Numerical and Laboratory Study of a Horizontally Evolving Convective Boundary Layer. Part I: Transition Regimes and Development of the Mixed Layer

Abstract

Results are presented from a large eddy simulation (LES) and wind tunnel study of the turbulence regime in a horizontally evolving sheared atmospheric convective boundary layer (CBL) capped by a temperature inversion. The wind tunnel part of the study has been conducted in the thermally stratified tunnel of the University of Karlsruhe. For the numerical part a modified LES procedure that was originally designed for simulation of the horizontally homogeneous atmospheric CBL has been employed. The study focuses on the transition between the neutrally buoyant boundary layer in the initial portion of the wind tunnel flow and a quasi-homogeneous convectively mixed layer developing downwind. The character of the transition between the two boundary layers and the associated changes in the turbulence structure are found to be strongly dependent on the magnitude and distribution of disturbances in the flow at the entrance of the wind tunnel test section. For all simulated inflow conditions, the transition is preceded by accumulation of potential energy in the premixed CBL. The eventual energy release in the transition zone leads to turbulence enhancement that has a form of turbulence outbreak for particular flow configurations. The numerically simulated CBL case with temperature fluctuations introduced in the lower portion of the incoming flow appears to be the closest to the basic CBL flow case studied in the wind tunnel. Second-order turbulence statistics derived from the LES are shown to be in good agreement with the wind tunnel measurements. Main features of transition, including the turbulence enhancement within the transition zone, are successfully reproduced by the LES.