Variability of Surface Air Temperature over Gently-Sloped Terrain

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 a 120-by-320 m Micronet in gently sloped terrain. Two-year long temperature records from the Lake Thunderbird Micronet in Norman, Oklahoma were analyzed. This Micronet has 28 sites equipped with temperature/RH loggers (spacing roughly 30 m) and one 15-m tall tower with 5 sonic anemometers.

Spatial analysis of temperature data revealed that cold pools frequently formed 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. The strength of each cold pool event was classified according to a Cold Pool Index (CPI) based on nocturnal temperature perturbations (difference between temperatures in cold pool region and spatially averaged temperature at the Micronet). 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. Cold pools associated with katabatic flows tended to develop faster and persist longer than cold pools with non-katabatic flows. However, no significant differences in cold pool strength were observed between the katabatic cold pool cases and non-katabatic cold pool cases. Thus, drainage flow appears not being a dominant process in cold pool formation and, as the observed temperature patterns suggest, it might even cause transport of warmer air into the cold pool and counteract cold pool development. 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. The observations that significant temperature changes can occur even with moderate elevation changes have important applications in agriculture as well as data assimilation.