hide
Free keywords:
-
Abstract:
The heterogeneously heated free convective boundary layer (CBL) is
investigated by means of dimensional analysis and results from
large-eddy simulations (LES) and direct numerical simulations (DNS). The
investigated physical model is a CBL that forms in a linearly stratified
atmosphere heated from the surface by square patches with a high surface
buoyancy flux. Each simulation has been run long enough to show the
formation of a peak in kinetic energy, corresponding to the "optimal"
heterogeneity size with strong secondary circulations, and the
subsequent transition into a horizontally homogeneous CBL.
Scaling laws for the time of the optimal state and transition and for
the vertically integrated kinetic energy (KE) have been developed. The
laws show that the optimal state and transition do not occur at a fixed
ratio of the heterogeneity size to the CBL height. Instead, these occur
at a higher ratio for simulations with increasing heterogeneity sizes
because of the development of structures in the downward-moving air that
grow faster than the CBL thickness. The moment of occurrence of the
optimal state and transition are strongly related to the heterogeneity
amplitude: stronger amplitudes result in an earlier optimal state and a
later transition. Furthermore, a decrease in patch size combined with a
compensating increase in patch surface buoyancy flux to maintain the
energy input results in decreasing KE and a later transition. The
simulations suggest that a CBL with a heterogeneity size smaller than
the initial CBL height has less entrainment than a horizontally
homogeneous CBL, whereas one with a larger heterogeneity size has more.
