LES of Shear-Influenced, Unstable Atmospheric Boundary Layers and the Stable Free Troposphere Forced by Varying Geostrophic Wind Directions with Homogeneous and Heterogeneous Surface Heating

Large-eddy simulation (LES) runs are performed to understand the influence of a one-dimensional (1D) surface heating heterogeneity on organized vertical motions within and above the atmospheric boundary layer (ABL). Two knowledge gaps are of particular interest: i) how do updrafts develop in the low free troposphere, and ii) what parameters control the updraft location and strength within the ABL? LES runs are performed for a shear-influenced, unstable ABL driven by geostrophic winds of the same magnitude but in various directions relative to a prescribed 1D surface-heat-flux heterogeneity. Quasi-steady-state LES results are phase-averaged over time and the horizontal dimension perpendicular to the surface-heat-flux gradient to quantify secondary circulations. Regarding the first knowledge gap, results show that organized vertical motions in the low free troposphere can be modeled as two-dimensional (2D) stationary gravity waves, whose amplitudes depend on ABL updraft strength and instability development within the free troposphere. Regarding the second knowledge gap, results show that organized updrafts within the ABL may form either above relatively warm surfaces or downwind of warm-to-cool transitions. These different locations are well explained by both the relative contributions to secondary circulations from phase-averaged horizontal and vertical velocity fluctuations and the relative importance of horizontal advection and turbulent transport in the phase-averaged internal energy fluctuation equation. The main balances associated with each updraft location are used to propose empirical models of updraft strength, and it is shown that the presence of sufficiently strong organized vertical motions can cause a non-negligible reduction in near-surface eddy viscosity.