18 Jul 2022
18 Jul 2022
Status: this preprint is currently under review for the journal WCD.

Increased vertical resolution in the stratosphere reveals role of gravity waves after sudden stratospheric warmings

Wolfgang Wicker1, Inna Polichtchouk2, and Daniela I. V. Domeisen1,3 Wolfgang Wicker et al.
  • 1Université de Lausanne, Lausanne, Switzerland
  • 2European Centre for Medium-Range Weather Forecasts, Reading, UK
  • 3ETH Zurich, Zurich, Switzerland

Abstract. Sudden stratospheric warmings (SSW) have a long-lasting effect within the stratosphere as well as impacts on the underlying troposphere. However, sub-seasonal forecasts of the winter polar stratosphere fail to use their full potential for predictability as they tend to underestimate the magnitude and persistence of these events already within the stratosphere. The origin of this underestimation is unknown. Here, we demonstrate that the state-dependent stratospheric cold bias following SSW events in sub-seasonal hindcasts can be halved by increasing vertical model resolution, suggesting a potential sensitivity to gravity wave forcing. While the predictability of the planetary Rossby wave flux into the stratosphere at lead times longer than a week is limited, the existence of a critical layer for gravity waves with a small zonal phase speed caused by the disturbed polar vortex provides predictability to the upper stratosphere. Gravity wave breaking near that critical layer can, therefore, decelerate the zonal flow consistently with anomalous subsidence over the polar cap leading to warmer temperatures in the middle polar stratosphere. Since the spectrum of gravity waves involves vertical wavelengths of less than 4000 m, as estimated by wavelet analysis, a high vertical model resolution is needed to resolve the positive feedback between gravity wave forcing and the state of the polar vortex. Specifically, we find that at a spectral resolution of TCo639 (approximate horizontal grid-spacing of 18 km) at least 198 levels are needed to correctly resolve the spectrum of gravity waves in the ECMWF Integrated Forecasting System. Increasing vertical resolution in operational forecasts will help to mitigate stratospheric temperature biases and improve sub-seasonal predictions of the stratospheric polar vortex.

Wolfgang Wicker et al.

Status: open (until 30 Aug 2022)

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Wolfgang Wicker et al.

Wolfgang Wicker et al.


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Short summary
Sudden stratospheric warmings are extreme weather events where the winter polar stratosphere warms by about 25 K. An improved representation of small-scale gravity waves in sub-seasonal prediction models can reduce forecast errors since their impact on the large-scale circulation is predictable multiple weeks ahead. After a sudden stratospheric warming, vertically propagating gravity waves break at a lower altitude than usual which strengthens the long-lasting positive temperature anomalies.