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 Environmental Prediction in Canadian Cities

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)

Elgin Street Experiment, Vancouver, Canada September 14/15, 2008 FLIR ThermoVision® A40M Thermal scanner 15 m mobile pump-up tower PC stored data at 1 Hz Reflective tape

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.

Tower base Net radiometer 3-D Ultrasonic Anemometer-Thermometer Pyranometer Infrared thermometer Fine-wire thermocouples 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. Fixed FOV along canyon sampled with 1 Hz at 320 x 240 pixels. Continuous 10 Hz surface data from 3D ultrasonic anemometers, thermocouples, IRTs (synchronized with thermal scanner).

Visualizing wind in the thermal image sequence Horizontal wind vector Buildings Circle: 1 m/s Lawn Cars Street Bushes

Why are lawns showing a high variability in T? 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 (waves)

Can geometric effects explain T‘ patterns? IRT ground-based Thermal Scanner 1 pixel of 10 minutes of 1 Hz measurements CANYON WEST 2008-09-15 15:20-15:30

Correlation between wind and T’ 5 sec averages Surface temperature Wind

Correlation between wind and T’ 2008-09-15 10:30 (10 sec time step) Wind 30 cm above grass heats up cools down wind slows down wind speeds up

Sensible heat vs. correlation of du/dt and dTs/dt QH transports energy away from lawn Sensible heat transfer Efficiency of QH transports energy towards lawn

Extraction of two-point statistics spatial separation (m) temporal lag (sec) along canyon cross canyon wind 169º 0.57 m s-1

Two-point correlations RTT vs. separation

Phase lag of two-point correlations of T’

Summary 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.

Funding agencies Frederic Chagnon, Environment Canada Ben Crawford, UBC Sue Grimmond, King’s College, London Adrian Jones, UBC Rick Ketler, UBC Ivan Liu, UBC Fred Meier, TU Berlin Kate Liss, UBC Tim Oke, UBC Dieter Scherer, TU Berlin Chad Siemens, UBC Derek van der Kamp, UBC And residents of Elgin Street, Vancouver, Canada Acknowledgements for technical assistance, program code, or instrumentation