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.

The response of bora-type flow to sea surface temperature

Boundary layer dynamics in an urban coastal environment under sea breeze conditions

The influence of the main large-scale wind directions on thermally driven mesoscale circulations at the Baltic southwest coast, southeast of Sweden, is examined. The aim of the study is to highlight small-scale alterations in the coastal atmospheric boundary layer. A numerical three-dimensional mesoscale model is used in this study, which is focused on an overall behaviour of the coastal jets, drainage flows, sea breezes, and a low-level eddy-type flow in particular. It is shown that synoptic conditions, together with the moderate terrain of the southeast of Sweden (max. height h0 ≤ 206 m), governs the coastal mesoscale dynamics triggered by the land-sea temperature difference Δ T. The subtle nature of coastal low-level jets and sea breezes is revealed; their patterns are dictated by the interplay between synoptic airflow, coastline orientation, and Δ T.

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Small-Scale Variability in the Coastal Atmospheric Boundary Layer

A mesoscale numerical model is used to study the atmospheric boundary-layer (ABL) response to nonlinear orographic forcing with Coriolis effect, f, over a mountain with length (the cross-wind component) comparable to the Rossby radius of deformation, LR. The orographic-wave breaking occurring for Froude number Fr 0, intensifies on the northern flank for westerly flows, as also found in other recent studies. A cumulative effect occurs as the Coriolis force lifts the northern ABL top and generates a stronger low-level jet (LLJ) than on the southern side. A differential layering also appears, since the specific humidity is higher in the lower southern ABL than in the related northern ABL, and vice versa. By contrast, there are higher values of the turbulent kinetic energy and humidity in the upper northern ABL.



The breaking of flow symmetry around the orography due to f changes both the vertical vorticity and horizontal divergence field, (ζ, D), it modulates eddies and turbulence leading to the differential layering of the ABL. The stronger northern LLJ and its weaker southern counterpart, both meandering, together with the asymmetric wave breaking, induce strong lee-side fluctuations of the (ζ, D) field in the presence of f. The enhanced (ζ, D) production due to wave breaking over the distance ≈ LR, the primary atmosphere–orography resonance occurs mainly in the vertical, while the ‘f-enhancement’ occurs in the horizontal plane. In this way, the initial mesoscale forcing may extend its effects over the synoptic scale. Copyright © 2004 Royal Meteorological Society

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Boundary-layer variations due to orographic-wave breaking in the presence of rotation

An incompressible, mesoscale model is used to estimate wave drag (WD) profiles over inhomogeneous 2D terrain. The goal is twofold: to evaluate the WD based on the model's fields and to analyze the atmospheric boundary layer (ABL) response to wave breaking. The model employs a simplified higher-order closure scheme for turbulent fluxes. A sponge layer mimics a radiative upper boundary condition (BC). Due to the no-slip lower BC for dissipative flows, the Eliassen-Palm theorem is not fulfilled and WD is generally not constant with height. Within the lower troposphere, the model's mean WD values compare to those from theory and other simulations. Above the ABL, where no physical processes dissipate waves, WD attenuates with height to roughly one-third of its theoretical value. This is mainly due to numerical dissipation of the used first-order advection scheme. However, the acceleration identified with the wave pattern reduction is small compared to governing accelerations. Two simulation sets are p...

Pär Holmgren

Papers

An advection algorithm and an atmospheric airflow application