Abstract/Summary

Recent work has shown that the sharpness of the extratropical tropopause declines with lead 15 time in numerical weather prediction models, indicating an imbalance between processes act16 ing to sharpen and smooth the tropopause. In this study the systematic effects of processes 17 contributing to the tropopause sharpness are investigated using daily initialised forecasts 18 run with the Met Office Unified Model over a three-month winter period. Artificial tracers, 19 each forced by the potential vorticity tendency due to a different model process, are used 20 to separate the effects of such processes. The advection scheme is shown to result in an ex21 ponential decay of tropopause sharpness towards a finite value at short lead times with a 22 timescale of 20-24 hours. The systematic effect of non-conservative processes is to sharpen 23 the tropopause, consistent with previous case studies. The decay of tropopause sharpness 24 due to the advection scheme is stronger than the sharpening effect of non-conservative pro25 cesses leading to a systematic decline in tropopause sharpness with forecast lead time. The 26 systematic forecast errors in tropopause-level potential vorticity are comparable to the inte27 grated tendencies of the parametrized physical processes suggesting that the systematic error 28 in tropopause sharpness could be significantly reduced through realistic adjustments to the 29 model parametrization schemes.