Turbulence in a Convective Marine Atmospheric Boundary Layer

Abstract

The structure and kinetic energy budget of turbulence in the convective marine atmospheric boundary layer as observed by aircraft during a cold air outbreak have been studied using mixed layer scaling. The results are significantly different from those of previous studies under conditions closer to free convection. The normalized turbulent kinetic energy and turbulent transport are about twice those found during the Air Mass Transformation EXperiment (AMTEX). This implies that, for a given surface heating, the present case is dynamically more active. The difference is mainly due to the greater importance of wind shear in the present case. This case is closer to the roll vortex regime whereas AMTEX observed mesoscale cellular convection, which is closer to free convection. Shear generation is found to provide a significant energy source in addition to buoyancy production to maintain a larger normalized turbulent kinetic energy and to balance a larger normalized dissipation. The interaction between turbulent pressure and divergence (i.e., pressure scrambling) is also found to transfer energy from the vertical to the horizontal components, and expected to be stronger in roll vortices than in mesoscale cells. The sensible heat flux is found to fit well with a linear vertical profile in a clear or subcloud planetary boundary layer (PBL), in good agreement with that of Lenschow. The heat flux ratio between the PBL top and the surface, derived from the linear fitted curve, is approximately ?0.14; this is in good agreement with that derived from the lidar data for the same case. Near the PBL top, the heat flux profiles are consistent with those of Deardorff and Deardorff et al.