The impact of the representation of the stratosphere on tropospheric weather forecasts
PublisherUniversity of Reading
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The mid-winter polar stratosphere has great potential as a source of additional predictability for mediumand extended-range weather forecasts. Several studies have demonstrated that during particularly dynamicallyactive periods in the stratosphere known as stratospheric sudden warmings (SSW), troposphericforecasts can be sensitive to the stratospheric evolution on timescales of 10 days. A key limitationof these studies has been their idealised nature. Similarly, there has been little investigation ofhow and why changes to the stratosphere influence tropospheric behaviour and what this means for thedesign of future numerical weather prediction systems.This thesis addresses the above issues by performing a series of ensemble experiments using the MetOffice Unified Model with a range of different vertical resolutions. These include a "low top" modelwith an upper boundary in the mid-stratosphere and two "high top" models with a similar upper boundaryin the mesosphere, but with differing stratospheric vertical resolutions. The forecasts were run atthese resolutions around the SSW of February 2010. The focus is on short to medium range timescalesand so the forecasts are only out to 30 days. Statistically significant differences in surface fields, withmean differences in surface pressure of up to 3hPa are found between "high top" and "low top" simulationsas soon as 5 days into the forecast. These tropospheric differences resemble a negative NorthAtlantic Oscillation pattern, and are likely related to the inability of the "low top" model to effectivelycapture the SSW. No statistically significant differences are detected in surface fields between the two"high top" models, suggesting that the extra vertical resolution does not influence the surface forecastat this timescale.The second part of this thesis investigates whether the dynamical interaction between the stratosphereand the troposphere is mediated by changes to the development of baroclinic eddies. Two innovativewave breaking detection methods are developed to assess Rossby wave breaking in the troposphere.