The bullet points below provide a summary of the ongoing UGAMP work on the impact of the stratosphere on tropospheric climate.
A multi-year simulation of the L49 (49 levels in the vertical) troposphere-middle atmosphere configuration of the Unified Model (UM) has been done and compared with a climatology produced from the UK Meteorological (UKMO) analyses. Model performance is good (Fig. 1). However, the model has some shortcomings, notably a cold bias in the stratosphere which is due to a cold bias in the long-wave part of the radiation scheme. This work has been published in the Quarterly Journal. Future runs will involve different configurations of the UM and will incorporate changes to the radiation scheme and the physics parametrizations. An AMIP II run will be carried out in due course with the L58 version.
The UM has been used for short-term deterministic forecasts to study the effect of vertical resolution and location of model lid and model top level on predictive skill. Results show that predictive skill in the SH lower stratosphere is improved with higher resolution and a raised model top level (Fig. 2). This work was documented in a UGAMP technical report and has been submitted to the Quarterly Journal: the paper is being revised in accordance to the referees's comments. An assessment of the predictive skill of the UM in the NH lower stratosphere in the short-term is also being done, focusing on February 1994 (when there was a minor wave-2 warming), and on January 1998 (when there was a strong wave-1 warming). Results show that flow history is just as important as resolution, and that it is not always the case that the model with the better represented stratosphere performs better in the lower stratosphere (Fig. 3). In this instance, the differences in skill are associated with the orientation of the polar vortex, and can be explained by noting that the model with higher vertical resolution has more degrees of freedom.
Ensemble forecasts will help determine whether a troposphere-middle atmosphere configura- tion of the UM has advantages as a climate prediction model at the seasonal time-scale. This work is to be done with L58 version of the UM in collaboration with Mike Harrison at the UKMO and the EU-funded PROVOST project.
An UM configuration (L74) with very high vertical resolution in the stratosphere (~0.8 km) is being run in an attempt to simulate a realistic QBO. A five-year run is envisaged. Preliminary results suggest that the model has a better representation of the stratospheric tropics than the L49 version, but is not capturing a QBO (Fig. 4). This run will be analysed in detail to assess the wave spectrum in the tropics; the extra-tropics will also be investigated and compared with results from the L49 version. This version of the model incorporates the Edwards-Slingo radiation scheme as well as other changes to the physical parametrizations. Comparison with the L49 version discussed above shows that the new radiation scheme ameliorates the stratospheric cold bias in the L49 version. Work is underway to extend the UM up to 0.01 hPa and incorporate the Hines gravity wave drag scheme.
The AMIP II run will carry a number of idealised passive tracers to study stratospheric trans- port and stratosphere-troposphere exchange. Future plans include the incorporation of elements of the adaptive mesh refinement technique for tracer advection in the UGAMP Stratosphere- Mesosphere Model (USMM) and the UM.
The impact of radiative feedback from the Antarctic ozone hole continues to be investigated with the USMM (coupled to a chemistry-transport model). A paper has been submitted to the Quarterly Journal. Recent studies concentrate on interannual variability and the role of the lower boundary forcing.
New correlated-k coefficients for the CO2 15 mm-band have been incorporated into the Ed- wards-Slingo scheme; tests indicate that the long wave (LW)cooling rates in the middle atmosphere are much improved. The Schumann-Runge band of oxygen, important for mesospheric heating rates, has been included in the short wave (SW). Testing of the enhanced SW scheme is in progress.
UGAMP GCM (UGCM) experiments have been carried out to study how changes in solar radiation and stratospheric ozone can affect the tropospheric circulation. A paper has been submitted to the Quarterly Journal.
Stratospheric variability is being studied using Halogen Occultation Experiment (HALOE) data and a mechanistic model.