Colloquium, October 7, 2008, 3:00 pm, EN6085A
TUESDAY

By means of large eddy simulations of clear and cloudy boundary layers, devoid of explicit mesoscale forcings, we show that buoyancy driven convection autonomously creates spatial fluctuations with a lateral size far exceeding the boundary layer depth. If the lateral domain is wide enough and if one waits long enough, these fluctuations will eventually dominate the variance of nearly all (thermo)dynamic variables.

In search for the cause of this phenomenon, we determine in the simulations the spectral decomposition of all terms contributing to the budgets of turbulence kinetic energy and variances of the involved (thermo)dynamical variables. These terms include the spectral transfer (cascade) and spectrally decomposed production/dissipation and pressure-correlation terms, etc. This analysis yields a direct insight into those processes that favor and those processes that inhibit the formation of large scale fluctuations. We conduct the analysis on a dry convective boundary layer and on a cloud topped boundary layer (Sc), and discuss the generic underlying mechanism.