Coriolis effects in homogeneous and inhomogeneous katabatic flows

Abstract

Katabatic flows along a planar slope in a viscous stably-stratified fluid are investigated analytically and numerically, with an emphasis on flow features arising from the action of the Coriolis force. Two idealized flow types are considered: turbulent flow along a uniformly cooled slope, and two-dimensional laminar flow induced by a cold strip of finite width running down the slope. In the case of turbulent flow along a uniformly cooled slope, the downslope velocity exhibits a boundary-layer structure, but the cross-slope velocity and buoyancy fields spread inexorably upward. This behaviour is qualitatively similar to that found in laminar flows along a uniformly cooled slope. In contrast, in the two-dimensional strip flow, a steady state is reached, in which the cross-slope wind and buoyancy fields vanish far above the slope, but the downslope and slope-normal velocity fields do not vanish. These latter two flow components comprise two purely horizontal along-isentrope counter-flowing currents: an upslope current entering the top of the boundary layer on one side of the strip, and a downslope current flowing out of the boundary layer into the environment on the other side of the strip. The slope-normal vorticity associated with these currents originates in the stretching of planetary vorticity in a broad zone of convergent flow over the cold strip.

Energetics of Slope Flows: Linear and Weakly Nonlinear Solutions of the Extended Prandtl Model

“…This type of flow is often observed in regions of complex orography and substantially affects the weather and climate in these regions (e.g., Poulos and Zhong, 2008). The topic of katabatic and anabatic wind is being actively explored and the work on its understanding includes the application of numerical models (direct numerical simulations (DNS): (e.g., Shapiro and Fedorovich, 2008); large eddy simulations (LES): e.g., Skyllingstad, 2003;Smith and Porté-Agel, 2013); mesoscale models: (e.g., Smith and Skyllingstad, 2005;Zammett and Fowler, 2007); and analytical models (e.g., Prandtl, 1942;Defant, 1949;Grisogono and Oerlemans, 2001;Zardi and Serafin, 2014). Continued interest in katabatic and anabatic winds stems from the important effects of this type of orographic flows on visibility and fog formation, air pollutant dispersion, agriculture and energy use, fire-fighting operations, sea-ice formation, etc.…”

MicroHH 1.0: a computational fluid dynamics code for direct numerical simulation and large-eddy simulation of atmospheric boundary layer flows

“…Mean profiles of the along-slope velocity component and buoyancy, as well as profiles of second-order turbulence statistics, such as kinematic turbulent fluxes of momentum and buoyancy, and velocity component and buoyancy fluctuation variances, were evaluated by averaging the simulated flow fields spatially over the along-slope planes and temporally over five oscillation periods beyond the transition stage. For comparison, the same katabatic flow case was reproduced using the numerical code (hereafter referred to as FS09) that was employed to simulate turbulent slope flows in Shapiro and Fedorovich (2008) and Fedorovich and Shapiro (2009). In that code, the time advancement was performed with an Asselin-filtered secondorder leapfrog scheme (Durran, 2013).…”

The Cessation of Continuous Turbulence as Precursor of the Very Stable Nocturnal Boundary Layer

“…In the latter case, we may speak of a so-called radiative SBL because in the absence of vigorous turbulent heat transport, clear-air radiation is the dominant thermodynamic processes (Van de Wiel et al 2003). Recently, considerable effort has been made to understand various processes within such VSBLs, such as longwave radiative cooling (Sun et al 2003b;Edwards 2009), subtle turbulent activity on the small and mesoscale (Mahrt 2011;Mahrt et al 2012), low-level jet formation (Banta 2008), and katabatic effects over sloping terrain (e.g., Shapiro and Fedorovich 2008). Apart from this relatively calm ''background behavior,'' disturbances may suddenly appear in the form of infrequent turbulent mixing events of various origin (Poulos et al 2002;Sun et al 2003a).…”

“…As before, momentum conservation facilitates the existence of a level with constant velocity. Indeed, also in the atmosphere, existence of such a velocity crossing point can be anticipated: high-level nocturnal winds (say, .100 m) are often subject to accelerations in response to decoupling, which leads to inertial oscillations (Blackadar 1957;Shapiro and Fedorovich 2010). In contrast, near-surface winds (say, ,20 m) usually become weaker during the evening.…”