Studies of Nocturnal Boundary Layer Processes from CASES99

Dr. Larry Mahrt

Oregon State University

This study analyzes 8 levels of sonic anemometer data collected on a 60-m tower during CASES99, toward the goal of understanding the vertical structure of the nocturnal boundary layer. Several different regimes are found. Thin boundary layers are often observed where fluxes decrease with height and approximately vanish between 20 and 30 m above ground. The flow above the thin boundary layer accelerates and increasing shear often generates significant turbulence in the middle of the night. This shear-generated turbulence is often stronger than that near the surface corresponding to an upside-down boundary layer. During these conditions, the turbulent transport of turbulence is downward toward the surface.

The vertical structure of the nocturnal boundary layer is also complicated by shallow drainage flows, which develop in the early evening even over gentle small-scale slopes.