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A major focus of this work 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).

A related strand of work is ozone assimilation. The radiative impact of ozone, particularly in the stratosphere, is such that an improved representation of it (i.e., via assimilation) should improve temperatures in this region and may have an impact on the quality of weather forecasts both in this region and in the troposphere. The expertise gained in the ozone assimilation work can easily be applied to the assimilation of any constituent. A dust data assimilation scheme is in development. Improved dust analyses and forecasts are important for a number of customers who perform operations in desert areas and also, potentially, can improve weather forecasts as a whole through more accurate representation of dust radiative heating effects.

Key Aims

• To improve the representation of the middle atmosphere in the Met Office NWP system via assimilation of new observations and improvements in assimilation methods.

• To improve modelling of the middle atmosphere through the use of improved parametrization schemes or model resolutions.

• To understand the impact of the above on tropospheric weather forecasts and to make recommendations for the future development of the Met Office NWP system.

• To develop an ozone assimilation scheme and demonstrate its impact on tropospheric weather forecasts and other Met Office products (e.g., UV forecasts).

• To develop other applications of constituent assimilation (e.g., dust assimilation) to produce new or improved forecast products for Met Office customers.

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 Parametrization (USSP) scheme, which parametrizes 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.

• Dust data assimilation scheme: A dust data assimilation scheme is being developed to assimilate aerosol optical depth observations from the SEVIRI satellite instrument. The assimilation will be applied in the Met Office's Southern Asia Model. Future research will focus on developing the background error covariances used, improving the dust analysis control variable, and extending the assimilation to other observation types and to the global domain.