MIDDLE ATMOSPHERE VARIABILITY IN THE UK METEOROLOGICAL-OFFICE UNIFIEDMODEL

This paper describes the seasonal evolution of the stratospheric circu lation simulated by a stratosphere-troposphere configuration of the UK Meteorological Office (UKMO) Unified Model. Results are shown from a five-year integration of the model. The model's simulation of the stra tosphere and its seasonal evolution compares well with global analyses produced by the UKMO troposphere-stratosphere data-assimilation syste m. The contrast between the winter circulation in the two hemispheres is well simulated. The zonal-mean winds show strong interannual variab ility in northern winter, while the southern hemisphere winterjet is m uch less variable. In northern winter the model spontaneously produces two major warmings and a number of minor warmings. In southern winter and spring the model reproduces well the break-up of the polar vortex and elements of the flow regime that often precedes this break-up. Th e model does, however, exhibit a number of shortcomings. Lack of conse rvation of potential vorticity prevents the model from capturing some of the ingredients of the flow regimes associated with stratospheric w armings and with the merger of anticyclones. There is a cold bias in t he stratosphere throughout the year, with a maximum cold bias over the winter pole near the stratopause. This temperature bias appears to be due to a cooling bias in the long-wave part of the radiation scheme. The model also has unrealistically strong planetary waves in the upper stratosphere, although amplitudes in the upper troposphere and lower stratosphere are in good agreement with those derived from the UKMO an alyses. This paper discusses possible future model improvements that s hould alleviate these problems.