The response of bora‐type flow to sea surface temperature

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 wave‐breaking (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−1). 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 O(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.