SEMINAR SERIES and COLLOQUIA

Previous studies have shown that in complex terrain cold pools typically develop in regions with local elevation minima shortly after sunset. Radiative cooling and reduced mixing, caused by vegetation and flow-decoupling in valleys and basins, have been identified as main mechanisms conducive for cold pool formation. While a few studies indicate that smaller-scale nocturnal temperature variations can also be observed in regions with moderate terrain, the frequency, strength and formation processes of such cold pool events is still largely unknown. This study thus focuses on the role of atmospheric conditions and terrain features in cold pool development across the 120-by-320 m Lake Thunderbird Micronet which is located in gently sloped terrain. Spatial analysis of temperature data from 26 Micronet sites (spacing roughly 30 m) revealed that cold pools frequently form in a region of relatively low elevation that is also sheltered by trees. Temperature differences between the cold pool and the near-environment of the order of 5°C were frequently observed. Using data from a nearby Oklahoma Mesonet site, Richardson numbers were computed, which demonstrated that cold pool formation frequently occurs under strongly stable conditions for which minimal vertical turbulent mixing is expected. Wind data collected with sonic anemometers on a 15-m tall tower for a period of three months (Spring 2005) suggest that flow sheltering by vegetation plays an important role in cold pool formation. The wind data also show signatures of katabatic flow for about 50% of the observed strong cold pool events. However, no significant differences in cold pool strength were observed between the katabatic and non-katabatic cold pool cases, and a heat budget analysis suggests that the katabatic flows were actually associated with warm air advection along the slope. Thus, if a katabatic jet had penetrated the cold pool, it would have produced substantial warming in the region of the cold pool. Since such warming was not observed, it is concluded that the katabatic jets did not actually penetrate the cold pool but likely flowed over it. The observations that significant temperature changes can occur even with moderate elevation changes have important applications in agriculture as well as data assimilation.

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