THE RESPONSE OF BORA-TYPE FLOW TO SEA-SURFACE TEMPERATURE

A non-linear, two-dimensional, hydrostatic, incompressible numerical m odel with a higher-order turbulence closure scheme is used to study th e effect of sea surface temperature on the severe downslope wind calle d bora at the Adriatic coast. A non-linear large-amplitude mountain wa ve is generated and is broken beneath and within its critical layer, d ue to resonant tuning between the initially single-layer atmosphere an d the terrain. The tuning is governed by the Froude number. A qualitat ive and sometimes quantitative analogy exists between the wave-breakin g (unsteady, stratified) flow and the hydraulic jump (steady, two-laye r flow). It is also known that the strongest Adriatic bora appears dur ing the winter season, when the sea surface temperature is typically l arger than the ground surface temperature. Firstly, a relatively highe r (lower) sea surface temperature means an additional distortion (mode ration) of the mountain wave and consequently a larger (smaller) area with bore 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 s trongest bera cases, there is a relatively large, elevated area-i.e. t he critical layer-with substantial flow decelerations and generally lo w wind speeds. The wave-breaking area has a Rossby number approximate to O(1). Hence, the earth's rotation appears to be an important part o f bora evolution. The simulations presented consider generalized bora cases which may pertain to other similar orographic flows.