Weather forecasts are influenced by the Middle Atmosphere. This is generally a predictable region which is punctuated by some of the most dramatic and rapid events seen anywhere in the atmosphere. In addition the majority of the ozone which absorbs harmful UV radiation is found in the stratosphere. Therefore, it is important to analyse and study this region of the atmosphere.

Analyses for the middle atmosphere have been produced daily at the Met office since 1991 and have been part of the operational forecasting suite since 1994. In 2006 increased computer power prompted an increase in the horizontal and vertical resolution of the global model, the merger of the stratospheric model with the standard global forecasting model and thus the investigation of the stratosphere in more detail than ever before.

Below is an example of the daily middle atmosphere analysis output illustrating the Southern Hemispheric major warming in September 2002. The left hand plot shows the geopotential height field, while the right hand plot shows temperature, both are on the 10hPa pressure level (around 30km altitude). The blue colours on the left hand plot show the polar vortex shifted away from the pole and split in two. The right hand plot highlights the unusually high temperatures in polar regions which have displaced the cold temperatures which are found within the polar vortex. This is the first, and only, time a major warming has been observed in the Southern Hemisphere. Major warmings are winter-time middle atmosphere events in which the temperatures in polar regions can increase by tens of degrees in the space of a few days. This reverses both the temperature gradient to the pole (usually temperatures decrease towards the poles) and the westerly mean zonal wind.

The original stratospheric data assimilation system used the Analysis Correction data assimilation scheme in conjunction with a 42-level configuration of the Unified Model. In November 2000 an upgraded stratospheric assimilation system was implemented to use the 3D variational (3D-Var) data assimilation system used at that time in the global forecast suite. These improvements included the assimilation of radiances (rather than temperature retrievals) from the operational polar orbiter satellites.

In October 2003 the stratosphere model was further upgraded to use a new semi -Lagrangian dynamical core, usually referred to as "New Dynamics", also used in the standard global model. This upgraded stratospheric model had 50 levels with the lowest levels the same as those used in the global system. This model was run to demonstrate the benefits of additional stratospheric levels on the processing of satellite temperature soundings and the forecast. In December 2005 the resolution of the operational global model was enhanced with a reduction in the horizontal spacing from 60km to 40km and an increase in the number of vertical levels from 38 to 50. This added 12 levels in the stratosphere raising the top of the global model to 63 km and allowed the merge of the stratospheric model and the standard global model. Consequently the stratospheric model was retired in March 2006 and the production of stratospheric analysis is now completed by the standard operational global forecast model.

The stratospheric analyses consist of fields of temperatures, wind components and geopotential heights on a global grid of resolution 0.5625° latitude by 0.375° longitude, at levels from 1000 hPa to 0.1 hPa (approximately 0-63 km). The analyses have been used in a number of research studies of stratospheric dynamics, for example a study of the tropical quasi-biennial oscillation. They have also been used to help validate and interpret measurements from various instruments on board UARS and also ILAS-2 (Improved Limb Atmospheric Spectrometer), an instrument on the Japanese ADEOS satellite.

The Met Office is collaborating with the NERC Data Assimilation Research Centre (DARC), in order to assimilate measurements from current and future research satellites. This collaboration includes the assimilation of ozone and water vapour measurements from the ESA Environment Satellite, Envisat.

The middle atmosphere produced regular SSU analyses from measurements made from NOAA polar orbiter satellites for the period December 1978 to June 1998.

The analyses used measurements from the Stratospheric Sounding Unit (SSU), Microwave Sounding Unit (MSU) and High-Resolution Infrared Sounder (HIRS) flown on satellites in the TIROS-N series; together these three instruments make up the TIROS Operational Vertical Sounder (TOVS). Radiance measurements from the stratospheric TOVS channels were used to derive deep layer-mean thickness values. The thicknesses were then interpolated onto a 5° resolution global grid, and added to the operational analysis of 100 hPa height, to produce height fields up to 1 hPa. In turn, temperatures, winds and potential vorticity can be derived from the height fields.

Stratospheric analyses produced at the Met Office are available for bona fide research purposes. Commercial or business use is not permitted without the written authority of the Met Office. The analysis and forecast data may be obtained through the British Atmospheric Data Centre (BADC), which is based at the Rutherford Appleton Laboratory.

More information on the stratosphere