Turbulence Processes in the Stable Marine Atmospheric Boundary Layer

Abstract

LONG-TERM GOALSThe long-term goals of this study are to improve our understanding of turbulence processes within the stable marine atmospheric boundary layer in spatially heterogeneous coastal environments with the aim of improving numerical representation of such environments. OBJECTIVESThe study has two distinct objectives. The first is the study of the evolving turbulence structure of the stable boundary layer as the mesoscale flow rounds a coastal headland and forms an expansion fan in which the flow accelerates and the boundary layer collapses. The region around the headland is one of extreme spatial heterogeneity -there are large changes in wind speed an order of magnitude increase in the surface wind stress, and boundary layer depth decreases by a factor of about 5. A large surface wind-stress curl drives enhanced upwelling of cold water creating a cold pool just downwind of major headlands; this increases the local stability further modifying the overlying boundary layer structure.The second objective is to characterize the entrainment zone structure at the top of the boundary layer. Previous studies of entrainment zone structure have focused almost entirely on convective conditions; this is the first extensive study of entrainment zone structure for the stable marine boundary layer. APPROACHAn extensive set of aircraft measurements from the Coastal Waves 96 (CW96) field campaign are being utilized for this study. Turbulence measurements from straight and level flight legs within the surface layer (30 m) define the surface forcing; measurements from extensive series of sawtooth profiles extending from approximately 15 m to above the inversion are used to derive both the mean and turbulent vertical structure following the approach of Tjernström (1993). The observations are being studied alongside numerical simulations being conducted by Michael Tjernström and Stefan Söderberg of Stockholm University, Sweden. Analysis of the aircraft data and integration of results from the modeling study is being undertaken by Ian Brooks at SIO.