Boundary layer

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The last force that has an important effect on air movement is that due to friction from the earth's surface. Towards the surface (i.e. below about 500 m for flat terrain), friction begins to reduce the wind velocity below its geostrophic value. The slowing of the wind towards the surface modifies the deflective force, which is dependent on velocity, causing it also to decrease. Initially, the frictional force is opposite to the wind velocity, but in a balanced state - when the velocity and therefore the Coriolis deflection decrease - the vector sum of the Coriolis and friction components balances the pressure gradient force (Figure 6.3B). The friction force now acts to the right of the surface wind vector. Thus, at low levels, due to frictional effects, the wind blows obliquely across the isobars in the direction of the pressure gradient. The angle of obliqueness increases with the growing effect of frictional drag due to the earth's surface averaging about 10 to 20° at the surface over the sea and 25 to 35° over land.

In summary, the surface wind (neglecting any curvature effects) represents a balance between the pressure-gradient force and the Coriolis force perpendicular to the air motion, and friction roughly parallel, but opposite, to the air motion.

The layer of frictional influence is known as the planetary boundary layer (PBL). Atmospheric profilers (lidar and radar) can routinely measure the temporal variability of PLB structure. Its depth varies over land from a few hundred metres at night, when the air is stable as a result of nocturnal surface cooling, to 1 to 2 km during afternoon convective conditions. Exceptionally, over hot dry surfaces, convective mixing may extend to 4 to 5 km. Over the oceans, it is more consistently near 1 km deep and in the tropics especially is often capped by an inversion due to sinking air. The boundary layer is typically either stable or unstable. Yet, for theoretical convenience, it is often treated as being neutrally stable (i.e. the lapse rate is that of the DALR, or the potential temperature is constant with height; see Figure 5.1). For this ideal state, the wind turns clockwise

Ekman Boundary Layer Convergence

Figure 6.5 The Ekman spiral of wind with height, in the northern hemisphere. The wind attains the geostrophic velocity at between 500 and 1000 m in the middle and higher latitudes as frictional drag effects become negligible. This is a theoretical profile of wind velocity under conditions of mechanical turbulence.

Figure 6.5 The Ekman spiral of wind with height, in the northern hemisphere. The wind attains the geostrophic velocity at between 500 and 1000 m in the middle and higher latitudes as frictional drag effects become negligible. This is a theoretical profile of wind velocity under conditions of mechanical turbulence.

(veers) with increased height above the surface, setting up a wind spiral (Figure 6.5). This spiral profile was first demonstrated in the turning of ocean currents with depth (see Chapter 7D1.a) by V. W. Ekman; both are referred to as Ekman spirals. The inflow of air towards the low-pressure centre generates upward motion at the top of the PBL, known as Ekman pumping.

Wind velocity decreases exponentially close to the earth's surface due to frictional effects. These consist of 'form drag' over obstacles (buildings, forests and hills), and the stress exerted by the air at the surface

Table 6.1 Typical roughness lengths (m) associated with terrain surface characteristics.

Terrain surface characteristics Roughness length (m)

Groups of high buildings 1-10

Temperate forest 0.8

Groups of medium buildings 0.7

Suburbs 0.5

Trees and bushes 0.2

Farmland 0.05-0.1

Grass 0.008

Bare soil 0.005

Snow 0.001

Smooth sand 0.0003

Water 0.0001

Source: After Troen and Petersen (1989).

interface. The mechanism of form drag involves the creation of locally higher pressure on the windward side of an obstacle and a lateral pressure gradient. Wind stress arises from, first, the molecular resistance of the air to the vertical wind shear (i.e. increased wind speed with height above the surface); such molecular viscosity operates in a laminar sub-layer only millimetres thick. Second, turbulent eddies, a few metres to tens of metres across, brake the air motion on a larger scale (eddy viscosity). The aerodynamic roughness of terrain is described by the roughness length (z0), or height at which the wind speed falls to zero based on extrapolation of the neutral wind profile. Table 6.1 lists typical roughness lengths.

Turbulence in the atmosphere is generated by the vertical change in wind velocity (i.e. a vertical wind shear), and is suppressed by an absence of buoyancy. The dimensionless ratio of buoyant suppression of turbulence to its generation by shear, known as the Richardson number (Ri), provides a measure of dynamic stability. Above a critical threshold, turbulence is likely to occur.

Divergence And Convergence Meteorology

Figure 6.6 Convergence and divergence. (A) Plan view of horizontal flow patterns producing divergence and convergence - the broken lines are schematic isopleths of wind speed (isotachs). (B) Schematic illustration of local mass divergence and convergence, assuming density changes. (C) Typical convergence-stretching and divergence-shrinking relationships in atmospheric flow.

Figure 6.6 Convergence and divergence. (A) Plan view of horizontal flow patterns producing divergence and convergence - the broken lines are schematic isopleths of wind speed (isotachs). (B) Schematic illustration of local mass divergence and convergence, assuming density changes. (C) Typical convergence-stretching and divergence-shrinking relationships in atmospheric flow.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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  • roan hughes
    How is global warming related to wind deflection?
    2 years ago

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