Numerical study of nocturnal low-level jets over gently sloping terrain

Abstract

Nocturnal low level-jets (LLJs) over gently sloping terrain typical of the U.S. Great Plains are investigated by means of direct numerical simulation. Such LLJs develop in a tilted atmospheric boundary layer as a result of inertia-gravity oscillations initiated by a change of the surface thermal forcing during the evening transition. External parameters are the free-atmospheric geostrophic wind, ambient atmospheric stratification, surface buoyancy forcing, and slope angle. The governing momentum and buoyancy balance equations are written in slope-following coordinates, and solved numerically in the Boussinesq approximation. The surface forcing is prescribed in a form of surface buoyancy or buoyancy flux, which are both slope-uniform but change in time. LLJs over slopes are contrasted with LLJs over flat terrain. Slope-induced effects essentially modify the entire structure of nocturnal LLJs. The shape of the LLJ wind-profile over a slope is characterized by a sharper and larger-magnitude maximum. The presence of the slope causes the along-slope advection of environmental potential temperature during the night. This advection can reignite static instability in the LLJ flow developing after the evening transition. The resulting turbulence leads to a complete or partial remix of the boundary layer flow and drastically changes the appearance of the LLJ in terms of its shape and vertical position. A pronounced nighttime jet can also develop from the daytime CBL in the absence of any free-atmospheric geostrophic forcing. The daytime flow preconditioning, an important precursor of the nocturnal LLJ development, plays an especially important role in LLJs over a slope.