Planetary and Gravity Waves as Drivers of Sudden Stratospheric Warmings (PEGASUS)
Lead Research Organisation:
University of Bath
Department Name: Electronic and Electrical Engineering
Abstract
Sudden stratospheric warmings (SSWs) are some of the most dramatic events in the entire atmosphere. Over just a few days, the high-altitude jet stream stops and sometimes even reverses, and polar stratospheric temperatures can shoot up as much as 50 degrees Celsius. Their effects propagate upwards, where they modulate the near-Earth space environment of the ionosphere, and downwards, where they can cause extreme winter weather in densely-populated regions such as Europe and North America.
SSWs occur on average twice every three winters, but may occur several times in one winter and then not at all for several years afterwards. Forecasting them more than a few days in advance is extremely challenging. Their effects are also very difficult to predict - while (for example) the 'Beast From The East' of February 2018 in Europe and the 'Polar Vortex Winter' of January 2014 in the eastern United States were directly attributable to SSWs which happened a few days earlier, many SSWs have occurred with almost no effect on surface weather.
In PEGASUS, we will use new satellite measurement techniques and advanced computer models to better understand the physics of how SSWs develop, and of how and why they affect both surface weather and space weather. We will (i) test a recent theory that changes our understanding of how and why SSWs happen, (ii) investigate the details of how, when and where SSWs affect surface weather and (iii) measure the effects of SSWs on the global upper atmosphere, with implications for GPS and radio communications.
(i) Traditionally, we thought that SSWs were triggered by extremely large and unusually intense 'planetary waves' travelling through the atmosphere. These large waves seriously disrupt the jet stream, making it collapse and triggering an SSW. However, recent work has shown that this conceptual model does not properly explain the observed SSW record. Instead, a new theory challenges this model at a fundamental level. This new theory is that smaller-scale 'gravity waves' over the weeks before the SSW nudge the jet stream into a less robust state, weak enough that normal winter weather can be enough to trigger the start of an SSW. The precise distribution of these gravity waves, in space, time and intensity, may also affect how severe the surface effects of the SSW are. There is thus an important need to test this new theory. PEGASUS will do so. We will use advanced new satellite methods of measuring both the large planetary waves and the much smaller gravity waves to study the development of every SSW in the last sixteen years. We will also study idealised mathematical models (i.e. models which strip away unnecessary details) to understand the underlying physics and mathematics of how SSWs evolve and develop. This will provide a robust and critical test of the new theory.
(ii) This combination of observational and theoretical insight will let us test and assess how well forty leading climate models reproduce SSWs. We will use this information to select the best such models, tested against both observations and theory from (i). We will then study these selected models in close detail to understand what features of SSWs cause them to affect the surface and the upper atmosphere, with the aim of better predicting both SSW development and surface effects in future. In particular, we will closely study the differences between the surface effects of two different types of SSW, known as 'splits' and 'displacements' based on how they affect the jet stream.
(iii) Finally, we will quantify how SSWs affect global GPS signals and radio communications, allowing us to understand not just the surface weather effects of SSWs but also their space-weather effects. This will use a chain of five state-of-the-art radars spanning from pole-to-pole, and global measurements of upper-atmospheric composition from satellite measurements.
SSWs occur on average twice every three winters, but may occur several times in one winter and then not at all for several years afterwards. Forecasting them more than a few days in advance is extremely challenging. Their effects are also very difficult to predict - while (for example) the 'Beast From The East' of February 2018 in Europe and the 'Polar Vortex Winter' of January 2014 in the eastern United States were directly attributable to SSWs which happened a few days earlier, many SSWs have occurred with almost no effect on surface weather.
In PEGASUS, we will use new satellite measurement techniques and advanced computer models to better understand the physics of how SSWs develop, and of how and why they affect both surface weather and space weather. We will (i) test a recent theory that changes our understanding of how and why SSWs happen, (ii) investigate the details of how, when and where SSWs affect surface weather and (iii) measure the effects of SSWs on the global upper atmosphere, with implications for GPS and radio communications.
(i) Traditionally, we thought that SSWs were triggered by extremely large and unusually intense 'planetary waves' travelling through the atmosphere. These large waves seriously disrupt the jet stream, making it collapse and triggering an SSW. However, recent work has shown that this conceptual model does not properly explain the observed SSW record. Instead, a new theory challenges this model at a fundamental level. This new theory is that smaller-scale 'gravity waves' over the weeks before the SSW nudge the jet stream into a less robust state, weak enough that normal winter weather can be enough to trigger the start of an SSW. The precise distribution of these gravity waves, in space, time and intensity, may also affect how severe the surface effects of the SSW are. There is thus an important need to test this new theory. PEGASUS will do so. We will use advanced new satellite methods of measuring both the large planetary waves and the much smaller gravity waves to study the development of every SSW in the last sixteen years. We will also study idealised mathematical models (i.e. models which strip away unnecessary details) to understand the underlying physics and mathematics of how SSWs evolve and develop. This will provide a robust and critical test of the new theory.
(ii) This combination of observational and theoretical insight will let us test and assess how well forty leading climate models reproduce SSWs. We will use this information to select the best such models, tested against both observations and theory from (i). We will then study these selected models in close detail to understand what features of SSWs cause them to affect the surface and the upper atmosphere, with the aim of better predicting both SSW development and surface effects in future. In particular, we will closely study the differences between the surface effects of two different types of SSW, known as 'splits' and 'displacements' based on how they affect the jet stream.
(iii) Finally, we will quantify how SSWs affect global GPS signals and radio communications, allowing us to understand not just the surface weather effects of SSWs but also their space-weather effects. This will use a chain of five state-of-the-art radars spanning from pole-to-pole, and global measurements of upper-atmospheric composition from satellite measurements.
Planned Impact
SSWs are major stratospheric weather events, which have knock-on effects throughout the atmosphere above and below them, from the Earth's surface to the edge of space.
At the surface, SSWs are an important driver of unusually extreme winter weather at surface level, especially the densely-populated regions of Europe and North America. Such weather has a major and direct impact on a vast range of industries, from logistics and transport to hospitality, oil and gas supply and public service. It also has broad implications for human health and mortality both during and after the events. PEGASUS will improve our scientific knowledge of how and why SSWs occur, facilitating advances in forecasting both when they happen and, when they do occur, how they will affect the surface.
Furthermore, a range of recent studies (e.g Sigmond et al, Nat. Geosci. 2013; Tripathi et al, Environ. Res. Lett 2015) have suggested that our limited ability to predict SSWs is a key roadblock on the path to producing meaningful weather forecasts at sub-seasonal and seasonal timescales, i.e. timescales of roughly a month or longer. Our work will directly address this block on the pathway to this highly challenging goal, with broad implications on long-term planning in industries such as farming and civil engineering.
At the surface, SSWs are an important driver of unusually extreme winter weather at surface level, especially the densely-populated regions of Europe and North America. Such weather has a major and direct impact on a vast range of industries, from logistics and transport to hospitality, oil and gas supply and public service. It also has broad implications for human health and mortality both during and after the events. PEGASUS will improve our scientific knowledge of how and why SSWs occur, facilitating advances in forecasting both when they happen and, when they do occur, how they will affect the surface.
Furthermore, a range of recent studies (e.g Sigmond et al, Nat. Geosci. 2013; Tripathi et al, Environ. Res. Lett 2015) have suggested that our limited ability to predict SSWs is a key roadblock on the path to producing meaningful weather forecasts at sub-seasonal and seasonal timescales, i.e. timescales of roughly a month or longer. Our work will directly address this block on the pathway to this highly challenging goal, with broad implications on long-term planning in industries such as farming and civil engineering.
Publications
Achatz U
(2023)
Atmospheric Gravity Waves: Processes and Parameterization
in Journal of the Atmospheric Sciences
Banyard T
(2021)
Atmospheric Gravity Waves in Aeolus Wind Lidar Observations
in Geophysical Research Letters
Banyard T
(2021)
Atmospheric Gravity Waves in Aeolus Wind Lidar Observations
Battalio J
(2023)
Martian Gravity Waves Observed by the Thermal Emission Imaging System (THEMIS) During Northern Summer
in Journal of Geophysical Research: Planets
Cullens C
(2023)
Observations of Typhoon Generated Gravity Waves From the CIPS and AIRS Instruments and Comparison to the High-Resolution ECMWF Model
in Journal of Geophysical Research: Atmospheres
Dempsey S
(2021)
Winds and tides of the Antarctic mesosphere and lower thermosphere: One year of meteor-radar observations over Rothera (68°S, 68°W) and comparisons with WACCM and eCMAM
in Journal of Atmospheric and Solar-Terrestrial Physics
Dempsey S
(2022)
Interannual Variability of the 12-hr Tide in the Mesosphere and Lower Thermosphere in 15 Years of Meteor-Radar Observations Over Rothera (68°S, 68°W)
in Journal of Geophysical Research: Atmospheres
Esler J
(2019)
Dynamical Elliptical Diagnostics of the Antarctic Polar Vortex
in Journal of the Atmospheric Sciences
Hall R
(2021)
Persistent Model Biases in the CMIP6 Representation of Stratospheric Polar Vortex Variability
in Journal of Geophysical Research: Atmospheres
Hall R
(2023)
Surface hazards in North-west Europe following sudden stratospheric warming events
in Environmental Research Letters
Hall R
(2022)
How Well Are Sudden Stratospheric Warming Surface Impacts Captured in CMIP6 Climate Models?
in Journal of Geophysical Research: Atmospheres
Hall R
(2021)
Tracking the Stratosphere-to-Surface Impact of Sudden Stratospheric Warmings
in Journal of Geophysical Research: Atmospheres
Hindley N
(2022)
Radar observations of winds, waves and tides in the mesosphere and lower thermosphere over South Georgia island (54° S, 36° W) and comparison with WACCM simulations
in Atmospheric Chemistry and Physics
Hindley N
(2019)
Gravity waves in the winter stratosphere over the Southern Ocean: high-resolution satellite observations and 3-D spectral analysis
in Atmospheric Chemistry and Physics
Hindley N
(2020)
An 18-Year Climatology of Directional Stratospheric Gravity Wave Momentum Flux From 3-D Satellite Observations
in Geophysical Research Letters
Hindley N
(2021)
Stratospheric gravity waves over the mountainous island of South Georgia: testing a high-resolution dynamical model with 3-D satellite observations and radiosondes
in Atmospheric Chemistry and Physics
Krisch I
(2022)
On the derivation of zonal and meridional wind components from Aeolus horizontal line-of-sight wind
in Atmospheric Measurement Techniques
Lawrence Z
(2022)
Quantifying stratospheric biases and identifying their potential sources in subseasonal forecast systems
in Weather and Climate Dynamics
Liu L
(2023)
Concentric Traveling Ionospheric Disturbances (CTIDs) Triggered by the 2022 Tonga Volcanic Eruption
in Journal of Geophysical Research: Space Physics
Narayanan V
(2024)
Observations of Mesospheric Gravity Waves Generated by Geomagnetic Activity
in Journal of Geophysical Research: Space Physics
Noble P
(2024)
Interannual Variability of Winds in the Antarctic Mesosphere and Lower Thermosphere Over Rothera (67°S, 68°W) During 2005-2021 in Meteor Radar Observations and WACCM-X
in Journal of Geophysical Research: Atmospheres
Okui H
(2023)
A Comparison of Stratospheric Gravity Waves in a High-Resolution General Circulation Model With 3-D Satellite Observations
in Journal of Geophysical Research: Atmospheres
Perrett J
(2021)
Determining Gravity Wave Sources and Propagation in the Southern Hemisphere by Ray-Tracing AIRS Measurements
in Geophysical Research Letters
Ramesh K
(2024)
Long-Term Variability and Tendencies in Mesosphere and Lower Thermosphere Winds From Meteor Radar Observations Over Esrange (67.9°N, 21.1°E)
in Journal of Geophysical Research: Atmospheres
Wright C
(2021)
Dynamical and surface impacts of the January 2021 sudden stratospheric warming in novel Aeolus wind observations, MLS and ERA5
in Weather and Climate Dynamics
Wright C
(2021)
Using vertical phase differences to better resolve 3D gravity wave structure
in Atmospheric Measurement Techniques
Wright C
(2020)
Multidecadal Measurements of UTLS Gravity Waves Derived From Commercial Flight Data
in Journal of Geophysical Research: Atmospheres
Wright CJ
(2023)
Using sub-limb observations to measure gravity waves excited by convection.
in NPJ microgravity
Wright CJ
(2022)
Surface-to-space atmospheric waves from Hunga Tonga-Hunga Ha'apai eruption.
in Nature
Wu X
(2021)
Stratospheric Gravity Waves as a Proxy for Hurricane Intensification: A Case Study of Weather Research and Forecast Simulation for Hurricane Joaquin
in Geophysical Research Letters
Description | Significant media interest in Hall et al (2021), including articles from ABC, the Daily Mail, the Express, the Independent, Metro, Newsweek, and Russia Today. Significant impact in Wright et al (2022), including articles in over 70 media sources, including CNet, Eos, The Independent, Nature, and The New York Times, and radio/TV including BBC Points West and The World Press release by European Space Agency focusing on work carried out by Wright using ESA-Aeolus data to study sudden stratospheric warmings. |
First Year Of Impact | 2021 |
Sector | Environment,Other |
Impact Types | Cultural |
Description | URF Renewal |
Amount | £793,000 (GBP) |
Funding ID | URF\R\221023 |
Organisation | The Royal Society |
Sector | Charity/Non Profit |
Country | United Kingdom |
Start | 09/2022 |
End | 09/2025 |
Description | Collaboration on spectral analysis with Joan Alexander and Laura Holt |
Organisation | Northwest Research Associates |
Country | United States |
Sector | Public |
PI Contribution | We provided the software techniques and wrote the resulting paper. |
Collaborator Contribution | They provided the underlying concepts and an environment free of distraction to do the work. |
Impact | New spectral analysis ("the 2D+1 S-Transform") that we have used in several studies since it provides improved vertical discrimination. |
Start Year | 2019 |
Description | Collaboration on sudden stratospheric warmings as driven by gravity waves with Dr Ales Kuchar, University of Leipzig |
Organisation | University of Leipzig |
Country | Germany |
Sector | Academic/University |
PI Contribution | We provided the gravity wave data for comparison. |
Collaborator Contribution | They provided the vortex edge data for the comparison. |
Impact | Paper in prep. |
Start Year | 2022 |