Thermal Circulations in Complex Terrain

 

About 70% of the earth surface is covered with complex topography. Local airflow is driven by pressure gradients and diurnal variation of solar heating causing mesoscale thermal circulation (valley/slope flows). Recent studies have identified anabatic flows as playing a key role in air pollution dispersion and transport of urban pollutants. Pollutants carried by anabatic flows can be detrained into the upper part of the BL or can be injected into the synoptic flow possibly showering downwind cities with pollutants. In urban areas with high ozone, the daytime upslope flow can contain embedded plumes of high ozone concentration which are advected upslope toward the higher elevations (Figure 1A).

 

In the mornings, anabatic winds breakup cold pools in mountain valleys, advecting the moisture upslope, leading to the initiation of mountain generated cumuli and their continued growth into cumulus congestus and cumulonimbus clouds (Figure 1B).

 

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Figure 1: A) Daytime convection and upslope dispersion of pollutants. B) Morning cold pool breakup by anabatic flow, an important process to cumulus initiation.

 

Our laboratory experiments investigate the mechanisms governing anabatic/katabatic flow above a uniformly heated/cooled slope to obtain critical flow parameters of consequence (Figure 2). These measurements include numerous slope angles and buoyancy flux settings with and without stratification and the presence of a synoptic flow, allowing identification of new processes and verification of theoretical predictions.

 

Laboratory techniques used include an in-house developed Particle Tracking Velocimetry (PTV) system including Feature Tracking Visualizations (FTV), (a method of animating color mapped PTV tracks by length or velocity to gain a time history of the flow) (Figure 3) and Stereoscopic Particle Image Velocimetry (Stereo PIV).

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Figure 2: Insulated water tank with electric foil acting as heated slope.

 

Figure 3: In-house developed Particle Tracking Velocimetry (PTV) systems with Feature Tracking Visualization (FTV) shown where tracks are color mapped by length or velocity.