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Atmospheric stability is the capacity of the atmosphere to enhance or suppress turbulent motions. Atmospheric flow can be smooth in parallel layers or turbulent, in which the flow frequently changes speed and direction, creating gusts and eddies.

Changes in atmospheric stability can change laminar flow to turbulent, and turbulent flow to laminar. Such changes in flow patterns affect not only the turbulence characteristics, but also the wind shear in the atmospheric boundary layer (ABL), the lowest layer of the atmosphere that is in contact with Earth’s surface.

In terms of stability, the atmosphere can be classified as stable, neutral or unstable. This concept can be understood by considering the simple example of the balls in Figure 1. The blue ball is in stable equilibrium because if it is pushed up the slope and released, it will roll back to its original position. In contrast, if the red ball is pushed, it will roll down the slope, away from its original position and, therefore, is in unstable equilibrium. The green ball is in neutral equilibrium because if it is pushed, it will come to a rest instead of rolling back toward or away from its original position.

Now, consider an air parcel floating in the atmosphere that is slightly displaced by an external force, also known as a trigger. This parcel will either move back to its original position, stay in the new position or continue to move away from its original position in the direction of the force. The motion will depend on whether the atmosphere is stable, neutral or unstable, respectively. There are numerous triggers in the ABL, including topography, solar heating and flow obstructions, such as buildings and trees. The disturbances caused by these triggers are damped out in a stable environment.

However, in an unstable environment, disturbances grow over time, thereby changing the turbulence and wind-shear characteristics of the ABL and giving rise to atmospheric phenomena such as thermals and billow clouds. Turbulent motion in the atmosphere is generated due to buoyancy and wind shear. Atmospheric stability is classified as static or dynamic, based on which factor is dominant.

Static stability. Static stability demonstrates whether the atmosphere will encourage buoyant convection. Consider a dry air parcel that is lifted upwards by a trigger from position A to position B (see Figure 2). Because the parcel is moving to a place of lower pressure, it will expand and cool as it rises. The rate of cooling, known as the Dry Adiabatic Lapse Rate (DALR), is -9.8°C/km for the atmosphere.

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The actual observed rate at which the atmosphere cools with increasing altitude is known as the Environmental Lapse Rate (ELR). The ELR varies in space and time depending on atmospheric conditions. In a statically stable environment, the ELR is greater (less negative) than the DALR (i.e., a rising parcel cools faster with height than its environment). Therefore, the parcel at position B will be cooler than its surroundings and, consequently, will sink back to its original position (position A).

In a statically neutral environment, the ELR is equal to the DALR. Here, a displaced parcel maintains the same temperature as that of its environment as the air parcel is displaced. When brought to rest, the air parcel will stay in its new position. In an unstable environment, the ELR is less (more negative) than the DALR (i.e., the environment cools faster with height than the rising parcel does).

In this case, the parcel moved to position B will be warmer than its surroundings and will continue to rise. This tendency for vertical acceleration of displaced parcels in an unstable layer creates turbulent “mixing” that works to eliminate the unstable condition, which is why the ELR is rarely lower than the DALR for long periods of time.

The ELR changes over time due to many factors, the most important of which is the diurnal cycle. Solar heating during the daytime warms the ground. This, in turn, warms near-surface air, which leads the temperature to decrease more quickly with height and, thus, decreases stability. Static stability dramatically changes over the course of the day. The atmosphere is usually stable at night and unstable during the day. The atmosphere can be nearly neutral in the early afternoon when the ABL is well mixed. Even under this condition, a shallow, unstable layer always exists near the ground. That is why a perfectly neutral environment across the rotor plane of a wind turbine is rare and lasts for only a few moments. Stability is also strongly affected by large-scale weather patterns.

For example, an influx of cool air near the surface or warm air arriving aloft can stabilize an otherwise unstable environment. Finally, as described in the next section, turbulence can also be generated in a statically stable environment at night due to dynamic instability.

Dynamic stability. Dynamic stability is the capability of the atmosphere to generate turbulence in the presence of strong winds. A statically unstable flow is always dynamically unstable. Dynamic instability can also occur when the flow is statically stable.

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For example, consider a statically stable environment with a layer of warm air overlying colder air and strong wind shear across the interface (see Figure 3). Initially, the flow is laminar, but if the shear is high enough, the flow becomes dynamically unstable. Waves form and grow in amplitude and eventually break. These breaking waves are called Kelvin-Helmholtz (KH) waves.

During the breaking process, warm air gets folded below cold air within each wave, giving rise to a localized zone of static instability. This combination of dynamic and static instabilities leads to rapid growth of turbulence, such that the entire breaking layer becomes turbulent within a few seconds.

Usually, dynamically generated turbulence is intermittent and confined to small regions. Because of the wave-like shape, this turbulence is referred to as coherent, unlike statically generated turbulence, which has no typical spatial pattern. Nocturnal jets, such as the Great Plains low-level jet, can lead to KH waves that produce a sustained, vigorously turbulent layer hundreds of kilometers wide but only tens of meters thick.

 

Stability and wind power

The effects of stability on turbulence and wind shear are relevant to wind power applications. By definition, atmospheric turbulence is strongly correlated with stability. A stable atmosphere that suppresses turbulent triggers is the least turbulent, while an unstable atmosphere that allows triggers to grow is the most turbulent.

Turbulence generated in unstable environments – especially KH waves caused by dynamic instability – can affect turbine load and performance. Studies show that sudden bursts of coherent turbulence in KH waves can generate high damage-equivalent loads in the turbine blades and drivetrains, as well as high vibrations that cause turbine SCADA systems to shut turbines down in order to prevent damage. Therefore, KH waves could be responsible for many unexplained turbine failures that occur in the evening and early morning.

Turbulence increases mixing between air parcels at different heights, thereby reducing the vertical variability of wind. Therefore, a stable atmosphere with low turbulence leads to high wind shear, both in direction and magnitude, while an unstable atmosphere produces the least amount of shear. High wind shear implies a rapid change in wind speed with height. Thus, in more stable environments, hub-height wind speeds tend to be higher than the surface winds.

There is no ideal stability condition for wind farm operations because the effects of turbulence and shear often offset each other. A very stable environment creates torque across the rotor that could cause fatigue. An unstable environment yields less shear, but the concurrent high turbulence and gustiness place other stresses on wind generators, causing more wear and tear in the pitch and yaw systems.

Information about atmospheric stability conditions and their impact on turbulence and wind shear can help wind farm operators in many different ways. For instance, the information can improve predictions of wind speed, power production and the load on rotors and drivetrains. Analysis of stability impacts on an individual wind farm’s production can provide guidance for long-term operation and maintenance plans. Improving the representation of stability in computer models can significantly improve wind-speed and power forecasts.

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When meteorologists talk about stability, they typically mean static stability. In fact, most meteorology textbooks discuss only static stability, and hardly even mention dynamic stability. Static instability can lead to many severe weather phenomena. Therefore, many industries are interested in real-time static-stability data that is readily available in the public domain. Dynamic instability affects the wind power industry even more than it impacts other industries. However, dynamic-instability data is not widely available.

The wind industry can benefit significantly from collecting its own data on dynamic instability by deploying a network of surface weather stations, meteorological towers and, if budgets permit, remote sensing instruments, such as LIDAR. This data can help wind farm owners and operators better prepare for the possible sudden onset of KH waves.

 

Somnath Baidya Roy teaches atmospheric sciences at the University of Illinois. He can be reached at sbroy@atmos.uiuc.edu. Justin Sharp is a principal and owner of Sharply Focused, a wind energy consulting firm. He can be reached at justin@sharply-focused.com.

Industry At Large: Wind Assessment

Why Atmospheric Stability Matters In Wind Assessment

By Somnath Baidya Roy & Justin Sharp

Understanding atmospheric stability can help wind farm owners and operators improve predictions of wind speed, power production and the load on rotors and drivetrains.

 

 

 

 

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