1. Linking atmospheric turbulence and surface temperature fluctuations in a street canyon
Andreas Christen, University of British Columbia
James Voogt, University of Western Ontario
ICUC-7, Yokohama June 29 to Jul 3, 2009

2. Hypothesis
Surface temperatures at the urban-atmosphere interface do not vary only as a consequence of annual and diurnal cycles, surface materials, shading and anthropogenic heat releases.
At higher-frequencies - in the order of seconds to minutes - selected urban facets might further experience abrupt temperature changes due to intermittent (turbulent) energy exchange.

3. Why is this of interest?
In contrast to the study of atmospheric turbulence - where spatial measurements are nearly impossible - spatial fields of surface temperatures can be easily recorded at high-frequency using thermal imaging.
Surface temperatures might indirectly assist the study of turbulent exchange processes in street canyons, i.e. changes in surface temperature in a street canyon could be used as a tracer to track turbulent eddies that dominate mixing processes of the canyon air.
Our objective is to evaluate if there are links between fluctuations in surface temperature and turbulent air movements in a street canyon.
Can we use time-sequences of surface temperature fluctuations to infer the movement of turbulent eddies in a complex environment?

4. Elgin Street, Vancouver, Canada
To explore feasibility and limitations of this approach, a thermal scanner (Flir ThermoVision® A40M, operated at 1 Hz) was coupled with turbulence measurements (ultrasonic-anemometers, at 10 Hz) within a vegetated suburban street canyon of Vancouver, BC, Canada. The thermal scanner was mounted on a 15 m tower in the center of the canyon and monitored a representative cross-section of the street, lawns and buildings on each side. Two tripods in the field of view were equipped with ultrasonic-anemometers, infrared thermometers and fine-wire thermocouples and provide in-situ data from the atmosphere 30 cm above the surface and from the surface itself.

5. FLIR ThermoVision® A40M Thermal scanner
with in pan and tilt device
15 m mobile pump-up tower
PC stored data at 1 Hz
Reflective tape

6. Surface station on both sides of canyon (East, West) in FOV of scanner
Tower
Net radiometer
3-D Ultrasonic Anemometer-Thermometer
Pyranometer
Infrared thermometer
Fine-wire thermocouples
Surface station on both sides of canyon (East, West) in FOV of scanner

7. Summary - Field activities ‘Elgin Street’
26 hours of thermal image sequences during a clear-sky situation (Sept 14 14:00 – Sept 15 16:00 PST, 2008):
Fixed FOV towards North sampled at 1 Hz rate at 320 x 240 pixels (Geometrically corrected).
Continuous 10 Hz surface data from 3D ultrasonic anemometers, thermocouples, IRTs (synchronized with thermal scanner).

8. Thermal Scanner - Field of View
24h cycle of mean surface temperatures

10. Visualizing wind in the thermal image
Horizontal wind vector
North-South Axis
East-West Axis
Circle: 1 m/s

11. Surface temperature fluctuations day

12. Surface temperature fluctuations night

13. Fluctuations - Integral standard deviation for each pixel
Night
Day
Shadow of
street light
15-Sep :30
15-Sep :30
Lawns
high fluctuations
Sidewalk / Street little fluctuation

14. Why are lawns showing such a high variability in Ts, but not streets, paths, and walls do not?
Turbulent energy exchange?
Geometric effects?
Sensible heat flux
Latent heat flux
Movement in wind
Low µ
Water availability
Grass is flexible + Anisotropy
Low µ
Dewfall (night waves)

15. Can geometric effects explain Ts patterns?
Anisotropy?
IRT
ground-based
FLIR
1 pixel of
10 minutes of
1 Hz measurements
CANYON WEST :20-15:30

16. Correlation between wind and temperature?
:30 (10 sec time step)
Wind 30 cm above grass
wind slows down
wind speeds up

17. Spectral energy at 0.32 Hz (3 sec)
Spectral analysis
Spectral energy at Hz (3 sec)

18. Summary
We observed significant high-frequency fluctuations on materials with low thermal admittance - mainly lawns in our canyon.
Fluctuations are controlled by near-surface wind.
Exchange of heat (and likely water vapor) from urban lawn surfaces is driven by intermittent, lager-scale coherent eddies, moving through the canyon (time scale 30 sec to 2 min).
Promising visualization tool for turbulence.

21. Funding agencies
Acknowledgements
Frederic Chagnon, Environment Canada
Ben Crawford, UBC
Adrian Jones, UBC
Rick Ketler, UBC
Fred Meier, TU Berlin
Kate Liss, UBC
Tim Oke, UBC
Dieter Scherer, TU Berlin
Chad Siemens, UBC
And residents of Elgin Street, Vancouver, Canada