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Robert D. Bornstein

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.

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Robert D. Bornstein

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.

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Robert D. Bornstein
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Tim Loose and Robert D. Bornstein

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.

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Tim Morgan and Robert D. Bornstein

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.

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Robert D. Bornstein and Alan D. Robock

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.

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Robert D. Bornstein and William T. Thompson

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.

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workshop summary

Workshop on Modeling the Urban Boundary Layer, Las Vegas, Nevada, 5 May 1975

Robert L. Lee, Robert W. Bergstrom, and Robert D. Bornstein
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Julie Pullen, Teddy Holt, Alan F. Blumberg, and Robert D. Bornstein

Abstract

Multiply nested urbanized mesoscale model [Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS)] simulations of the New York–New Jersey metropolitan region are conducted for 4–11 July 2004. The simulations differ only in their specification of sea surface temperatures (SSTs) on nest 4 (1.33-km resolution) and nest 5 (0.44-km resolution). The “control SST” simulation (CONTROL-SST) uses an analyzed SST product, whereas the “New York Harbor Observing and Prediction System (NYHOPS) SST” simulation (NYHOPS-SST) uses hourly SSTs from the NYHOPS model hindcast. Upwelling-favorable (southerly) winds preceding the simulation time period and continuing for much of the first 5 days of the simulation generate cold water adjacent to the New Jersey coast and a cold eddy immediately outside of the harbor in the New York Bight. Both features are prominent in NYHOPS-SST but are not pronounced in CONTROL-SST. The upwelled water has a discernible influence on the overlying atmosphere by cooling near-surface air temperatures by approximately 1°–2°C, slowing the near-surface winds by 15%–20%, and reducing the nocturnal urban heat island effect by up to 1.3°C. At two coastal land-based sites and one overwater station, the wind speed mean bias is systematically reduced in NYHOPS-SST. During a wind shift to northwesterly on day 6 (9 July 2004) the comparatively cooler NYHOPS-SSTs impact the atmosphere over an even broader offshore area than was affected in the mean during the previous 5 days. Hence, air temperature evolution measured at the overwater site is better reproduced in NYHOPS-SST. Interaction of the offshore flow with the cool SSTs in NYHOPS-SST induces internal boundary layer (IBL) formation, sustained and deepened by turbulent kinetic energy advected from adjacent land areas; IBL formation did not occur in CONTROL-SST.

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Ning An, Jingjing Dou, Jorge E. González-Cruz, Robert D. Bornstein, Shiguang Miao, and Lin Li

Abstract

The focus of this study is an intense heat episode that occurred on 9–13 July 2017 in Beijing, China, that resulted in severe impacts on natural and human variables, including record-setting daily electricity consumption levels. This event was observed and analyzed with a suite of local and mesoscale instruments, including a high-density automated weather station network, soil moisture sensors, and ground-based vertical instruments (e.g., a wind profiler, a ceilometer, and three radiometers) situated in and around the city, as well as electric power consumption data and analysis data from the U.S. National Centers for Environmental Prediction. The results show that the heat wave originated from dry adiabatic warming induced by the dynamic downslope and synoptic subsidence. The conditions were aggravated by the increased air humidity during subsequent days, which resulted in historically high records of the heat index (i.e., an index representing the apparent temperature that incorporates both air temperature and moisture). The increased thermal energy and decreased boundary layer height resulted in a highly energized urban boundary layer. The differences between urban and rural thermal conditions throughout almost the entire boundary layer were enhanced during the heat wave, and the canopy-layer urban heat island intensity (UHII) reached up to 8°C at a central urban station at 2300 local standard time 10 July. A double-peak pattern in the diurnal cycle of UHIIs occurred during the heat wave and differed from the single-peak pattern of the decadal average UHII cycles. Different spatial distributions of UHII values occurred during the day and night.

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