Atmospheric Gravity Waves

Last updated on September 16, 2023

Atmospheric Gravity Waves: Exploring the Forces Shaping Our Skies

When we drop a stone into a pool of water, the resulting ripples are known as gravity waves. These waves occur when there are stable layers of fluids with different densities. Disturbing the boundary of the fluid causes buoyancy forces to restore equilibrium, resulting in oscillations that propagate as waves. This phenomenon is aptly named "gravity waves" due to the role gravity plays in their formation.

While gravity waves are commonly observed in water, they also abound in the upper atmosphere's stable density layering. Their effects become visible in various atmospheric phenomena, such as the intricate curls of nacreous clouds in the stratosphere, the mesmerizing patterns of noctilucent clouds in the mesosphere, and the slowly shifting bands of airglow in the thermosphere.

But what triggers these atmospheric gravity waves? The disturbances that set them in motion often originate far below in the troposphere. For example, wind flow over mountain ranges and violent thunderstorms can act as catalysts. Jet stream shear and solar radiation are also sources of these disturbances. Initially, these waves have small amplitudes at the tropopause, but as they propagate upwards, their amplitudes increase until they eventually break in the mesosphere and lower thermosphere. Remarkably, their wavelengths can span thousands of kilometers, while their periods range from a few minutes to several days.

Beyond their aesthetic value in shaping clouds, atmospheric gravity waves play a crucial role in transferring energy, momentum, and chemical species between different atmospheric layers. These transfers have significant implications for upper atmosphere winds, turbulence, temperature, and chemistry.

To delve further into the world of atmospheric gravity waves, let's explore some fascinating aspects of these phenomena:

1. Propagation and Dissipation

Atmospheric gravity waves propagate vertically through the atmosphere, carrying energy and momentum as they travel. As they encounter variations in temperature, pressure, and wind speed, they may undergo dispersion, refraction, and reflection. These interactions can lead to the dissipation of the waves or their transformation into other types of atmospheric waves.

2. Classification

Atmospheric gravity waves can be classified based on their source and the mechanism responsible for their generation. Some common classifications include mountain waves, lee waves, and internal gravity waves. Each type has distinct characteristics and can produce unique atmospheric effects.

3. Influence on Weather Patterns

The presence of atmospheric gravity waves can have a profound impact on weather patterns. These waves can alter wind patterns, induce vertical motion in the atmosphere, and even trigger the formation of clouds and precipitation. By understanding the behavior of gravity waves, meteorologists can gain insights into the dynamics of weather systems.

4. Global Distribution

Atmospheric gravity waves are not limited to specific regions but are found worldwide. They can be observed in diverse geographical locations, from mountainous regions to coastal areas. By studying the global distribution of these waves, scientists can gain a comprehensive understanding of their prevalence and characteristics across different climates.

5. Remote Sensing Techniques

To study atmospheric gravity waves, researchers employ various remote sensing techniques. These include satellite observations, radar systems, lidar (light detection and ranging), and radiosonde measurements. By analyzing the data obtained through these techniques, scientists can gather valuable insights into the properties and behavior of gravity waves.

6. Impact on Aviation

Atmospheric gravity waves can have significant implications for aviation. Pilots often encounter these waves during flights, particularly when flying over mountainous regions or areas prone to thunderstorm activity. The presence of gravity waves can cause turbulence, affecting the comfort and safety of passengers. Understanding these waves is crucial for improving aviation forecasting and mitigating potential hazards.

In conclusion, atmospheric gravity waves are captivating natural phenomena that shape our skies in mesmerizing ways. From their origins in disturbances far below in the troposphere to their effects on weather patterns and aviation, these waves play a crucial role in the dynamics of our atmosphere. By studying and understanding the behavior of gravity waves, scientists can unlock valuable insights into the intricate workings of our planet's atmospheric systems.

Drop a stone into a pool of water. The spreading ripples are gravity waves. The waves occur between any stable layers of fluids of different density. When the fluid boundary is disturbed, buoyancy forces try to restore the equilibrium. The fluid returns to its original shape, overshoots and oscillations then set in which propagate as waves. Gravity or buoyancy is the restoring force hence the term - gravity waves.

These waves (internal gravity or buoyancy waves) abound in the stable density layering of the upper atmosphere. Their effects are visibly manifest in the curls of the stratosphere’s nacreous clouds, in the moving skein-like and billow patterns of the mesosphere’s noctilucent clouds and in the slowly shifting bands of the thermosphere’s airglow.

What triggers them? The ‘stones into the pond’ are disturbances far below in the troposphere, for example, wind flow over mountain ranges and violent thunderstorms. Jet stream shear and solar radiation are other sources. An initial small amplitude at the tropopause increases with height until the waves break in the mesosphere and lower thermosphere. Their wavelengths can range up to thousands of kilometres. Their periods range from a few minutes to days.

They do more than give clouds interesting shapes. They are vital in their role of transferring energy, momentum and chemical species between the different atmospheric layers and in the subsequent influence on upper atmosphere winds, turbulence, temperature and chemistry.

Images:

Nacreous clouds, Antarctica - Cherie Ude

Noctilucent clouds, England - Chris Terran

Airglow, Kansas - Doug Zubenel

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