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Stratosphere and Large Scale Dynamics

Time / pressure sections of temperature (top) and zonal wind (bottom) showing the major stratospheric sudden warming of January 2009.

Correctly representing the interactions between the lower, middle and upper atmosphere is vital for longer range weather and climate forecasts.

Due to the action of solar heating, the Earth's atmosphere is divided into a number of discrete layers. The bottom layer is the Troposphere, from the Latin word trope meaning turning or overturning. This layer is heated from below by solar radiation reaching the Earth's surface, making this layer unstable giving rise to the overturning processes of convection on a wide range of spatial scales, which make up the day-to-day weather we experience. The Troposphere ranges in depth from up to 20km in the Tropics to only 7km at the poles.

Above the Troposphere is the Stratosphere, from the Ancient Greek word strata meaning layer. The Stratosphere is heated from above by the absorption of high energy Ultra-Violet (UV) solar radiation in the ozone layer, and this heating from above limits any convection or vertical mixing in the Stratosphere. The Stratosphere extends to roughly 50km in height, where the Mesosphere begins, from the Greek word meso for middle. In the Mesosphere the air becomes so thin that it becomes a poor absorber of UV radiation and begins to get colder with height again making vertical movement possible again. Noctilucent clouds are observed in the Mesosphere because of this overturning. The Mesosphere extends to roughly 90km in height. Above this is Thermosphere where the air is heated increasingly with height by absorbing the most energetic of the incoming solar radiation. The Thermosphere extends to many hundreds of kilometers in height, into the Exosphere at the very edge of the atmosphere and the Magnetosphere beyond. These layers extend out many thousands of kilometers and their interactions with the Solar Wind create so called Space Weather.

A major focus work in this area is to improve the analysis and modelling of the middle atmosphere. This is important because it has been demonstrated that an improved representation of the middle atmosphere in the NWP system can lead to better weather forecasts in the atmosphere as a whole; for example, via more accurate assimilation of satellite radiance observations. Furthermore, it is becoming increasingly recognised that the interaction between the stratosphere and troposphere may lead to improved tropospheric forecasts for longer forecast periods (greater than 10 days). For example, Stratospheric Sudden Warming events have a complex interaction with Tropospheric blocking events which lead to long lasting settled weather conditions.

Key Aims

  • To monitor and evaluate the performance of key middle atmosphere processes in the Unified Model at all timescales.
  • To develop an ozone assimilation scheme and demonstrate its impact on tropospheric weather forecasts and other Met Office products (e.g., UV forecasts).
  • To improve the representation of the middle atmosphere in the Met Office NWP system via assimilation of new observations and improvements in assimilation methods.

Current Projects

  • Assimilation of Special Sensor Microwave Imager Sounder (SSMIS) radiance observations: SSMIS observations from channels that are sensitive to the lower atmosphere have been assimilated at the Met Office since 2006. There are also other SSMIS channels that are sensitive to higher levels of the atmosphere (approximately 45-80 km). However, to date these data have not been assimilated because they are strongly affected by the Zeeman effect, which is currently not included in the the fast radiative transfer model used in the assimilation scheme. Current research focuses on extending the radiative transfer model to include the Zeeman effect, to assimilate the upper level data, and to investigate the impact on Met Office analyses and forecasts.
  • Impact of the representation of the stratosphere on tropospheric weather forecasts: The interaction between the stratosphere and troposphere may lead to improved tropospheric forecasts for longer forecast periods (greater than 10 days). The current Met Office NWP forecast model extends up to 80 km, but the vertical resolution above the lower stratosphere is very low. Comparisons of extended range forecasts made with this model and other model versions with enhanced middle atmosphere resolution are being made to investigate if this improved resolution impacts on tropospheric forecast skill.
  • Model developments in the middle atmosphere: Parametrizations pertaining to the middle atmosphere in the Unified Model are being developed. Areas of development include the Ultra Simple Spectral gravity wave Parameterization (USSP) scheme, which parameterises sub-grid scale gravity waves that are important in the middle atmosphere for the accurate simulation of the extratropical jets and the Quasi-Biennial Oscillation. In addition, modifications to the model radiation scheme, which are aimed at reducing temperature biases in the stratosphere, are also being tested.
  • Ozone data assimilation scheme: An ozone data assimilation scheme has already been developed and is capable of assimilating ozone data from both operational and research satellites. Further improvements to the assimilation scheme (e.g., improved ozone chemistry and background error covariances) are in development, and the impact of the improved representation of ozone on weather forecasts is being studied. In addition, ozone assimilation is being used as a technique to estimate stratospheric chemical polar ozone loss, as part of the European RECONCILE project on arctic stratospheric ozone loss.
  • The Met Office is also developing a Space Weather analysis and forecast capability.

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