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Final Report: The Energy Balance of Urban Microclimates
EPA Grant Number: CR830890C006Subproject: this is subproject number R830890C006 , established and managed by the Center Director under grant CR830890
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: Center for Environmental Science
Center Director: Frederick, John
Title: The Energy Balance of Urban Microclimates
Investigators: Barzyk, Timothy M , Frederick, John
Institution: University of Chicago
EPA Project Officer: Winner, Darrell
Project Period: July 1, 2003 through June 30, 2004 (Extended to June 30, 2006)
RFA: Targeted Research Center (2004)
Research Category: Hazardous Waste/Remediation , Targeted Research
Description:
Objective:Urban microclimates are discrete areas where, as a consequence of urban development, surface temperatures and energy fluxes between the ground and atmosphere may differ from those in nearby regions. Construction materials and other urban landscape features can alter surface radiation components, regional weather and atmospheric flows. The consequences are air and surface temperatures in urban environments that may differ from those in less-developed settings characterized by extensive soil and vegetative cover.
The objective of this project is to document the effects of urban development on the fluxes of energy that act to heat and cool urban surfaces, together with the consequent effects on surface temperature. This is accomplished by combining a set of radiative and meteorological measurements with a quantitative model of the energy balance of the ground. We here describe the measurement sites and instruments, followed by a summary of an energy balance model for application to artificial surfaces. Energy fluxes from solar and terrestrial radiation, sensible heat transport, heat conduction and evaporative cooling are deduced for urban sites with varying characteristics. Comparison of results deduced for different locations illustrates the range of conditions that prevail in various urban microclimates.
Summary/Accomplishments (Outputs/Outcomes):Two net radiometers and weather stations collected concurrent measurements from the roofs of two urban locations located approximately 10 km apart. First, the “urban canyon site” is the 12-story rooftop of Chicago City Hall, which is surrounded by taller structures, most over 30 stories. This rooftop is approximately one-third the height of the urban canopy layer around it. The roof here is a white waterproof PVC-polymer with a high albedo (0.36). Next, tall structures do not surround the five-story “urban control site”, located on the University of Chicago campus, approximately 10 km south of the urban canyon. This roof is black pitch covered with gray pebbles (albedo of 0.23). The surrounding area is characterized by low-rise structures of similar height and composition, and more grassy areas exist here than near the canyon site.
The research uses the Kipp and Zonen CNR-1 net radiometer, which has upward- and downward-facing sensors that measure energy received from an entire hemisphere. The measurements encompass separate solar (0.3-3 μm) and terrestrial (5-50 μm) radiant energy fluxes. The ratio of reflected to incoming solar energy defines the surface albedo. The Davis Vantage Pro2 weather station includes temperature and humidity sensors, an anemometer (wind speed) and a solar radiation sensor. The temperature and humidity sensors have accuracies of ±0.5°C and ±3 percent, respectively. A fan-aspirated radiation shield reduces effects of solar radiation and rooftop heat on the air temperature readings. For quality assurance, the radiometers and weather stations were calibrated by running them side-by-side at the urban control site for one week. When simultaneous measurements were plotted against each other, the resulting slopes were very close to 1.0, indicating the compatibility of different sensors.
An energy balance model provides a framework for interpreting the measurements. Six processes lead to energy exchanges between an underlying surface and the atmosphere. These are described by fluxes, denoted by Q expressed in watts per square meter. These arise from: (1) net absorption of solar energy at the surface, being the difference between downward solar radiation and upward reflected radiation [QSOL(dn) - QSOL(up)]; (2) absorption of downward thermal longwave energy at the surface [QLW(dn)]; (3) emission of thermal longwave energy upward at the surface [QLW(up)]; (4) exchange of sensible heat between surface and atmosphere [QSENS]; (5) evaporative cooling, provided that liquid water is present [QEVAP]; and (6) conduction of heat into or out of the underlying surface [QCONDUC]. Conservation of energy requires:
0 = QLW(up) - QLW(dn) + QSOL(up) - QSOL(dn) + QSENS + QEVAP - QCONDUC (1)
The evaluation of terms in Equation 1 utilizes radiation data, surface and air temperatures and wind speed measured at 10-minute intervals. Measurements of QLW(up) specify the surface temperature TS via the Stefan-Boltzmann Law, while the weather station provides the air temperature TA within 2 meters of the ground. The model parameterizes the sensible heat flux QSENS in terms of the wind speed and surface-to-air temperature contrast TS-TA. Then the combination QEVAP - QCONDUC is expressed as a linear combination of TS and its time derivative dTS/dt. Equation 1 then functions as a statistical regression model in which the time series of radiation fluxes and temperatures can be used to infer values of QSENS and the difference QEVAP - QCONDUC. When applied to several days of data, the regression model typically explained 83-84 percent of the variance in net downward radiation [defined as QNET = QLW(dn) - QLW(up) + QSOL(dn) - QSOL(up)] at the urban canyon site and 96 percent of the variance at the control site.
Table 1 presents 24-hour averaged energy fluxes for the two sites based on data obtained in August 2005. Negative values of flux indicate a warming effect; positive values indicate cooling. The results are based on seven consecutive days of data that have been averaged into a single 24-hour period, starting at 4:00 a.m.
Incoming solar energy (Figure 1) is strongly affected by the presence of tall structures. Diurnally averaged downward solar flux at the canyon site is about 100 W m-2 less than at the control site. Tall structures block the direct solar beam; with the result that only diffuse solar energy strikes the canyon sensor in early morning and late afternoon. The canyon surface can reflect up to 36 percent of incoming solar because of its high albedo, as opposed to 23 percent at control, but when averaged diurnally, the control reflects 5-6 W m-2 more energy in absolute terms.
Table 1. Average Energy Fluxes Based on 7 Consecutive 24-hour Periods. QNET is the total incoming minus the total outgoing radiant energy flux. Negative values correspond to heating of the underlying surface.
Flux Component |
Value of Flux (W m-2) |
|
Control |
Canyon |
|
-QSOL(dn) |
-244.8 |
-146.9 |
QSOL(up) |
57.1 |
51.7 |
-QLW (dn) |
-389.8 |
-419.5 |
QLW (up) |
484.2 |
465.5 |
QEVAP - |
28.6 |
27.1 |
QSENS |
64.7 |
22.1 |
QNET |
-93.3 |
-49.2 |
Differences in solar heating are partially offset by enhanced downwelling thermal longwave radiation at the canyon, which is ~30 W m-2 greater than at the control site throughout the 24-hour period (Figure 2). This suggests that the walls of surrounding tall structures contribute a nearly constant source of background radiation. The control’s underlying surface emits 20 W m-2 more thermal radiation (diurnal average) due to its high surface temperatures (Figure 3).
The net effect of evaporative cooling and conduction (QEVAP - QCONDUC) at both sites is 27-29 W m-2, corresponding to a cooling effect. Since no precipitation occurred during the recording period, this likely represents heat conduction downward away from the surface into the cooler interior of the structure. To support this, an energy budget analysis based on data from the green garden rooftop potion of Chicago’s City Hall, where watering occurs daily, produced a value of QEVAP - QCONDUC greater than 120 W m-2 in a 24-hour averaged sense.
Sensible heat transport out of the canyon is nearly a factor of three less than at the control site, due mainly to two processes. One is a smaller diurnally-averaged surface-to air-temperature differential. The other is decreased wind speed due to wind blockage by tall structures. On average, winds are slower by 2.6 km hr-1 in the canyon (Table 2). Sensible heat transport is greater at the control, with a net cooling of 64.7 W m-2 compared to 22.1 W m-2 in the canyon.
Figure 3 illustrates air and surface temperatures at the two locations. Air temperatures are greater in the canyon after 9:00 p.m., with the greatest difference occurring just before dawn. At specific times between 9:00 a.m. and 9:00 p.m. the canyon’s surface temperatures are up to 12.4°C cooler than at the control site. In the evening, the canyon’s slower cooling rate led its surface to become warmer than the control’s. In spite of the above behavior, when air and surface temperatures are averaged over the entire 7-day period, there is little difference between the two sites (Table 2).
Figure 1. Incoming Flux of Solar Radiation: Seven Days of Data Averaged Into a 24-hour Period
Figure 2. Incoming Flux of Terrestrial Radiation: Seven Days Averaged Into a 24-hour Period
This project analyzed several microclimates by evaluating the energy fluxes that heat and cool urban surfaces together with measured meteorological variables. Significant differences appear in solar and thermal radiation, air and surface temperatures, and wind speeds. Tall structures at the urban canyon site account for some of these differences. They block the direct solar beam, decreasing incoming solar radiation by 100 W m-2 at the canyon. They increase incoming thermal energy by 30 W m-2, which decreases the surface cooling rate, particularly overnight. The canyon’s vertical development slows the wind by an average of 2.6 km hr-1 relative to the control site. Reduced wind speed and a low surface-to-air temperature differential decrease sensible heat transport out of the canyon, which is one-third that of the control site.
These results have implications for the design of energy efficient urban spaces. The model applied here can be used to examine how the physical environment of a surface (vertical structures, availability of water for evaporation) as well as the properties of the surface itself (albedo, thermal characteristics) influence the 24-hour cycle in surface temperature and the resulting demands for energy to heat or cool interior spaces.
Table 2. Average Air and Surface Temperatures and Wind Speed Based on 7 Days of Data
Control |
Canyon |
|
TSURF (°C) |
30.4 |
27.7 |
TAIR (°C) |
26.8 |
26.9 |
Wind (km hr-1) |
4.5 |
1.9 |
Figure 3. Air and Surface Temperatures: Seven Days of Data Averaged Into a 24-hour Period
No journal articles submitted with this report: View all 2 publications for this subproject
Supplemental Keywords:microclimates, urban heat island, energy balance,
,
Air, Scientific Discipline, Engineering, Chemistry, & Physics, Ecology and Ecosystems, Environmental Monitoring, urban microclimate, urban air , air modeling, climatology, atmospheric reactions
Relevant Websites:
Progress and Final Reports:
Original Abstract
Main Center Abstract and Reports:
CR830890 Center for Environmental Science
Subprojects under this Center:
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
CR830890C001 The Urban Measurements Project—The Urban Atmosphere Observatory
CR830890C002 The Urban Data Analysis and Modeling Project
CR830890C003 Attenuation of Ultraviolet Solar Radiation by Cloudy Skies: Links to Urban Air Quality
CR830890C004 Measurements of Black Carbon in Chicago: Implications for Controls on Diesel Emissions
CR830890C005 Attenuation of Visible Sunlight by Limited Visibility and Cloudiness
CR830890C006 The Energy Balance of Urban Microclimates