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Discussion papers
https://doi.org/10.5194/acp-2019-679
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-679
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 19 Aug 2019

Submitted as: research article | 19 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Adding value to Extended-range Forecasts in Northern Europe by Statistical Post-processing Using Stratospheric Observations

Natalia Korhonen1,2, Otto Hyvärinen1, Matti Kämäräinen1, David S. Richardson3, Heikki Järvinen4, and Hilppa Gregow1 Natalia Korhonen et al.
  • 1Weather and Climate Change Impact Research, Finnish Meteorological Institute, Helsinki, 00101, Finland
  • 2Doctoral Programme in Atmospheric Sciences, University of Helsinki, Finland
  • 3ECMWF, Reading, UK
  • 4Institute for Atmospheric and Earth System Research/Physics, University of Helsinki, 00014, Finland

Abstract. The skill scores of the Extended-Range Forecasts (ERF) of the European Centre for Medium-Range Weather Forecasts (ECMWF) are still quite modest for the forecast weeks 3–6 in Northern Europe. As there are known stratospheric precursors impacting the surface weather with potential to improve ERFs, we aim to quantify the effect of these predictors and post-process the ERFs with them.

During boreal winter the quasi-biennial oscillation (QBO) affects the stratospheric polar vortex; the easterly (westerly) QBO often coincides with weaker (stronger) than average polar vortex. Consequently, the weaker (stronger) than average stratospheric polar vortex is connected to negative (positive) Arctic Oscillation (AO) and colder (warmer) than average surface temperatures in Northern Europe. We developed a stratospheric wind indicator, SWI, based on the previous months' stratospheric wind observations and the phase of the AO during the following weeks. We demonstrate that there was a statistically significant difference in the observed surface temperature within the 3–6 weeks depending on the SWI at the start of the forecast. These temperature anomalies were underestimated by the ECMWF's reforecasts.

When our new SWI was applied in post-processing the ECMWF's two-week mean temperature reforecasts for weeks 3–4 and weeks 5–6 in Northern Europe during boreal winter, the skill scores of those weeks were slightly improved. This indicates there is some room to improve the ERFs, if the stratosphere-troposphere links were better captured in the modelling.

Natalia Korhonen et al.
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Natalia Korhonen et al.
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Short summary
During boreal winter the Arctic Oscillation index and the mean surface temperatures over Northern Europe are found to be statistically significantly lower 3–6 weeks after easterly Quasi-biennial Oscillation with wind speeds less than 10 m/s. This stratosphere-troposphere connection is used in post-processing of the extended range forecasts of the ECMWF. By this, the skill scores of the winter surface temperature forecasts for weeks 3–4 and 5–6 over Northern Europe are slightly improved.
During boreal winter the Arctic Oscillation index and the mean surface temperatures over...
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