This paper presents a turbulence closure for neutral and stratified atmospheric conditions. The closure is based on the concept of the total turbulent energy. The total turbulent energy is the sum of the turbulent kinetic energy and turbulent potential energy, which is proportional to the potential temperature variance. The closure uses recent observational findings to take into account the mean flow stability. These observations indicate that turbulent transfer of heat and momentum behaves differently under very stable stratification. Whereas the turbulent heat flux tends toward zero beyond a certain stability limit, the turbulent stress stays finite. The suggested scheme avoids the problem of self-correlation. The latter is an improvement over the widely used Monin–Obukhov-based closures. Numerous large-eddy simulations, including a wide range of neutral and stably stratified cases, are used to estimate likely values of two free constants. In a benchmark case the new turbulence closure performs indistinguishably from independent large-eddy simulations.
A non-linear, two-dimensional, hydrostatic, incompressible numerical model with a higher-order turbulence closure scheme is used to study the effect of sea surface temperature on the severe downslope wind called bora at the Adriatic coast. A non-linear large-amplitude mountain wave is generated and is broken beneath and within its critical layer, due to resonant tuning between the initially single-layer atmosphere and the terrain. The tuning is governed by the Froude number. A qualitative and sometimes quantitative analogy exists between the wavebrealung (unsteady, stratified) flow and the hydraulic jump (steady, two-layer flow). It is also known that the strongest Adriatic bora appears during the winter season, when the sea surface temperature is typically larger than the ground surface temperature.Firstly, a relatively higher (lower) sea surface temperature means an additional distortion (moderation) of the mountain wave and consequently a larger (smaller) area with bora wind maxima. For a relatively higher sea surface temperature a propagating hydraulic jump occurs. Typically bora maxima are about three to four times larger than the related geostrophic wind (8 m s-l). Secondly, the presence and importance of the inertial oscillation are indicated. Since the wave-breaking is the vital component of the strongest bora cases, there is a relatively large, elevated area-i.e. the critical layer-with substantial flow decelerations and generally low wind speeds. The wave-breaking area has a Rossby number xO(1). Hence, the earth's rotation appears to be an important part of bora evolution. The simulations presented consider generalized bora cases which may pertain to other similar orographic flows.
KEYWORDS:Downslope wind Mountain wave Numerical model * Corresponding author: Villavagen 16, S75 136 Uppsala, Sweden.t Smith and Sun (1987) extended the theory to nonlinear steady-state solutions for stratified two-layer flows; however, wind shear was not accommodated.
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