The importance of the stratosphere in operational weather forecasting

The Met Office operational global NWP system has recently significantly increased its horizontal and vertical resolution. The vertical domain has been extended from 38 to 50 levels with the new model upper boundary at 63km, while the horizontal resolution has increased from N216 to N320 (a change from 432 to 640 grid boxes around a latitude circle). As a consequence of this upgrade the global weather forecasting model now spans the whole depth of the stratosphere for the first time. This change has led to improved assimilation of satellite data and subsequently increased the accuracy of tropospheric weather forecasts.

The performance of the extended global NWP system has been shown to improve from the surface up to around 10hPa. The results indicate that the improvement has come predominantly from the increase in vertical resolution rather than the increase in horizontal resolution. However, above 1hPa there is degradation in performance. This problem has been alleviated by the inclusion of a gravity wave drag parameterization scheme. Initially, there were some numerical stability issues close to the winter poles which have subsequently been resolved.

This talk discusses the issues highlighted above and compares the new high resolution global NWP system with the low horizontal resolution stratospheric assimilation system which had previously been running operationally at the Met Office. In addition, early results from the planned upgrade to a 70 level model, with a top at 80km, will be presented.