An updated climatology of the troposphere-stratosphere configuration of the Met Office's Unified Model

In this paper results are presented from an improved version of the troposp here-stratosphere configuration of the Met Office Unified Model (UM). The n ew version incorporates a number of changes, including new radiation and or ographic gravity wave parameterization schemes, an interannually varying se a surface temperature and sea ice climatology, and the inclusion of convect ive momentum transport. The UM climatology is compared with assimilated dat a and with results from a previous version of the UM. It is shown that the model cold biases in the January winter stratosphere and the January and Ju ly summer stratosphere are reduced, chiefly because the new radiation schem e is more accurate. The separation between subtropical and polar night jets in July is also better simulated. In addition, in the current version stra tospheric planetary wave amplitudes in southern winter are less than half t hose in northern winter, which is in much better agreement with observation s than the previous model version. Despite these improvements, the model st ill has a cold bias in the winter polar stratosphere, which suggests that t he model representation of gravity wave drag is inadequate. Sensitivity tes ts were carried out and showed that the improved simulation of the separati on of subtropical and polar night jets in July is due both to the different sea ice climatology and to the inclusion of convective momentum transport. The improved simulation of stationary wave amplitudes in July cannot be at tributed to an individual model change, although it seems to be related to changed wave propagation and dissipation within the stratosphere rather tha n changes in the tropospheric forcing.