The Waves the Wind Blows: Gravity Waves and Their Role in Climate Change
Seminar by Dr Wright
What are Atmospheric Gravity Waves?
Atmospheric gravity waves are a natural phenomena. Current research is throwing new light on the role gravity waves play in climate change. Atmospheric gravity waves are a 3D physical mechanism which carry momentum and energy between the different layers of the atmosphere: from the troposphere to the stratosphere and mesosphere (Figure 1). They influence climate change and have important impacts on extreme weather events such as hurricanes and typhoons. They also cause problems in weather forecasting and climate prediction models, particularly in the Souther Hemisphere at the South Pole. They also influence ozone depletion and cause clear-air turbulence which effects aircraft, making them an important and relevant area of current study.
What Causes Gravity Waves?
Gravity waves are created from a range of processes. This includes orography (wind flow over mountains), fluctuations in the jet stream, and from high energy meteorological sources, such as violent storms and convection. 70% of atmospheric gravity waves are generated by orography and 20% are generated by convection, which leaves a remaining 10% from other sources.
When a parcel of air is disturbed in a stratified environment, as temperatures rise, the density of the parcel of air decreases, causing the air to rise due to increased buoyancy. As the parcel of air rises, the temperature drops as it enters colder regions, causing an increase in density. Hence, gravity exerts a stronger pull on the parcel of air, causing it to sink. The oscillation between the the parcel of air rising and sinking is what produces the gravity wave. The scale and time period of this oscillation can vary dramatically; the largest scale gravity wave systems can be hundreds of kilometres in length. Gravity waves are often visible as clouds, as when the air rises, water particles in the air cool and condense in the colder, higher regions of the atmosphere (Figure 2).
The Cold Pole Problem
The current inadequate understanding of atmospheric gravity waves impacts predictions of climate and weather patterns. This is a particular problem at the South Pole. Models of the polar atmosphere is too cold and consistently overestimate the speed of the Southern Hemisphere jet winds. Climate forecasting in Australia is also impacted. The mountains of the Southern Andes and the Antarctic Peninsula are some of the source of the gravity waves that perturb the jet stream in the Southern Hemisphere. The orographic gravity waves are generated as fast surface winds flow perpendicularly across these mountain belts, before being directed into the path of the Southern Hemisphere jet winds at 60◦S.
Hurricanes and Typhoons
There is evidence of gravity wave generation during the intensification phases of tropical cyclones. Evidence shows that gravity wave activity increases in this intensification phase as a cyclone builds up to its full intensity, before this activity drops sharply. However, though gravity waves are measurable in tropical cyclones, there is substantial background activity. Furthermore, the extensive cloud cover during tropical cyclones makes it difficult to measure convection at the core of the cyclone, which makes it challenging to predict which cyclones will become hurricanes. This is an important factor as gravity wave activity is increased by up to 15% by cyclones that intensify into hurricanes. An increased understanding of gravity wave activity will help advance weather and climate predictions.
Current Research
Progress is being made by researchers such as Dr Corwin Wright, who is working to constrain the characteristics and parameters of atmospheric gravity waves and solve these modelling errors in climate and weather forecasting. Dr Corwin Wright is working in collaboration with the MET Office and the European Centre for Medium-Range Weather Forecasts (ECMWF) to achieve this. With improved constraints on gravity waves, we can better understand extreme winter weather events triggered by upper-air winds, such as the ‘Beast from the East’ that affected Great Britain and Ireland in 2018. As climate changes in response to global warming, there will be more storms, increased convection and stronger surface winds, all of which generate gravity waves. This begs the question: to what extent will climate change impact gravity wave activity?
