The basic mechanisms for how temperatures change with elevation in mountain landscapes and how temperature inversions form in valleys have been understood for many years. Under “normal” circumstances, temperatures decrease at a known rate, the “lapse rate”, with elevation. Knowledge of the lapse rate allows meteorologists and scientists to extrapolate from a few measurement locations across a complex landscape. By definition, an inversion event turns the normal lapse rate on its head. During an inversion, air temperatures are cooler than expected near the surface. However, even though these phenomena are well known, the long term temperature records at the Andrews site, coupled with the interdisciplinary approach of the scientific research team, has revealed important new twists on these fundamental ideas.
In an analysis of temperatures measured continuously for several years on a ridge and a nearby valley at the H.J. Andrews Experimental Forest, Daly et al. (2009) found striking and highly dynamic differences at the two locations. More often than not, temperatures on the ridge were warmer than expected from the lapse rate, sometimes by more than 15 degrees C. However, occasionally the ridge site could be colder than expected. Closer analysis showed that the pooling and ponding of cool, dense air is the primary cause, and these airflow patterns are not only dynamic, but much more pervasive and persistent than previously thought. Furthermore, because the air pooling essentially “decouples” the valleys from the upper atmosphere, valleys are less sensitive to variations in weather and climate compared with exposed ridges. Using a model that links air circulation patterns in the upper atmosphere to temperature patterns on the ground, Daly et al. estimated that an overall rise in the regional climate of 2.5 degree C would cause daily maximum temperatures in December on exposed ridges to increase by up to8 degrees C compared with only about 3 degrees in nearby valleys.
Concurrent research at the Andrews has shown that nocturnal airflows associated with cold air drainage are deeper (more than 30m deep), swifter, and more turbulent in a forested basin than anyone previously imagined (Pypker et al. 2007a, b). The tall trees that cover the basin are an important part of the story. On clear evenings, heat radiates from the foliage to the sky, cooling the air around the canopy. The cool, dense air then falls to the ground, forming a deep river of air that tumbles and mixes along the same path as the stream system, carrying gases like carbon dioxide and other aerosols laterally. The surface of the canopy defines a “cap” to the inversion layer that limits vertical exchange with the atmosphere above. Essentially, this creates a transient, environmental “cocoon” within the valleys.
Because cold air pooling and consequent atmospheric decoupling occur in many mountain valleys, especially at high latitudes, these phenomena are likely to be important considerations in understanding the impacts of climate change and the climate responses of ecosystems in mountainous regions more generally.