Linking atmospheric turbulence and surface temperature fluctuations

in a street canyonAndreas Christen, University of British ColumbiaJames 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.

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

FLIR ThermoVision® A40MThermal scanner with in pan and tilt device

15 m mobile pump-up tower

PC stored data at 1 Hz

Reflective tape

Fine-wire thermocouples

Net radiometer

Pyranometer

3-D Ultrasonic Anemometer-Thermometer

Infrared thermometer

Tower

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

QuickTime™ and aGIF decompressor

are needed to see this picture.

24h cycle of mean surface temperatures

Horizontal wind vector

Circle: 1 m/sNorth-South Axis

East-West Axis

Visualizing wind in the thermal image

Surface temperature fluctuations day

Surface temperature fluctuations night

Fluctuations - Integral standard deviationfor each pixel

Night Day15-Sep-2008 12:3015-Sep-2008 01:30

Sidewalk / Streetlittle fluctuation

Lawnshigh fluctuations

Shadow of street light

Turbulent energy exchange? Geometric effects?

Sensible heat flux Latent heat flux

Why are lawns showing such a high variability in Ts, but not streets, paths, and walls do not?

Low µ Water availability

Low µGrass is flexible +

Anisotropy

Movement in wind

Dewfall (night waves)

Can geometric effects explain Ts patterns?

10 minutes of1 Hz measurements

CANYON WEST 2008-09-15 15:20-15:30

FLIR1 pixel ofIRT

ground-based

Anisotropy?

Correlation between wind and temperature?

2008-09-15 10:30 (10 sec time step)Wind 30 cm above grass

wind speeds upwind slows down

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.

Summary

Frederic Chagnon, Environment CanadaBen Crawford, UBC Adrian Jones, UBCRick Ketler, UBC Fred Meier, TU Berlin Kate Liss, UBC Tim Oke, UBC Dieter Scherer, TU BerlinChad Siemens, UBC

And residents of Elgin Street, Vancouver, Canada

Acknowledgements

Funding agencies