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35 years of urban climate research at the 'Vancouver-Sunset' flux tower Christen, Andreas; Oke, Tim; Grimmond, Sue; Steyn, Douw; Roth, Matthias Mar 31, 2013

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35 years of urban climate research at the ‘Vancouver-Sunset’ flux towerby Andreas Christen (1)with contributions from Tim Oke (1), Sue Grimmond (2), Douw Steyn (3) and Matthias Roth (4)(1) University of British Columbia, Department of Geography / Atmospheric Science Program, Vancouver, Canada(2) King’s College London, Environmental Monitoring and Modelling Group, Department of Geography, London, United Kingdom (3) University of British Columbia, Department of Earth and Ocean Sciences / Atmospheric Science Program, Vancouver, Canada(4) Department of Geography, National University of Singapore, SingaporeThe ‘Sunset’ neigborhood in Vancouver, BC, may be the most extensively studied area in urban climatology. Since 1977, about 50 papers have been published that focus on flux measurements, land-atmosphere exchange, and models developed using data gathered on and around the ‘Vancouver-Sunset’ micrometeorological tower. This newsletter contribution aims to provide a brief history of this unique flux tower, show how selected developments in urban climatology were linked to work at this site, and highlight some of the ongoing research on urban trace-gas flux measurements to validate fine-scale emission models.Establishing an urban micrometeorological tower (1977)One of the biggest conceptual challenges in urban climatology is the spatial heterogeneity of cities. From single lots (roofs, walls, streets, lawns) to the land-cover patchiness in an urban neighborhood (parks, low-density, high-density areas) up to the differences in surface properties between cities and the surrounding area (urban heat island, country breezes etc.) - urban systems are dominated by heterogeneity on many scales, and advection is the norm rather than the exception. From the start it was clear that micrometeorological approaches and theories developed for flat and homogeneous sites would be unlikely to apply a priori to the study of urban land-atmosphere interactions. Moreover, it is challenging to separate urban effects from other effects This article was published in slightly different form in the Fluxnet Newsletter at http://fluxnet.ornl.gov(local winds, land-cover differences, topography, synoptic effects) unless proper experimental control (Lowry, 1977) has been established. In the 1960s and 1970s the mostly unpublished trials to measure turbulent fluxes above urban surfaces were flawed because micrometeorological methods were applied with instruments situated generally too close (low) to the elements that constitute the urban surface e.g. on rooftops or on small masts within the layer that is now recognized as the roughness sublayer (Raupach and Thom, 1981). In the 1970s, there was growing evidence from pioneering work such as the turbulence studies using eddy covariance (EC) instruments mounted on masts during METROMEX in St. Louis, USA (Clarke et al., 1978) and in both Vancouver (Burnaby) and Uppsala by Oke (1978) that micrometeorological methods might work as long as the measurement height was well above the height of the buildings. In other words, in the inertial sublayer above extensive urban surfaces where individual flow-distorting effects of buildings are blended.Can fluxes be measured successfully using traditional micrometeorological approaches, if we are high enough above the surface? How high is high enough? What are site requirements that would allow this? What would be an appropriate site in the complex setting of a city like Vancouver? Those Questions were part of a graduate research seminar led by then Associate Professor Tim Oke at the University of British Columbia, in the mid-1970s. As an exercise, students were involved in the identification of a ‘homogeneous’ site in Vancouver at which it would be possible to make successful micrometeorological measurements. The criteria developed were: (a) no major changes in building and tree height and density around the site, (b) a relatively flat area, and (c) no significant land-cover boundaries (i.e. a sufficient fetch). One of the sites identified by his students, using maps and air photos was the ‘Sunset’ residential neighborhood. Follow-up work suggested it might 35 years of urban climate research at the ‘Vancouver-Sunset’ flux tower Page 2Figure 1 - Then graduate student Douw Steyn (left) and research assistant Brian Guy (right) celebrating having finally completed installation of the complete set of mast mounted instrumentation in July 1978. The set-up included a uvw propellor anemometer (top), a wind profile with cup-anemometers, a yaw-sphere and a Bowen-Ratio system (bottom right), all approaches adopted from work in agricultural and forest climatology.be possible to erect a tower in this area, inside a power substation of BC Hydro, where security was good. Subsequently, after gaining the approval of the city planning authorities and the local residents and with funding from the Natural Science and Engineering Research Council of Canada (NSERC) the ‘Vancouver Sunset’ tower was established in 1977 (123.0784ºW, 49.2261ºN). It was one of the first, and now the longest-running, micrometeorological tower in Canada, in an urban setting. With a height of 30 m this slim lattice structure allows relatively undisturbed measurements to be made at 4 to 5 times the gable height of the surrounding buildings (6-7m). But the perceived simplicity, was not as straight-forward from a practical perspective. It required about ten years to establish confidence in flux measurement at Vancouver –Sunset. During that period a number of the key approaches used today to conduct urban flux measurements were established by research completed at the tower. Pioneering research on the urban energy and water balances (1977 - 1987)In the late 1970s, there was essentially no experimental evidence concerning the magnitude of, and controls on, the energy and water balances in cities. Urban weather and hydrology models assumed that cities are largely impervious and extremely dry. During the first decade, retrieving energy and water balances and the diurnal and seasonal changes in their component fluxes was at the center of the research program at Vancouver-Sunset. How do we measure integrated turbulent fluxes of sensible and latent heat from an urban surface? What are the proper methods to do that? The first experiments applied instrument systems used over agricultural and forest stands, including the Bowen ratio energy balance (BREB) approach (Kalanda et al., 1980) and aerodynamic methods using gradients of air temperature, humidity and wind and the net all-wave radiation (Figure 1). Oke and McCaughey (1983) compared the summertime energy balance fluxes measured in 1980 using the BREB approach at Vancouver-Sunset, to a simultaneously operated system over a grassland site outside the built-up area of the city. The surprising results during the fairly dry summer season showed that the evapotranspiration was about 40% higher over the city (where lawns were irrigated) than the rural values, or at least about equal during cloudy days. Unfortunately, despite using high-quality instruments, calibrations, tests and the installation of multiple reversible Bowen ratio profile systems, the gradients recorded were often extremely small, and at or below the resolution of the instruments. The first studies to apply EC included measurements with a yaw-sphere thermometer system. The system measured turbulent sensible heat fluxes from wind and temperature fluctuations with a pressure sphere anemometer and a fine-wire resistance thermometer, respectively. It was built at UBC in the laboratory of Dr. Andy Black in Soil Science, and tested over an extensive grass site (Yap et al., 1974; Yap and Oke, 1974). It was first used at ‘Vancouver-Sunset’ in July-August, 1978 in the study of Steyn (1980). Steyn’s work at the tower also included the first analysis of surface roughness using morphometric analysis and turbulence spectra for the wind components. The yaw-sphere-thermometer system was superseeded by a commercial Campbell Scientific CA27 1D ultrasonic-anemometer and thermometer system in summer 1983 (Cleugh This article was published in slightly different form in the Fluxnet Newsletter at http://fluxnet.ornl.govand Oke, 1986) (Figure 2). Net all-wave radiation was measured directly, while storage was estimated and the latent heat flux was solved as a residual. Figure 3 shows the diurnal course of the energy balance terms in summer 1983 compared to simultaneously measured values at a rural grassland site within 7 km distance.Most early energy and water balance measurements were conducted in the dry summer season. The first work in other seasons was carried out in winter to spring 1987, and showed that latent heat flux was the most important turbulent flux of the energy balance in the wet wintertime (Grimmond, 1988, 1992). Winter and summer data from the tower were also used to develop an urban evapotranspiration-interception model based on the urban energy balance that considered storage heat flux, aerodynamic and surface resistances and drainage (Grimmond, 1988; Grimmond and Oke, 1991).Unlike the case of agricultural or forested surfaces, where the use of heat flux plates and temperature profiles was established, the complexity of the construction materials and the structure of an urban surface inhibited direct measurement of the storage heat flux density. Alternatively, different parameterizations were developed. Initially Oke et al. (1981) developed a model with a linear relationship between net radiation and storage. It was an area-weighted average of the equivalent relations of several urban surfaces (grass, concrete, tarmac, etc.). To account for the phase lag that exists in the diurnal course of each surface relation, Oke and Cleugh (1987) developed and Grimmond et al. (1991) applied the Objective Hysteresis Model (OHM) to the urban area. This simple approach has proved to be very practical to apply in connection with field observations, 35 years of urban climate research at the ‘Vancouver-Sunset’ flux tower Page 4Figure 2 - The flux instrumentation on top of the tower in 1986 looking towards south. Two net all-wave radiometers (Swissteco, left), a Krypton hygrometer (Campbell Scientific KH20, to the right of the radiometers) and 1-D ultrasonic anemometer (Campbell Scientific CA27T, right) with a fine-wire thermocouple were used to measure all terms of the urban energy balance except for storage which was quantified as a residual. it agrees well with other methods that often require much greater input (Roberts et al., 2006) and has been incorporated into simple urban energy balance models.The last term of the urban energy balance, the anthropogenic heat flux, i.e. the heat released by human activities including combustion, electricity, human metabolism is inaccessible to direct measurement. But a detailed inventory of sources around the tower site was compiled by Grimmond (1988). Her results suggested values of about 8 W m-2 on the annual average.By the end of the 1980s the methodology to measure turbulent fluxes was established, which greatly advanced capabilities to quantify, study and model the energy and water balance of the urban surface. The studies and subsequent models developed at 'Vancouver Sunset' demonstrated that urban areas are not comparable to the dry 'urban desert' postulated in early writing and models and that urban vegetation and irrigation can play a crucial role in the energy partitioning (Oke, 1989). Turbulence and source areas (1985 - 1995)The successful measurement of turbulent fluxes inspired discussions on whether the turbulent exchange of heat and mass above the urban surface can be described using standard surface layer scaling, specifically the Monin-Obukhov similarity theory (MOST). Previous work on turbulence and dispersion over extremely rough urban surfaces had focused on the transfer of momentum, but Roth et al. (1989) presented the first detailed analysis of the turbulence transfer (integral statistics and cospectra) that included fluctuations of temperature and water vapor. Roth and Oke (1993) and Roth (1993) gathered extensive measurements of velocity and scalar fluctuations, using a 10Hz - 3D sonic anemometer linked with thermocouples and fast hygrometers on the tower (Figure 4). They demonstrated that many aspects of the turbulent transfer can be successfully analyzed within the Monin-Obukhov scaling framework in the urban inertial sublayer. They also identified a number of fundamental differences from theory including increased efficiency of the turbulent transfer of momentum and sensible heat. Another This article was published in slightly different form in the Fluxnet Newsletter at http://fluxnet.ornl.govFigure 3 - Simultaneously measured averages of energy balance terms measured / estimated at the urban flux tower ‘Vancouver Sunset’ compared to ‘Vancouver-Airport’ which served as a rural reference. Shown is a 30-day ensemble average (from Oke 1987 based on Cleugh and Oke, 1986; reproduced with permission).interesting finding was that the dissipation of turbulent kinetic energy was smaller than predicted. A possible explanation was that transport processes of TKE were more relevant so close to the roughness elements. Finally, the source patterns for water vapor were patchy compared to the more uniform distribution of sensible heat sources and this caused the latent heat fluxes to disagree with MOST predictions (Roth, 1993).The biggest remaining challenge in the interpretation of the flux measurements at ‘Vancouver-Sunset’ was the question of whether the results reflect a representative sample of the urban surface. To study the spatial heterogeneity of the energy balance terms, a temporary mobile tower roved to 5 different locations around the fixed ‘Vancouver Sunset’ tower in order sample the radiative and turbulent fluxes at different locations (Schmid et al., 1990). Some sites were near parks, while others were at mid-block. At 4-5 times the height of the buildings, net all-wave radiation proved to be relatively uniform and compared well with the tower signal (1-5%) whereas the sensible heat fluxes within the neighborhood varied by 25 - 40%.Pioneering work on turbulent source areas was conducted at the Vancouver Sunset site (Schmid et al.,1988, Schmid and Oke,1990) based on the ideas of Pasquill (1972) and Gash (1986). Interest centered on the extent to which local measurements of a turbulent flux are representative of the surrounding area. The very patchy suburban surface around the site made it necessary to find if observations provided a sample that could be considered spatially representative. The source area model SAM (and later FLUXSAM) was developed using a probability density function plume diffusion relation. It made it possible to map the extent of the ellipsoid source area (sometimes called a 35 years of urban climate research at the ‘Vancouver-Sunset’ flux tower Page 6Figure 4 - Instrumentation at 27.5m on the 'Vancouver-Sunset' tower in 1989 used by then graduate student Matthias Roth to study turbulence spectra, scalar and momentum fluxes and dissimilarity of turbulent exchange mechanisms. Left is a EC system composed of a 1-D sonic anemometer (CSI CA27T) and Krypton hydrometer (CSI KH20), in the center is a 3-D sonic anemometer (Kaijo Denki DAT-310 TR-61C) was used to retrieve all 3-D turbulence statistics, and at the right is another 1-D sonic anemometer (CSI CA27T) combined with a Lyman-alpha hygrometer (ERC BLR) (Photo: M. Roth)‘footprint’) of a flux measurement made at a given height (see Figure 5). The technique has become a standard protocol to assess the representativeness of turbulent flux observations over all types of surface. It further allowed a source attribution of the signal. For example, Grimmond (1988) used source area estimates and overlaid them with a GIS of the nature of the tower surroundings to match the results given by a storage and an anthropogenic heat model.Monitoring long-term energy and water balances (since 2000)Recent advances in instrumentation and data processing enabled the collection of long-term datasets to study the seasonal variability of the energy and water balance partitioning at ‘Vancouver Sunset’. From 2001-2002, and again from 2008-2011 long-term flux measurements of the energy and water balance terms were conducted. A specific technical goal of the latter period was to support modeling efforts to incorporate the effect of vegetation into the Canadian Urban Flow and Dispersion Model (CUDM). The tower also contributed to an observational dataset to evaluate long-term hydrology models (e.g. Järvi et al., 2011). Additionally to the flux tower a detailed hydrological / bioclimatological set of measurements was collected including metering of water used for lawn irrigation, 8 homes and gardens wired up with soil sensors and leaf-level conductance measurements on urban trees. To provide comparisons with the urban site, a rural reference site was established. Table 1 shows a comparison of the energy balance terms over the full annual cycle between the urban and rural site. Notable are the seasonal differences in the partitioning of turbulent fluxes. An interesting feature is This article was published in slightly different form in the Fluxnet Newsletter at http://fluxnet.ornl.govFigure 5 - 0.5 level isopleths of the turbulent source areas at ‘Vancouver-Sunset’ for August 19, 1986 calculated by SAM (from Schmid, 1990).the ‘over closure’ of the annual energy balance at the urban site - the residual term at indicates a missing source of about 15 W m-2 (because storage should approximately vanish over an entire year). This missing source matches relatively well more recent estimates of the annual average anthropogenic heat flux density of 13 W m-2 in the source area (Van der Laan et al., 2010).Measuring and modelling greenhouse gas exchange (since 1993)With cities being the land-surface ecosystem that emits the largest fraction of greenhouse gases (GHG), attention has also been given to the measurement of GHG emissions into the urban atmosphere, with the goal to estimate local or regional emissions and what controls them. The first concentration measurements of CO2 at Vancouver-Sunset were made as early as June 1993, using a differential infrared gas analyzer (Licor 6262) with inlets at two heights on the tower. Reid and Steyn (1997) established that although emissions are highest during the day, the substantial depth of the mixed layer and the regional boundary layer dynamics caused lowest concentrations in the afternoon, and elevated concentrations (up to 80 ppmv) during the night. The observed CO2 concentrations nicely matched a simple local 2-D boundary layer model and allowed inference of an approximate source strength profile for the area upwind of the tower.35 years of urban climate research at the ‘Vancouver-Sunset’ flux tower Page 8Table 1 - Daily totals of the energy balance terms measured over an unmanaged grassland site South of Vancouver (“rural”, Westham Island) and simultaneously on the urban flux tower Vancouver-Sunset (“urban”) - β is the Bowen-ratio. Data from October 2008 to September 2009. Both sites were equipped with a similar EC system (CSI CSAT3 and Li 7500) and with similar 4-component net radiometers (Kipp & Zonen CNR-1) (Data from Christen et al. 2010).Net all-wave radiationQ*Sensible heat fluxQHLatent heat fluxQEResidualΔQS - QFRural Urban Rural Urban Rural Urban Rural UrbanSpring(MJ m-2 day-1)9.10 8.94 2.64 6.07 5.16β = 0.53.04β = 2.01.30 -0.17Summer(MJ m-2 day-1)13.44 12.53 4.40 9.56 8.39β = 0.53.31β = 2.90.65 -0.34Fall(MJ m-2 day-1)3.75 3.38 0.77 2.51 3.18β = 0.22.06β = 2.0-0.20 -1.19Winter(MJ m-2 day-1)0.28 -0.01 -0.46 1.01 1.33β = -0.31.10β = 0.9-0.59 -2.12Yearly total(GJ m-2 y-1)2.41 2.15 0.67 1.75 1.65 0.87 0.09 -0.47The first full year dataset of directly measured CO2 fluxes on the ‘Vancouver-Sunset’ tower was gathered in 2001-2002 with a closed path CO2/H2O analyzer (Li-6262) and a 3D sonic anemometer (Gill Instruments, R2A) (Walsh, 2006). It became obvious that what was previously considered a relatively homogeneous urban surface for the turbulent fluxes of sensible and latent heat, was actually a highly patchy and irregular arrangement of emission sources of CO2 which inhibited a straight-forward ‘ecosystem-level’ quantification of the exchange. Two arterial roads cross at a busy intersection only  180 m from the base of the tower. About 1400 detached homes are located within the 80% long-term source area (Figure 6), of which about 93% are heated by natural gas. Additionally areal and diffuse emissions originate from highly managed urban green This article was published in slightly different form in the Fluxnet Newsletter at http://fluxnet.ornl.govFigure 6 - Surface characterization of the Sunset neighborhood in a 2 x 2 km area derived from high-resolution LiDAR data. LiDAR can be used to characterize the built form (e.g volume, height and type of houses) as well as the characteristics of the urban vegetation (e.g. tree heights, LAI). Overlaid is the long-term cumulative turbulent source area climatology and the two major arterial roads E 49th Ave (21’000 veh. day-1) and Knight St. (48’000 veh. day-1). The long-term 80% source area covers an area of about 1 km2 and 1400 houses (LiDAR data from Goodwin et al., 2009 and source area climatology from Christen et al., 2011).space (respiration) and human metabolism. Trees (17.1 stems / ha) and lawns with an overall urban leaf area index of 1.8 m2 m-2 uptake a minor fraction of the CO2. Walsh (2005) presented her dataset as ensemble fluxes coming from eight different wind sectors, of which the two sectors containing the intersection often measured fluxes greater than 60 µmol m-2 s-1. The highest ensemble annual emissions were estimated to be 10.0 kg C m-2 y-1 (from the SSE sector) and the lowest were 2.0 kg C m-2 y-1 (from sectors, with no arterial roads, in the WNW area).Direct measurement of CO2 fluxes were re-established from 2008 to 2012, using a 3D ultrasonic anemometer (CSI-CSAT3) and an open path analyser (Li-7500). Continuous measurements since 2008 show a similar spatial behavior with little inter annual variability. Consistently, highest annual ensemble fluxes are found from the SE sector with the intersection (13.0±0.3 kg C m-2 y-1) and lowest from the NW sector that contains no arterial roads (3.0±0.3 kg C m-2 y-1). The two other sectors contain one arterial road segment each, at about the same distance from the tower. The NE sector with a road segment that has 48,000 veh. day-1 shows higher fluxes (6.7±0.1 kg C m-2 y-1) than the SW sector with a segment of 21,000 veh. day-1 (4.4±0.2 kg C m-2 y-1). The diurnal course of the fluxes is in-line with traffic counts, and there is on average a 25% reduction observed on weekends compared to weekdays. If the monthly ensemble averaged fluxes are shown against heating degree days (sum of temperatures below a 18ºC threshold, when heating systems are usually turned on), a clear relation can be discerned (Figure 7, see also Christen et al. 2011). 35 years of urban climate research at the ‘Vancouver-Sunset’ flux tower Page 10Figure 7 - Monthly CO2 fluxes measured at ‘Vancouver-Sunset’ (ensemble average from all four wind direction sectors) as a function of monthly heating degree days for the 4-year period May 2008 to April 2012.The spatial heterogeneity of CO2 sources is a challenge to the usefulness of these direct flux measurements and calls for detailed modeling of the processes contributing to emissions and uptake in the turbulent source area of the tower. A highly detailed LiDAR scan, flown in 2007 over the source area of the tower (Figure 6), has become an important data-source to model GHG emissions at fine scales. The LiDAR scan describes the detailed characteristics of urban building structure and vegetation characteristics (Goodwin et al., 2009). The data were used to inform building energy models and classical ecological models in order to quantify in detail the carbon cycle for a ~4 km2 area surrounding the tower (Figure 8, based on Christen et al. 2010). Numbers in Figure 8 denote carbon fluxes in kg C m-2 y-1 (arrows) and carbon pools in kg m-2 (boxes). Their corresponding size is proportional to the magnitude of the flux or pool. Fluxes carry carbon in different forms, such as carbohydrates in fuels, or CO2 between the surface and atmosphere. Fluxes entering the system in Figure 8 from the left are lateral fluxes of imported carbon supplying the neighborhood with fuels, food and materials. Lateral fluxes leaving the system on the right side are exports of carbon in the form of various waste products. Vertical fluxes are shown at the top, and are essentially the CO2 fluxes measured on the tower, they are a combination of emissions from buildings, traffic, human respiration and the net ecosystem exchange of urban vegetation and soils. The model predicts that 6.7 kg C m-2 y-1 is imported into the system by lateral fluxes, and about 90% of this carbon eventually leaves the system in the form of CO2 to the atmosphere (5.9 kg C m-2 y-1). Only a small part leaves the system as lateral output in the form of waste (0.8 kg C m-2 y-1). Readers familiar with similar carbon budgets for forests will recognize that pathways of carbon in an urban ecosystem are linear rather than cyclic and that the 'urban ecosystem' is characterized by a substantial throughput of carbon; most of the imported carbon are fossil fuels (>80%). The local storage of carbon is small. Vegetation and soils are moderate pools and most carbon is stored in buildings – this includes carbon in wooden buildings and furniture. Nevertheless, compared to the fluxes, pools in an urban ecosystem are relatively small. This carbon cycle model can be also defined in a spatial context because the location of roads (traffic counts) and buildings (heating systems) is well known in an urban area. Christen et al. (2011) used a gridded version of this model at a fine resolution of 50m and overlaid turbulent source areas of the EC system on the tower. In the annual total the modelled fluxes weighed by the turbulent source area (7.42 kg C m-2 y-1 ) matched reasonably well the measured flux of 6.71 kg C m-2 y-1 determined by means of EC on the tower (given the uncertainty of source area models). This opens up opportunities for flux towers to verify fine-scale pollutant and GHG emission inventories.Most recently, the approach has been extended to direct EC measurements of Methane (CH4) using a open-path CH4 analyzer (Li7700, Figure 9). Between Feb 2012 and April 2012 an average (net) emission of 17 nmol m-2 s-1 (23.7 mg CH4 m-2 day-1) was recorded. This flux density matches larger-scale estimates for cities and is comparable to emissions from wetlands. A higher CH4 flux density is measured during the daytime (maximum ~24 nmol m-2 s-1), a lower flux density is recorded in the late night (minimum ~9 nmol m-2 s-1). Most of the CH4 emitted in an urban area can be attributed to incomplete combustion (traffic, space heating / natural gas) and possibly some fugitive This article was published in slightly different form in the Fluxnet Newsletter at http://fluxnet.ornl.gov35 years of urban climate research at the ‘Vancouver-Sunset’ flux tower Page 12Figure 8 - Modeled urban carbon cycle for the Sunset neighborhood. See text for details (modified from Christen et al. 2010).emissions of the natural gas network that supplies homes. Uncertain to-date is the contribution of biogenic processes.ConclusionsUrban flux towers such as ‘Vancouver-Sunset’ can be a platform to not only quantify the exchange processes over a specific urban area, but more importantly are places to develop, translate and adapt experimental approaches to complex and extremely rough surfaces. The recent advances in the numerical modeling of the urban environment (building energy modeling, GHG emission modeling, dispersion and weather forecasting at increasingly finer scales) require detailed information on flow, turbulence and land-atmosphere interactions in cities. The knowledge and certainty that we can apply theories or use selected parameterizations and simplifications originate from experimental field research on micrometeorological towers in the urban atmosphere. Urban measurements are also needed to test and verify models. As such, results obtained at ‘Vancouver-Sunset’ are not only of local interest, but have more generally This article was published in slightly different form in the Fluxnet Newsletter at http://fluxnet.ornl.govFigure 9 - The current flux system on top of the tower (April 2012) looking towards the north-west, with downtown Vancouver and the North Shore Mountains in the background. A 3-D ultrasonic anemometer-thermometer (Campbell Scientific CSAT-3), an open path CO2/H2O IRGA (Li7500, center) and a four-component net radiometer (Kipp & Zonen, CNR-1, not shown) are operated year-round for more than 4 years, supplemented with additional short-term instruments such as an open path CH4-analyzer (Li-7700, right). Although it is from a different view direction to that of Figure 2, one can note an overall visible growth of trees in the 26 year period between them.advanced the modeling of weather, climate, hydrology and emissions in the urban environment.AcknowledgementsThe unique measurements at Vancouver-Sunset would not have been possible without the continuous financial support of the Natural Science and Engineering Research Council of Canada (NSERC) continuously since 1977, and more recently the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS) and the Canadian Foundation for Innovation / BCKDF. BC Hydro has provided the University of British Columbia with a long-term permission and commitment that have allowed us to operate this unique research facility.ReferencesClarke, J.F., Ching J.K.S., Binkowski F.S., Godowitch J.M., 1978: “Turbulent structure of the urban surface boundary layer”, Ninth NATO/CCMS International Technical Meeting on Air Pollution Modeling and its Application, Toronto, ON.Christen, A., Coops N., Kellett R., Crawford B., Heyman E., Olchovski I., Tooke R., van der Laan M., 2010: “A LiDAR-Based Urban Metabolism Approach to Neighbourhood Scale Energy and Carbon Emissions Modelling”. University of British Columbia, 2010 Technical report for Natural Resources Canada, 104pp.Christen A., Crawford B., Grimmond S., Voogt J., Oke T. (2010): “Year-long urban-rural differences of the surface radiation and energy balance observed in and near Vancouver, BC”, 44th Annual CMOS Congress / 36th Annual Scientific Meeting of CGU. 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