Coriolis effects in mesoscale flows with sharp changes in surface conditions

Abstract

SUMMARYA general linearized 'shallow-layer' perturbation model, where the approximately neutral lower layer of thickness h 0 is situated below a stable upper layer (i.e. an inversion with temperature change T ), is developed for steady, mesoscale atmospheric flows over low-lying topography whose height is less than h 0 . With the Coriolis parameter f , sharp changes in surface conditions (surface roughness, terrain elevation, heat flux) are modelled as a distributed body force through the lower layer. The Froude number of this layer is small. Typical cases of mesoscale discontinuities are examined. The results are compared with those of a continuously stratified model and observations, and with numerical mesoscale model results for a meteorological case-study over the Dover Straits region of the English Channel. The main results are:(i) If the wind direction is parallel to the edge-line separating the change in surface roughness, there are marked increases and decreases in these coastal winds whose maxima can occur over the sea within a distance of order h 0 (∼1 km) of a coast. The strength of these wind 'jets', which do not occur in the absence of Coriolis force, decrease away from the edge-line gradually over transverse length-scales of the order of the Rossby deformation radiusChanges to surface roughness lead to an increase in the wind speed perturbation in the downwind direction until limited by non-linear effects. When the wind is at an angle to a roughness change or coast, the maxima occur at the coastline.(ii) Where there are sharp changes in the orientation of contours of constant roughness length (e.g. at capes or bays on the coastline or wakes of high-drag areas), 'detached' jets are formed in the downwind direction.(iii) Changes in surface elevation at a coast produce effects different from those of roughness; a positive wind jet forms parallel to the coast in the direction of the wind when the coast is on the right (looking downwind) and a negative jet when the coast is on the left. These jets do not increase in strength along the flow and do not persist downwind.(iv) Coriolis effects also determine how the inversion height varies near coastlines and surface roughness changes; for example, increasing/decreasing inland over a distance L R when stable airflow approaches from the sea and the coast is on the right/left of an observer looking downwind (opposite in the southern hemisphere). This mechanism is consistent with observed increasing/decreasing cloudiness inland from a coast.(v) Other effects occur where the surface elevation changes gradually over a distance of order L R (e.g. a wide, shallow valley); frictional effects are comparable with buoyancy and Coriolis forces, and flows perpendicular to the elevation change are deflected to the left (in the northern hemisphere), as observed in the Rhine valley.(vi) The shallow-layer model simulates the major features of the low-level flow field computed using the numerical mesoscale model with a horizontal resolution of 2 km, i.e. of order h 0 . Broad ...

Characteristics of Stable Flows over Southern Greenland

“…The 10-m wind ECMWF data for 00UTC show the flow being deflected as it passes the tip, though it gives a marginally stronger speed up in the wake of around 18 ms À1 . Note that in other synoptic situations over southern Greenland with a more uniform approach flow (e.g., HUNT et al, 2004-9 November, 2000, westerly flows lead to southerly winds in the wake of the easterly side of Greenland (as in the idealised and numerical results of Fig. 3(b)(i)).…”

Coriolis effects on wind jets and cloudiness along coasts

“…The overall wind profile across the channel is consistent with it being formed by two main elements, namely the coastal jet in the northern side and a detached jet from Cap Griz Nez. When the wind backs and is not exactly parallel to the Kent coast, the coastal wind jet on the northern side is transformed into a detached jet which forms along the streamline passing the tip of Kent (see Hunt et al 2004). Examination of the vertical structure of the jet showed that the maximum wind was at a height of about 200 m and about 16-17 m s -1 (not shown).…”

“…the Coriolis force induces a wind jet parallel to the coastline and perpendicular to the sloping inversion height. This is explained by the equation |g′∂h/∂n|~|-f _ u S | (from equation (7(f )) of Hunt et al (2004)), where s and n are the directions parallel and perpendicular to the coastline respectively. Here _ u S denotes the average change in wind speed over the depth of the inversion layer, parallel to the coast.…”

Boundary-layer development and gravity waves in conventionally neutral wind farms

“…A number of authors have extended the traditional surface-layer approach to meso-scale flows, i.e. including Coriolis effects and the full depth of the ABL, and considered larger downwind fetches (Taylor 1969;Jensen 1978;Wright et al 1998;Hunt et al 2004). These studies predicted that the change in surface stress is accompanied by a change in surface wind direction.…”

“…the turbulent stress adapts rapidly to the new roughness and spreads up from the surface, whereas the changes in wind direction occur more uniformly across the boundary layer and take place at larger downwind fetches. Moreover, Hunt et al (2004) showed that variations in surface roughness also affect the inversion height and the pressure field. In the current work, we choose a sufficiently large simulation domain to study these effects, and their influence on wind farm power extraction in the context of a wind farm-induced roughness change.…”