Effects of Initial Temperature and Velocity Perturbations on the Development of Convection in the Atmospheric Boundary Layer

Early stages in the development of the inversioncapped atmospheric convective boundary layer (CBL) were studied by means of numerical large eddy simulation (LES) in conjunction with data from the University of Karlsruhe (UniKa) wind tunnel model of the atmospheric CBL (Fedorovich et al. 1996, 2001). The LES experiments were conducted for two CBL flow types. The first investigated type was the spatially evolving quasi-stationary CBL modeled in the wind tunnel. The second type was the nonsteady, horizontally quasi-homogeneous CBL in a periodic domain. The latter CBL type is commonly considered in meteorological applications of LES. Thus, the dynamics of convective entrainment in a pure case of spatial CBL evolution was analyzed versus the entrainment dynamics in a pure case of CBL temporal development. In the LES of atmospheric convection, (see, e.g. Nieuwstadt et al. 1993), convective motions are traditionally initiated by prescribing randomly distributed velocity and temperature fluctuations throughout an originally homogeneous flow region beneath the inversion. The subsequent development of convection passes through a transition phase, properties of which (in particular, the duration of transition) essentially depend on the depth of the perturbed layer and characteristics of the imposed fluctuations. In order to avoid an arbitrary choice for the initial perturbed layer depth, the resolved-scale temperature and/or velocity perturbations in the present study were generated only in the immediate vicinity of the underlying surface. The effects of magnitude and correlation of such perturbations on the stimulation of convection in shearfree and sheared clear boundary layers driven by surface buoyant forcing were systematically investigated.