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

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.

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?

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.

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

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

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).

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

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

Surface temperature fluctuations day

Surface temperature fluctuations night

Fluctuations - Integral standard deviation for each pixel Night Day Shadow of street light 15-Sep-2008 01:30 15-Sep-2008 12:30 Lawns high fluctuations Sidewalk / Street little fluctuation

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)

Can geometric effects explain Ts patterns? Anisotropy? IRT ground-based FLIR 1 pixel of 10 minutes of 1 Hz measurements CANYON WEST 2008-09-15 15:20-15:30

Correlation between wind and temperature? 2008-09-15 10:30 (10 sec time step) Wind 30 cm above grass wind slows down wind speeds up

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

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.

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