Glossary

11.1 Gravity waves

9 min read•august 21, 2024

Gravity waves are crucial atmospheric oscillations that transfer energy and momentum vertically. These waves, caused by buoyancy forces, play a vital role in weather patterns, climate dynamics, and atmospheric circulation. Understanding gravity waves helps explain various phenomena and improves predictions.

From their generation by topography and convection to their impact on cloud formation and atmospheric mixing, gravity waves influence multiple layers of our atmosphere. Their effects span from local weather to global climate patterns, making them a key focus in atmospheric physics research.

Fundamentals of gravity waves

  • Gravity waves play a crucial role in atmospheric dynamics by transferring energy and momentum vertically
  • Understanding gravity waves helps explain various atmospheric phenomena and improves weather and climate predictions
  • Gravity waves occur in both the atmosphere and oceans, demonstrating the interconnectedness of Earth's fluid systems

Definition and characteristics

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  • Oscillations in fluid systems where buoyancy acts as the restoring force
  • Characterized by vertical displacement of air parcels and associated pressure perturbations
  • Wavelengths range from a few kilometers to hundreds of kilometers
  • Periods typically vary from minutes to several hours
  • increases with altitude due to decreasing air density

Sources of gravity waves

  • Topographic features (mountains, hills) generate waves as air flows over them
  • Convective activity (thunderstorms, frontal systems) produces gravity waves
  • Wind shear in jet streams can trigger wave formation
  • Geostrophic adjustment processes in large-scale atmospheric flows
  • Tsunamis and earthquakes can induce atmospheric gravity waves

Wave propagation mechanisms

  • Vertical propagation occurs when waves transport energy upward through the atmosphere
  • Horizontal propagation allows waves to travel long distances from their source
  • Ducting happens when waves become trapped between atmospheric layers
  • Critical level filtering occurs when background wind speed matches wave
  • Wave reflection can happen at sharp density or wind speed gradients

Gravity wave dynamics

  • dynamics govern how these waves behave and interact with the atmosphere
  • Understanding these dynamics is crucial for predicting wave impacts on atmospheric circulation
  • Gravity wave dynamics involve complex interactions between wave properties and background atmospheric conditions

Dispersion relation

  • Describes the relationship between wave frequency and wavenumber
  • For gravity waves, dispersion relation is given by ω2=N2k2k2+m2+f2m2k2+m2\omega^2 = N^2\frac{k^2}{k^2 + m^2} + f^2\frac{m^2}{k^2 + m^2}
  • ω\omega represents wave frequency, NN is the Brunt-Väisälä frequency
  • kk and mm are horizontal and vertical wavenumbers, respectively
  • ff is the Coriolis parameter, important for large-scale waves

Energy and momentum transport

  • Gravity waves carry energy and momentum vertically through the atmosphere
  • Energy flux is proportional to the product of pressure and vertical velocity perturbations
  • Momentum flux is given by u′w′‾\overline{u'w'}, where u′u' and w′w' are horizontal and vertical velocity perturbations
  • Wave breaking deposits energy and momentum into the background flow
  • This transport mechanism drives important atmospheric circulation patterns (QBO, mesospheric jets)

Wave breaking processes

  • Occurs when wave amplitude grows large enough to cause instability
  • Convective instability happens when wave-induced temperature gradient exceeds adiabatic lapse rate
  • Dynamic instability occurs when wave-induced wind shear becomes too strong
  • Wave breaking leads to and mixing in the atmosphere
  • Breaking waves deposit momentum, altering the mean flow in the upper atmosphere

Atmospheric impacts

  • Gravity waves significantly influence atmospheric structure and dynamics across various scales
  • Their effects are particularly pronounced in the middle and upper atmosphere
  • Understanding these impacts is crucial for accurate weather and climate modeling

Vertical mixing and transport

  • Enhance vertical mixing of atmospheric constituents (trace gases, aerosols)
  • Contribute to the transport of water vapor and other greenhouse gases to the upper atmosphere
  • Facilitate the exchange of heat and momentum between atmospheric layers
  • Influence the vertical distribution of ozone in the stratosphere
  • Play a role in the formation and maintenance of the atmospheric boundary layer

Temperature and wind variations

  • Induce local temperature fluctuations through adiabatic heating and cooling
  • Create periodic wind speed and direction changes in the mesosphere and lower thermosphere
  • Contribute to the formation of temperature inversions in the stratosphere
  • Generate small-scale turbulence, affecting local heat distribution
  • Influence the occurrence of sudden stratospheric warmings

Cloud formation and precipitation

  • Trigger cloud formation by lifting air parcels to their condensation level
  • Create wave cloud patterns (lenticular clouds, billow clouds)
  • Enhance precipitation in mountainous regions through orographic lifting
  • Influence the development and organization of convective systems
  • Contribute to the formation of polar stratospheric clouds in the winter stratosphere

Observation techniques

  • Observing gravity waves is challenging due to their wide range of scales and transient nature
  • Multiple observation techniques are employed to capture different aspects of gravity waves
  • Combining various observation methods provides a comprehensive understanding of wave characteristics

Ground-based instruments

  • Radar systems (MST radar, MF radar) measure wind velocities and wave structures
  • Lidar technology detects temperature and density perturbations associated with gravity waves
  • Airglow imagers capture wave-induced variations in atmospheric emissions
  • Infrasound arrays detect low-frequency acoustic waves generated by large-scale gravity waves
  • Radiosondes provide vertical profiles of temperature, pressure, and wind affected by waves

Satellite measurements

  • Limb-sounding instruments measure temperature and trace gas profiles, revealing wave structures
  • Microwave sensors detect temperature fluctuations associated with gravity waves
  • GPS radio occultation techniques provide high-resolution vertical profiles of atmospheric properties
  • Imaging spectrometers observe wave-induced variations in airglow emissions
  • Synthetic aperture radar captures ocean surface patterns influenced by atmospheric gravity waves

Balloon and aircraft observations

  • High-altitude research balloons measure in-situ temperature and wind variations
  • Constant-level balloons track horizontal wave propagation in the stratosphere
  • Research aircraft equipped with specialized instruments detect wave-induced turbulence
  • Dropsondes released from aircraft provide vertical profiles of wave activity
  • Airborne lidar systems measure wave-induced temperature and wind perturbations

Modeling gravity waves

  • Accurate representation of gravity waves in atmospheric models is crucial for realistic simulations
  • Modeling approaches range from parameterizations to high-resolution explicit simulations
  • Ongoing research aims to improve gravity wave representation in weather and climate models

Parameterization in climate models

  • Gravity wave drag parameterizations account for unresolved wave effects on large-scale flow
  • Orographic gravity wave schemes represent waves generated by subgrid-scale topography
  • Non-orographic gravity wave parameterizations account for waves from other sources (convection, fronts)
  • Tuning parameters in these schemes helps improve model performance and climate simulations
  • Recent developments include stochastic parameterizations to represent wave variability

High-resolution simulations

  • Mesoscale models with grid spacings of a few kilometers can resolve larger gravity waves
  • Large Eddy Simulations (LES) capture small-scale wave dynamics and breaking processes
  • Global cloud-resolving models are beginning to explicitly represent a wide spectrum of gravity waves
  • High-resolution simulations provide insights into wave generation, propagation, and dissipation mechanisms
  • Results from these simulations inform the development of improved parameterizations for coarser models

Challenges in wave representation

  • Wide range of spatial and temporal scales makes it difficult to capture all relevant wave processes
  • Intermittency and localized nature of gravity wave sources pose challenges for global models
  • Nonlinear interactions between waves and mean flow are complex to represent accurately
  • Limited observations make it challenging to validate model simulations of gravity waves
  • Computational constraints restrict the resolution of global models, necessitating parameterizations

Gravity waves in different layers

  • Gravity waves exhibit distinct characteristics and impacts in various atmospheric layers
  • Understanding these layer-specific behaviors is crucial for comprehending the overall atmospheric dynamics
  • Wave interactions between different layers contribute to vertical coupling in the atmosphere

Tropospheric gravity waves

  • Often generated by topography, convection, and frontal systems
  • Influence local weather patterns and precipitation distribution
  • Contribute to clear-air turbulence, affecting aviation safety
  • Play a role in organizing convective systems and squall lines
  • Interact with boundary layer processes, affecting surface weather conditions

Stratospheric gravity waves

  • Important for driving the quasi-biennial oscillation (QBO) in the tropical stratosphere
  • Contribute to the formation and breakdown of the polar vortex
  • Influence the transport and mixing of ozone and other trace gases
  • Play a role in the development of sudden stratospheric warming events
  • Affect the propagation of planetary waves from the troposphere to the mesosphere

Mesospheric gravity waves

  • Drive the mesospheric circulation, including the reversal of the zonal mean flow
  • Contribute to the cold summer mesopause phenomenon
  • Influence the formation and dynamics of noctilucent clouds
  • Play a crucial role in the vertical coupling between the lower and upper atmosphere
  • Affect the distribution of meteoric dust and metal layers in the upper mesosphere

Interactions with other phenomena

  • Gravity waves interact with various atmospheric phenomena, influencing global circulation patterns
  • These interactions often involve complex feedback mechanisms and energy exchanges
  • Understanding these relationships is crucial for accurate atmospheric modeling and prediction

Gravity waves vs planetary waves

  • Gravity waves have smaller scales and higher frequencies compared to planetary waves
  • Both wave types contribute to momentum transport in the middle atmosphere
  • Planetary waves can modulate gravity wave propagation and breaking
  • Gravity waves influence the vertical propagation of planetary waves
  • Interactions between these wave types affect the timing of sudden stratospheric warmings

Gravity waves and atmospheric tides

  • Gravity waves can be modulated by atmospheric tides, affecting their propagation characteristics
  • Tidal winds filter gravity waves, leading to longitudinal variations in wave activity
  • Both gravity waves and tides contribute to the momentum budget of the mesosphere and lower thermosphere
  • Interactions between gravity waves and tides influence the variability of the ionosphere
  • Gravity wave breaking can generate secondary waves that interact with tidal structures

Coupling with ionospheric processes

  • Gravity waves propagating from the lower atmosphere can reach the ionosphere
  • Wave-induced perturbations in the neutral atmosphere affect ionospheric plasma dynamics
  • Contribute to the formation of traveling ionospheric disturbances (TIDs)
  • Influence the development of equatorial plasma bubbles and spread F phenomena
  • Play a role in the day-to-day variability of the ionosphere, affecting radio wave propagation

Climate and weather implications

  • Gravity waves have significant impacts on both short-term weather and long-term climate patterns
  • Their effects span from local to global scales, influencing various atmospheric processes
  • Understanding these implications is crucial for improving weather forecasts and climate projections

Role in general circulation

  • Drive the quasi-biennial oscillation (QBO) in the tropical stratosphere
  • Contribute to the maintenance of the middle atmosphere circulation
  • Influence the strength and variability of the polar vortex
  • Play a role in the Brewer-Dobson circulation, affecting ozone distribution
  • Impact the mesospheric residual circulation, driving the cold summer mesopause

Influence on weather patterns

  • Affect the development and intensity of extratropical cyclones
  • Contribute to the formation and evolution of frontal systems
  • Influence the distribution and intensity of precipitation, especially in mountainous regions
  • Play a role in the initiation and organization of convective storms
  • Impact the formation and dissipation of marine stratocumulus clouds

Long-term climate effects

  • Modulate the transport of heat and momentum in the atmosphere, affecting global energy balance
  • Influence the distribution of greenhouse gases, particularly water vapor in the upper troposphere
  • Contribute to the variability of the stratospheric polar vortex, affecting surface climate patterns
  • Play a role in the coupling between the troposphere and stratosphere, impacting climate modes (NAO, AO)
  • Affect the vertical distribution of ozone, influencing long-term trends in stratospheric temperatures

Current research topics

  • Gravity wave research remains an active field with numerous ongoing investigations
  • Current studies aim to address knowledge gaps and improve our understanding of wave processes
  • Advancements in observation techniques and modeling capabilities drive new research directions

Gravity wave hotspots

  • Investigating regions of intense gravity wave activity (Southern Andes, Antarctic Peninsula)
  • Studying the impact of these hotspots on global atmospheric circulation patterns
  • Examining the role of gravity wave hotspots in stratospheric warming events
  • Analyzing seasonal and interannual variability of wave activity in these regions
  • Developing improved parameterizations to account for hotspot effects in global models

Climate change impacts

  • Assessing how changing temperature gradients affect gravity wave generation and propagation
  • Investigating the potential feedback between gravity waves and polar stratospheric clouds
  • Studying the role of gravity waves in the response of the middle atmosphere to increased CO2
  • Examining how changes in convective patterns influence gravity wave spectra
  • Analyzing the impact of gravity waves on the lifting of the tropopause in a warming climate

Improved detection methods

  • Developing new satellite instruments for high-resolution gravity wave observations
  • Advancing machine learning techniques for identifying gravity wave signatures in data
  • Improving ground-based networks for continuous monitoring of gravity wave activity
  • Enhancing data assimilation methods to incorporate gravity wave observations in models
  • Exploring the potential of using commercial aircraft as platforms for gravity wave detection

Key Terms to Review (18)

Amplitude: Amplitude refers to the maximum extent of a vibration or oscillation, measured from the position of equilibrium. It plays a crucial role in understanding various atmospheric phenomena, influencing factors like intensity and energy propagation in sound and wave systems. Higher amplitudes generally indicate greater energy and intensity, which can impact weather patterns and atmospheric behavior significantly.
Buoyancy frequency: Buoyancy frequency, also known as the Brunt-Väisälä frequency, is a measure of the stability of a fluid and how quickly it can return to equilibrium after being displaced. It quantifies the oscillation frequency of a parcel of fluid that is displaced vertically in a stratified fluid environment, which is crucial for understanding the propagation of gravity waves.
Gravity wave: A gravity wave is a type of wave generated in a fluid medium when the force of gravity tries to restore equilibrium after the fluid has been disturbed. These waves are commonly observed in atmospheric conditions and can occur in both oceans and the atmosphere, playing a crucial role in energy transfer and the dynamics of weather systems.
Henri Poincaré: Henri Poincaré was a French mathematician, theoretical physicist, and philosopher of science, recognized for his foundational contributions to various fields, including dynamical systems and the theory of relativity. His work laid the groundwork for understanding gravity waves, particularly in how they relate to the behavior of physical systems and the mathematical framework that governs them.
Hydrostatic balance: Hydrostatic balance refers to the state where the gravitational force acting on an air column is exactly balanced by the pressure gradient force within that column. This equilibrium is crucial in understanding how atmospheric pressure varies with altitude and plays a significant role in phenomena such as weather patterns and temperature distribution. By maintaining this balance, the atmosphere achieves stability, which affects the behavior of air masses and wave propagation.
Inertia-gravity wave: An inertia-gravity wave is a type of wave in fluid dynamics that arises from the interplay between gravity and the inertia of the fluid. These waves are characterized by oscillations that occur when a displaced fluid parcel returns to its equilibrium position due to gravitational forces, while also being affected by the inertia of the fluid as it tries to resist that displacement. In atmosphere dynamics, these waves play a crucial role in understanding phenomena such as atmospheric stability and wave propagation.
Internal waves: Internal waves are gravity waves that occur within a fluid medium, such as the ocean or atmosphere, where the density varies with depth. These waves arise due to the influence of buoyancy forces acting on different layers of the fluid, leading to oscillations and energy transfer within the medium. Internal waves can significantly affect ocean circulation and atmospheric dynamics, impacting everything from climate patterns to marine ecosystems.
John F. McCready: John F. McCready was a prominent meteorologist known for his pioneering research on gravity waves and their significance in atmospheric science. His work greatly contributed to the understanding of how these waves interact with weather patterns and atmospheric processes, establishing foundational theories that are still relevant today.
Modulation: Modulation refers to the variation in a wave's amplitude, frequency, or phase to convey information or energy transfer in different contexts. In the realm of atmospheric physics, it plays a significant role in understanding gravity waves, where modulation can indicate changes in atmospheric conditions, energy propagation, and the interaction between waves and background flows.
Mountain waves: Mountain waves are a type of atmospheric wave that occurs when stable air flows over mountain ranges, causing the air to oscillate vertically. These waves can lead to significant turbulence and are important for understanding weather patterns in mountainous regions, as they interact with gravity waves, which are related to the stability of the atmosphere.
Phase Speed: Phase speed is the rate at which a wave phase propagates through space, defined as the speed at which a particular point of constant phase travels. This concept is crucial for understanding how different types of waves behave in various atmospheric conditions, impacting their energy and momentum transfer. The phase speed varies depending on wave type and the properties of the medium through which it travels.
Radar observations: Radar observations refer to the use of radar technology to detect and analyze atmospheric phenomena, including precipitation, wind patterns, and the presence of various weather systems. This technology enables meteorologists to gather real-time data about the atmosphere, which is crucial for understanding and predicting weather conditions such as mountain waves and gravity waves.
Satellite measurements: Satellite measurements refer to the collection of data from space using satellites equipped with various instruments that monitor atmospheric conditions and environmental phenomena. These measurements provide valuable insights into weather patterns, climate changes, and atmospheric dynamics, enabling scientists to analyze large-scale phenomena like gravity waves with precision and accuracy.
Shear instability: Shear instability refers to the phenomenon where differences in wind speed and direction at different altitudes lead to the development of turbulence and chaotic air motions. This occurs when a stable layer of the atmosphere becomes disturbed, often due to strong vertical wind shear, resulting in the formation of waves or eddies. Understanding shear instability is crucial in atmospheric physics as it plays a significant role in weather patterns and the behavior of gravity waves.
Topographic forcing: Topographic forcing refers to the influence of the Earth's surface features, such as mountains and valleys, on atmospheric motion and weather patterns. These surface irregularities can cause changes in wind direction, speed, and turbulence, leading to the formation of phenomena like gravity waves, which are significant in understanding how terrain affects local and regional climate conditions.
Turbulence: Turbulence refers to the chaotic and irregular motion of fluids, particularly in the atmosphere, resulting from the interaction of various forces and temperature differences. This phenomenon is crucial for understanding the behavior of air parcels in the atmosphere, influencing stability, mixing processes, and the structure of different layers within the atmosphere. It plays a key role in weather patterns, air quality in urban settings, and the propagation of gravity waves.
Wave equation: The wave equation is a mathematical representation that describes the propagation of waves through a medium. This equation captures the relationship between the wave's displacement, its velocity, and the medium's properties, such as density and tension. Understanding the wave equation is crucial for analyzing various phenomena like wave propagation in fluids, acoustic waves in the atmosphere, and the dynamics of gravity waves.
Wavelength: Wavelength is the distance between consecutive crests or troughs of a wave, commonly used to describe electromagnetic waves, including light. It plays a crucial role in determining the energy and behavior of waves, influencing how they interact with matter and their propagation through different media. In various contexts, such as light scattering and remote sensing, understanding wavelength helps explain phenomena like color perception and atmospheric measurements.
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