environmental prediction in canadian cities inferring turbulent exchange processes in an urban...

Environmental Prediction in Canadian Cities

Inferring turbulent exchange processes in an urban

street canyon from high-frequency thermography

A. Christen(1), and J. A. Voogt(2)

(1) Department of Geography, University of British Columbia, Vancouver, BC, Canada(5) Department of Geography, University of Western Ontario, London, ON, Canada

AMS 19th Symposium on Boundary Layers and TurbulenceChristen and Voogt / Aug 3 2010

Objectives

Previous work has demonstrated that we observe high-frequency temperature fluctuations of selected urban facets in IRT data and in time-sequential thermal imagery1.

Our hypothesis is that observed high-frequency temperature fluctuations are linked to turbulent sensible heat exchange of a selected facet (surface renewal).

Application - to quantify the scale and/or shape of the dominating eddies in exchange processes (spatial, duration) and their pahse velocities.1 Christen et al. 2005, Christen and Voogt, 2009 (ICUC-9)


FLIR ThermoVision® A40MThermal scanner

15 m mobile pump-up tower

PC stored data at 1 Hz

Elgin Street Experiment, Vancouver, CanadaSeptember 14/15, 2008

Reflective tape


Fine-wire thermocouples

Net radiometer

Pyranometer

3-D Ultrasonic Anemometer-Thermometer

Infrared thermometer

Tower base

Surface wind / turbulence measurements on both sides of canyon (East, West) in FOV of scanner


Elgin Street - Experiment

• 24 hours of thermal image sequences of surface temperature T recoded at 1 Hz (320 x 240 pixels) in a fixed field of view.

• 10 Hz surface data from two 3D ultrasonic anemometers, thermocouples, and IRTs in the FOV.

• Instead of absolute T, we use the deviation of surface temperature T‘ of each pixel from its temporal mean.


Visualizing wind in the thermal image sequenceHorizontal wind vector

Circle: 1 m/s

Street

Lawn

Buildings

Cars

Bushes


Turbulent energy exchange?

Why are lawns showing a high variability in T?

Sensible heat flux

Low µ

Latent heat flux

Water availability

Low µ

Dewfall (waves)

Geometric effects?

Grass is flexible + Anisotropy

Movement in wind


Can geometric effects explain T‘ patterns?

10 minutes of1 Hz measurements

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

Thermal Scanner1 pixel ofIRT

ground-based


Correlation between wind and T’

Surface temperature

Wind

5 sec averages


Correlation between wind and T’

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

wind speeds upwind slows down

cools down

heats up


Sensible heat vs. correlation of du/dt and dTs/dt

QH transportsenergy

towards lawn

QH transportsenergy away

from lawn

Effi

ciency

of

Sensi

ble

heat

transf

er


Extraction of two-point statisticsspatial separation (m)

temporal lag (sec)

alo

ng c

anyon

cross canyon

wind169º

0.57 m s-1


Two-point correlations RTT vs. separation At τ = 0


Phase lag of two-point correlations of T’


• Fluctuations in surface temperature can be measured over facets with low thermal admittance (lawns) and show coherent patterns over time.

• Fluctuations in surface temperature are shown to be correlated to change in near-surface wind.

• Exchange of sensible heat from urban lawn surfaces is driven by elongated turbulent structures moving along the canyon. The phase velocity of T’ fluctuations is faster than mean local subcanopy wind.

Summary


Frederic Chagnon, Environment CanadaBen Crawford, UBCSue Grimmond, King’s College, London Adrian Jones, UBCRick Ketler, UBC Ivan Liu, UBCFred Meier, TU Berlin Kate Liss, UBC Tim Oke, UBC Dieter Scherer, TU BerlinChad Siemens, UBCDerek van der Kamp, UBC

And residents of Elgin Street, Vancouver, Canada

Acknowledgements for technical assistance, program code, or instrumentation

Funding agencies

environmental prediction in canadian cities inferring turbulent exchange processes in an urban...

Documents

wind slide

th symposium

boundary layers

canada slide

t surface temperature

fov of scanner slide

reflective tape slide

visualizing wind