ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 5: 103–107 (2004) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/asl.67
Influence of topography on the Phoenix CO2 dome: a computational study Ge Wang* and Martin Ostoja-Starzewski Mechanical Engineering Department, McGill University, Montreal, Quebec, Canada
*Correspondence to: Ge Wang, Mechanical Engineering Department, McGill University, 817 Sherbrooke Street West, Montreal, Quebec, H3A 2K6, Canada. E-mail:
[email protected] Received: 3 September 2003 Revised: 19 December 2003 Accepted: 25 February 2004
Abstract Recent measurements reveal that atmospheric carbon dioxide (CO2 ) concentrations in the urban core of Phoenix, Arizona, are often 200 ppmv above the surrounding areas. This increase is up to two orders of magnitude higher than comparable values in other cities. In this investigation, we examine the role of topographically induced circulation as a contributor to the CO2 dome. A three-dimensional time-dependent regional model shows that frequently occurring katabatic winds generate a mesoscale circulation in Phoenix conducive to the accumulation of CO2 in the center of the city. We conclude that local topography and associated circulation regimes are important in understanding the high levels of CO2 in the Phoenix metropolitan area. Copyright 2004 Royal Meteorological Society Keywords:
mesoscale model; ozone study; nested grid; complex terrain; CO2 dome
1. Introduction Over the past several years, with the support of a grant from the Urban Research Initiative of the National Science Foundation, the atmospheric carbon dioxide (CO2 ) ‘dome’ over the growing Phoenix, Arizona, metropolitan area has been the focus of an intense multidisciplinary study. The combination of large emissions of CO2 throughout a city highly dependent on private passenger vehicles (over 100 million vehicle km per day), a relatively stable atmosphere in the desert climate, the valley setting of a city surrounded by mountains, and a relatively low vegetation cover produces atmospheric CO2 levels in the center of the city that routinely exceed 500 ppmv, as opposed to approximately 380 ppmv outside the city (Idso and Balling, 1998). A series of experiments is underway to determine the role of local meteorology and human activities on the shape and intensity of the urban CO2 dome in Phoenix. Measurements taken to date reveal that the CO2 levels are greatest during the winter morning periods when the atmosphere is most stable and Phoenix vehicular traffic is increased substantially by its many winter visitors (Idso and Balling, 1998). Spatial variations in the CO2 dome appear to be most strongly related to the level of urbanization. Vegetation appears to slightly modulate the spatial pattern, particularly in the high-sun months, by absorbing CO2 via photosynthesis during the day and releasing CO2 via respiration at night. While these experiments have been useful in defining temporal and spatial variations in the urban CO2 dome, they have not specifically addressed the role Copyright 2004 Royal Meteorological Society
of topographically induced circulation as a major contributor to the dome’s development. During unstable periods in the afternoon, particularly in the summer, strong vertical mixing in the atmosphere increases the dispersion of CO2 . Furthermore, the vertical transfer of momentum couples with the winds over the surface, thereby promoting stronger horizontal dispersion of CO2 concentrations. As a result, the largest urban CO2 values tend to occur in the early morning hours, in winter (Idso and Balling, 1998), during weekdays (Diem, 2000), and in periods without significant synoptic activity. Comrie and Diem (1999) drew similar conclusions from a study of the Phoenix area carbon monoxide levels. In this paper, we explore the role that topography plays in promoting the large CO2 dome in the Phoenix metropolitan area (Figure 1). Although Phoenix does have the PRISMS (Phoenix Real-time Instrumentation for Surface Meteorological Studies) system with high-resolution 5-minute wind measurements for 16 stations on the chosen case-study days, we believe that Idso and Balling’s (1998) statement is still applicable: ‘Determining the influence of meteorological factors on the nature of the urban CO2 dome is considerably more difficult than quantifying its presence.’ With this observation in mind, we elected to focus on the numerical modeling in this paper. Such a model capable of simulating circulation patterns over a relatively fine grid spacing in a nested coordinate system — that is, of 1 km, as was already applied successfully in the ozone study in the El Paso area (Brown et al. 2001) — would allow us to more accurately determine the role of topography in generating a convergent wind regime in the Phoenix area, which in turn gives rise to the large CO2 dome over the city.
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2. Model description The simulations for this investigation were carried out using the HOTMAC (Higher-Order Turbulence Model for Atmospheric Circulation) mesoscale numerical model. It is a three-dimensional time-dependent meteorological model utilizing 1 12 -order turbulence closure scheme (Yamada and Bunker, 1988). Using a hydrostatic approximation, a gradient diffusion closure scheme for the horizontal turbulence components, and a terrain following coordinate system, the governing equations for mass, momentum, heat and moisture are solved numerically using the alternating-direction implicit (ADI) finite difference scheme. This scheme permits relatively large time steps, thus reducing computational expense. A four-point diffusive smoothing scheme is used to remove spurious waves. A nudging scheme for a local wind correction scheme is also employed. HOTMAC includes solar and terrestrial radiation effects as well as drag and radiation effects for 14 different kinds of forest canopies (Figure 2). The lower boundary conditions are defined by surface energy balance and similarity theory. The soil heat flux is obtained by solving a five-level heat conduction equation in the soil, which ignores lateral heat transfer. The computations are initialized using the vertical temperature and relative humidity data taken from the National Weather Service (NWS) radiosonde launches in Tucson, Arizona, at 0500 LST and 1700 LST. The data are assimilated into the computation at every 12 hours, based on NWS radiosonde data. There exist only two NWS radiosonde data available for the Phoenix area with 12-hourly intervals. One is for Tucson and another for Flagstaff (see Figure 1). Comparing their distance and terrain characteristics to Phoenix, we chose Tucson. This also explains why we use a 12-hourly nudging. The flow computed by HOTMAC can be used to drive a Monte-Carlo dispersion and transport code: RAPTAD (RAndom Particle Transformation And Diffusion) (Williams and Yamada, 1990). That model combines the attributes of random walk and puff skills, Copyright 2004 Royal Meteorological Society
wherein pseudo-particles are transported with instantaneous velocities, which include the mean field and turbulent velocities. The location of each pseudo-particle represents the center of mass of a concentration distribution for each puff. The total concentration of any point is obtained by adding the concentration contribution of each puff at that point (kernel method). Each puff is assumed to have a Gaussian distribution where variances are determined by the time integration of the velocity variance encountered over the history of the puff. The variances are estimated based on the random force theory of turbulent diffusion (Lee and Stone, 1982). The HOTMAC-RAPTAD modeling system has many advantages for air pollution applications involving complex terrain over a multi-day period. The use of higher-order diffusion modeling permits the computation of horizontally inhomogeneous turbulence fields, typically found in complex terrain. In contrast to normal Gaussian Puff models, RAPTAD works well in situations where the wind direction changes with height, as in the case of the Phoenix airshed (see Figure 1).
3. Results In contradistinction to Idso and Balling (1998) who chose January 1998 as the study date, we selected October 8 1999 for our simulation. There are several reasons in support of that choice: 1. Diem and Brown (2003) said that urban effects should be greatest in the summer season, resulting from a decreased synoptic-scale forcing (eg, fronts) and less concealment of mesoscale progresses (eg, mountain–valley circulation). Using a Reanalysis Circulation Map (CRM) site http://www.cdc.noaa. gov/cdc/reanalysis/reanalysis.shtml, we can also show that large-scale synoptic forcing was minimal on the chosen day. Overall, choosing a period of time not within the monsoon months (from July to September of every year) contributes to a better understanding of the Phoenix region. Atmos. Sci. Let. 5: 103–107 (2004)
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2. As Idso and Balling (1998) observed, the CO2 dome and its high concentration evaluation exist in the winter season in the Phoenix area. In this paper we are most interested to see if this phenomenon indeed exists in the non-monsoon summertime season in the Phoenix area from the viewpoint of the influence of topography. For the study of the plume — knowing that the urbanization levels, traffic and vegetation contribute to CO2 emissions (Idso and Balling, 1998) — we set up eight pseudo-release locations (see Figure 2a) in the Phoenix metropolitan area to represent the actual contaminant release as contributing factors to the CO2 emissions. As seen in Figures 3(a) and 4(a), the HOTMAC model predicts how the down-slope flows are drained Copyright 2004 Royal Meteorological Society
into the basin at early morning and how they flow up-slope during the daytime heating. In the absence of synoptic forcing, this pattern continues day after day (Davis and Gay, 1993; Ellis et al., 1999, 2000), thus sloshing the contaminants released in the airshed upand down-slope directions, causing the accumulation of these contaminants in the airshed. As pointed out by Hunt et al. (2000), a part of the air mass will be transported over the mountains out of the airshed every day, but this removal is quite inefficient due to the flow reversal every night. This local thermal circulation, which is highly dependent on the topography of the Phoenix area, is conducive for the build-up of gases like CO2 in the airshed. Indeed, results of the RAPTAD routine in Figures 3(b) and 4(b) clearly show that pseudo-particles emitted Atmos. Sci. Let. 5: 103–107 (2004)
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throughout the metropolitan area become entrapped in the airshed in the early morning, and increase in concentration over the central portion of the city, and this ‘dome’ disappears in the daytime via the removal of the airshed out of the basin. If it were not for the lack of exact experimental emission data of CO2 at those sites, using HOTMAC/RAPTAD we could give precise concentration evaluation predictions.
4. Closure The concentration of CO2 in the urban atmosphere likely reflects a combination of human activities, vegetation patterns, and local meteorology. The high levels of CO2 found in the central core of Phoenix are certainly related to the vehicular traffic throughout the city and to the relatively sparse vegetation in the desert setting. However, as seen in this study, the CO2 Copyright 2004 Royal Meteorological Society
levels are further elevated in the city due to the local topography and associated airflow. Results from three-dimensional time-dependent meteorological and particle dispersion models, HOTMAC/RAPTAD, show that frequently occurring katabatic winds generate a mesoscale circulation in Phoenix, conducive to the accumulation of CO2 in the center of the city. The mountains that surround most of Phoenix, coupled with frequent inversions, provide a setting for the development of katabatic winds that ultimately act to increase the concentration of anthropogenerated gases such as CO2 . We conclude that local topography and associated circulation regimes are important in understanding the high level of CO2 in the Phoenix metropolitan area.
Acknowledgements We thank Andrew W. Ellis (Arizona State University) and an anonymous referee for comments which helped improve this Atmos. Sci. Let. 5: 103–107 (2004)
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Figure 4. (a) Spatial velocity distribution, and (b) plume trajectory as of 0900 LST, October 8 1999 paper. Work of the first author was supported by the Southwest Center for Environmental Research and Policy (SCERP) and the National Science Foundation (Fluid Mechanics and Atmospheric Sciences Program). The second author acknowledges the support by the Canada Research Chairs program.
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