Impacts of Increased Vertical Resolution in the Stratosphere on Dynamical Extended-Range Forecasts

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  • 力学的延長予報に対する成層圏の鉛直解像度の影響
  • 力学的延長予報に対する成層圏の鉛直解像度の影響〔英文〕
  • リキガクテキ エンチョウ ヨホウ ニ タイスル セイソウケン ノ エンチョク

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Abstract

o examine the impacts of increased vertical resolution in the stratosphere of a numerical weather prediction model, 30-day forecast experiments starting from 14 initial conditions for the Northern Hemisphere winter were carried out using a control model and a high-resolution model. It was found that forecast skill for the stratosphere was increased by increasing the vertical resolution. In particular, the climatological Aleutian high was much better simulated. However, zonal mean wind was not improved as a whole due to an easterly bias of the polar night jet of the high-resolution model. Although no clear impacts were seen in the forecast skill of daily 500 hPa height, the prediction of l0-day mean 500 hPa height was improved after around Day 15. In particular, low-frequency modes on the Pacific side, such as the Western Pacific pattern and Eastern Pacific pattern, were better predicted. Systematic errors in the troposphere were not much changed. The forecast for the zonal wavenumber-1 component of the tropospheric circulation was not improved in spite of the improvement of the same wave component in the stratosphere. The reduction of prediction error of the tropospheric wavenumber-2 component after around Day 15 was followed by that of the corresponding stratospheric component with a lag of 1-2 days. These results suggest that the stratospheric impact on the troposphere in terms of forecast skill results from local interactions between the troposphere and the stratosphere. They also indicate that higher vertical resolution in the stratosphere is necessary for better forecasting of tropospheric low-frequency variabilities.

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