The Potential Impact of Upper Stratospheric Measurements on Sub-seasonal Forecasts in the Extra-Tropics

This chapter examines the potential improvements in tropospheric weather forecasts that might arise from an enhanced representation of the upper stratospheric state. First, the chapter reviews current operational practice regarding observation of the atmosphere and the relative paucity of observations in the altitude range 40–70 km. Then, we describe some idealised model calculations to quantify the potential gain in skill available from improved monitoring in this region. The idealised model experiments use a relaxation technique with the Hadley Centre General Environment Model, to assess the potential gain in skill from observations both of the whole stratosphere and the upper stratosphere. At weather forecasting timescales (up to forecast day 30), better knowledge of the stratosphere, close to the onset of a sudden stratospheric warming, improves forecasts of the tropospheric northern annular mode. Whole-stratosphere information significantly improved average surface temperature anomalies over northern North America, whilst upper stratosphere information improved anomalies over Central Siberia. These results suggest any new observational technique which can contribute to monitoring of the 40–70 km region would likely benefit tropospheric forecast skill during wintertime.