Impact of the stratosphere on the troposphère on weather forecast time-scales
Jackson [D.R.]
The impact of different representations of the stratosphere on the troposphere on weather forecast timescales is investigated by comparing results from two versions of the Unified Model (UM), with upper boundaries at 10 and 0.1 hPa. Five pairs of 30 day runs were made for both January and July conditions. Root mean square differences in the geopotential height fields at 464 hPa for the two model versions are largest in the winter storm track regions and in general increase with time. However, a comparison of the monthly mean fields shows no statistically significant difference. There is, however, clear evidence of reflection of planetary waves off the 10 hPa upper boundary which leads to differences in the vertical and meridional phase structure of the waves and thus in the eddy heat and momentum fluxes. These differences do not generally appear until about 10 days into the model simulations and thus the reflections are unlikely to influence the troposphere on short range (less than five days) forecast timescales. However, they many be important on longer-range forecast and climate timescales.
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