CASES-99: A Comprehensive Investigation of the Stable Nocturnal Boundary Layer

Abstract

The Cooperative Atmosphere-Surface Exchange Study?1999 (CASES-99) refers to a field experiment carried out in southeast Kansas during October 1999 and the subsequent program of investigation. Comprehensive data, primarily taken during the nighttime but typically including the evening and morning transition, supports data analyses, theoretical studies, and state-of-the-art numerical modeling in a concerted effort by participants to investigate four areas of scientific interest. The choice of these scientific topics is motivated by both the need to delineate physical processes that characterize the stable boundary layer, which are as yet not clearly understood, and the specific scientific goals of the investigators. Each of the scientific goals should be largely achievable with the measurements taken, as is shown with preliminary analysis within the scope of three of the four scientific goals. Underlying this effort is the fundamental motivation to eliminate deficiencies in surface layer and turbulent diffusion parameterizations in atmospheric models, particularly where the Richardson number exceeds 0.25. This extensive nocturnal boundary layer (NBL) dataset is available to the scientific community at large, and the CASES-99 participants encourage all interested parties to utilize it. These preliminary analyses show that during nights where weak (< 2 m s-1) surface winds and strong static stability near the surface (exceeding 150 C km-1 to 20 m AGL) might otherwise indicate essentially nonturbulent conditions, that various, sometimes undefined, atmospheric phenomena can generate significant turbulent mixing, and therefore significant turbulent fluxes. In many cases, a jet structure will form in the NBL between 50 and 200 m AGL, resulting in strong shear between the surface and jet maximum. Consequently, though surface winds are weak, turbulence can be a significant feature in the stable NBL. Further, contrary to some previous work studying nocturnal jets over the Great Plains, the wind direction in the jet is often influenced by an inertial oscillation and seldom confined to the southerly quadrant (e.g., the Great Plains low-level jet).