Inferring turbulent exchange processes in an urban street canyon from high-frequency thermography

From the surface renewal approach we know that time series of surface temperatures often follow ramp-like patterns. The abrupt replacement of progressively warmed or cooled near-surface air with well-mixed air from aloft by organized sweep-ejection cycles is reflected in changing surface temperatures, in particular on surfaces with a low thermal admittance. In an urban ecosystem, those surfaces are roofs and dry lawns that respond to turbulent sensible heat exchange within less than 10 seconds. Those changes in surface temperatures are measurable by conventional thermography.

We report results from a field experiment within a suburban street canyon of Vancouver, BC, Canada in September 2008. A thermal scanner (Flir ThermoVision A40M, operated at 1 Hz) was mounted on a 15 m tower in the center of the canyon and monitored a representative cross-section of the street canyon continuously over 24 hours. The field of view of the scanner included street, short-cut lawns and similar buildings on each side (see figure). Two tripods in the field of view were equipped with synchronized ultrasonic-anemometers (Campbell Scientific Inc., at 10 Hz), infrared thermometers and fine-wire thermocouples. Those systems provided in-situ data from 30 cm above two dry lawn surfaces.

Exchange of heat from the urban lawn surfaces is shown to be driven by intermittent, 'honami'-type, and spatially coherent eddies and associated small-scale turbulence, that are clearly visible in thermography images if properly filtered from mean trends. Eddies are moving through the canyon at a time scale between 30 sec to 2 min. The combination of thermography and in-situ turbulence measurements underlines that temperature fluctuations visible in the thermography image are associated with eddies that drive the exchange of energy and mass between lawns and the street canyon air. We have therefore applied spatial statistics to the thermography time-series to quantify characteristics (turbulent length-scales etc.) of sensible heat exchange.

We conclude that surface temperatures indirectly assist the study of turbulent exchange processes, i.e. changes in surface temperature in a street canyon can be used as a tracer to track and extract characteristics that describe turbulent sensible heat exchange.