration events at GCNP.
The meteorological data were used as input into a three-dimensional mass-consistent wind model, whose output was used as input into a horizontal forward-trajectory model. Calculated polluted air locations were compared with observed surface-tracer concentration values.
Results show that complex-terrain features affect local wind-flow patterns during winter in the Grand Canyon area. Local channeling, decoupled canyon winds, and slope and valley flows dominate in the region when synoptic systems are weak. Direct NGS plume transport to GCNP occurs with northeasterly plume-height winds, while indirect transport to the park is caused by wind direction shifts associated with passing synoptic systems. Calculated polluted airmass positions along the modeled streak lines match measured surface-tracer observations in both space and time.
Abstract
This study investigates interactive effects from the Beijing urban area on temperature, humidity, wind speed and direction, and precipitation by use of hourly automatic weather station data from June to August 2008–12. Results show the Beijing summer urban heat island (UHI) as a multicenter distribution (corresponding to underlying land-use features), with stronger nighttime than daytime values (averages of 1.7° vs 0.8°C, respectively). Specific humidity was lower in urban Beijing than in surrounding nonurban areas, and this urban dry island is stronger during day than night (maximum of −2.4 vs −1.9 g kg−1). Wind direction is affected by both a mountain–valley-breeze circulation and by urbanization. Morning low-level flows converged into the strong UHI, but afternoon and evening southerly winds were bifurcated by an urban building-barrier-induced divergence. Summer thunderstorms also thus bifurcated and bypassed the urban center because of the building-barrier effect during both daytime and nighttime weak-UHI (<1.25°C) periods. This produced a regional-normalized rainfall (NR) minimum in the urban center and directly downwind of the urban area (of up to −35%), with maximum values along its downwind lateral edges (of >15%). Strong UHIs (>1.25°C), however, induced or enhanced thunderstorm formation (again day and night), which produced an NR maximum in the most urbanized area of up to 75%.
Abstract
This study investigates interactive effects from the Beijing urban area on temperature, humidity, wind speed and direction, and precipitation by use of hourly automatic weather station data from June to August 2008–12. Results show the Beijing summer urban heat island (UHI) as a multicenter distribution (corresponding to underlying land-use features), with stronger nighttime than daytime values (averages of 1.7° vs 0.8°C, respectively). Specific humidity was lower in urban Beijing than in surrounding nonurban areas, and this urban dry island is stronger during day than night (maximum of −2.4 vs −1.9 g kg−1). Wind direction is affected by both a mountain–valley-breeze circulation and by urbanization. Morning low-level flows converged into the strong UHI, but afternoon and evening southerly winds were bifurcated by an urban building-barrier-induced divergence. Summer thunderstorms also thus bifurcated and bypassed the urban center because of the building-barrier effect during both daytime and nighttime weak-UHI (<1.25°C) periods. This produced a regional-normalized rainfall (NR) minimum in the urban center and directly downwind of the urban area (of up to −35%), with maximum values along its downwind lateral edges (of >15%). Strong UHIs (>1.25°C), however, induced or enhanced thunderstorm formation (again day and night), which produced an NR maximum in the most urbanized area of up to 75%.
