Morning Breakup of a Nocturnal Cold Pool

Local flow patterns in areas of complex topography are driven by diurnal heating and cooling. If the topography is basin-shaped, then the drainage flows occurring at night accumulate in the lower valleys of the basin, causing the development of a pool of cold stable air that traps pollutants. These cold pools are destroyed during the day by rising convective boundary layer and upslope flows, leading to the dilution of pollutants. In order to understand mechanisms of cold pool destruction, a series of laboratory experiments were conducted, focusing on the identification of flow phenomena, rather than dynamic modeling of a particular atmospheric situation. The experimental configuration consisted of a V–shaped water tank, subjected to either bottom heating, cooling or a heating/cooling cycle. The working fluid was initially stably stratified (thermally) with a pronounced inversion layer. Bottom heating with a specified heat flux was initiated and dye visualization, temperature and particle tracking velocity (PTV) measurements were used to monitor the flow field. It was found that cold pools are destroyed by the combined influence of along-slope flow, horizontal intrusions emanating from the slope flows and entrainment of these intrusions into the growing bottom convective boundary layer. This scenario is different from previously held view of cold pool destruction. Field data taken during the VTMX field campaign (Salt Lake City, Utah, October 2000) were interpreted in light of laboratory observations.