Coastal Convergence - Meteorological-Physical Background

[Introduction]

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The coastline generally represents a marked discontinuity in surface roughness. The resulting mechanical forcing leads to a secondary circulation in the boundary layer, leading to localised vertical motion which may, in turn, have a strong influence on weather in the coastal zone.

This chapter describes a flow in which friction is the main driving force for cloud development. Stau Cloudiness and Lee Cloudiness conceptual models describe circulations primarily caused by more distinct orography (mountains and hills). It has to be kept in mind, however, that other factors besides friction, such as the differential heating between land and sea or the topography and shape of the coast have a marked influence on the dynamics of Coastal Convergence. Purely thermally driven circulation on the coast is called the Sea-Breeze.

[Description]

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The image above shows the ideal hodograph representing the well-known Ekman spiral. The wind profile is generally unstable for a neutrally buoyant atmosphere. The secondary circulations which develop as a result of this instability tend to flatten the above-shown hodograph, that is, to decrease the ageostrophic component. However, a distinct ageostrophic component of the wind (v/ug) still remains in the boundary layer. The velocity of this ageostrophic component depends on the roughness of the underlying surface. Over land the roughness length z₀ is in the order of few centimeters while for sea surface z₀ is only a few millimeters. Due to this the wind backs more over land than over sea.

Friction not only causes the wind to back but also slows down the air flow in the boundary layer. Wind blowing from sea to land is decelerated, while the opposite is true for wind blowing offshore. The adjustment of wind speed to changes in surface roughness is a much faster process than the adjustment of wind direction. Typically, a geostrophic wind having a speed of 10 m/s is adjusts itself at 20 km distance from coast. By contrast, wind direction is adjusted much slower, length scale typically being on the order of 500 km.

The differences in both the speed and direction of wind across the coast lead to small-scale convergent and divergent areas and associated vertical motions. The locations of convergence-divergence driven vertical motions naturally depend on the orientation of the coastline with respect to the flow. The basic types of frictional convergence can be described schematically as follows:

[Schematic Image(s) ]

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