MIDDLE ATMOSPHERE CLIMATOLOGIES FROM THE TROPOSPHERE-STRATOSPHERE CONFIGURATION OF THE UKMOS UNIFIED MODEL

A climatology of the middle atmosphere is determined from 11-yr integr ations of the U.K. Meteorological Office Unified Model and compared wi th 18 years of satellite observations and 5 years of data assimilation fields. The model has an upper boundary at 0.1 mb, and above 20 mb us es Rayleigh friction asa substitute for gravity wave drag. Many of the results are, however, found to be relatively insensitive to enhancing the damping above 0.3 mb. As with most general circulation models, th e polar night jet in both hemispheres is too strong and does not have the observed equator ward slope with height. The model suffers from th e common ''cold pole'' problem and, apart from a local warm pool cente red just below 100 mb in northern high latitudes in January, and anoth er at about 30 mb at 70 degrees S in July, has a cold bias throughout the stratosphere. Al the level where polar stratospheric clouds occur, the temperature bias is about -4 K in the Northern Hemisphere and up to +6 K in the Southern Hemisphere. For the majority of the southern w inters, local minimum temperatures in the lower stratosphere agree wel l with observations but in some years the behavior is more like the No rthern. Hemisphere with values rising rapidly in late winter This feat ure of the simulation is also seen in the South Pole temperatures at 1 0 mb with midwinter warmings occurring in two of the years. At 10 mb, midwinter warming behavior at the North Pole is quite well reproduced. as is the annual cycle in extratropical circulation. In the Tropics, there is no quasi-biennial oscillation, and the semiannual oscillation in the upper stratosphere has a poorly simulated westerly phase, whil e the easterly phase lacks the observed seasonal asymmetry. Simulated stationary wave amplitudes in the upper stratosphere lack a strong hem ispheric asymmetry and are overpredicted in both hemispheres despite h aving roughly the correct amplitudes at 100 mb. Interannual variabilit y in the winter stratosphere is underestimated, and again there is evi dence that the model does not produce the proper hemispheric asymmetri es.