Stratospheric Nudging And Predictable Surface Impacts (SNAPSI): a protocol for investigating the role of stratospheric polar vortex disturbances in subseasonal to seasonal forecasts

Hitchcock, Peter B.; Butler, Amy H.; Charlton‐Perez, Andrew; Garfinkel, Chaim I.; Stockdale, Tim; Anstey, James; Mitchell, Dann; Domeisen, Daniela I. V.; Wu, Tongwen; Lu, Yixiong; Mastrangelo, Daniele; Malguzzi, P.; Lin, Hai; Muncaster, Ryan; Merryfield, William; Sigmond, Michael; Xiang, Baoqiang; Jia, Liwei; Hyun, Yu‐Kyung; Oh, Jiyoung; Specq, Damien; Simpson, Isla R.; Richter, Jadwiga H.; Barton, Cory; Knight, Jeff; Lim, Eun‐Pa; Hendon, Harry H.

doi:10.5194/gmd-15-5073-2022

Cited by 18 publications

(8 citation statements)

References 86 publications

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“…Our findings highlight that more comprehensive and extended model outputs are useful for understanding model biases and their origin, including evaluating the realism of stratospheric processes and to quantify benefits from model developments. However, given the complex interactions between waves and mean flow in the stratosphere, nudging experiments (Hitchcock et al., 2022; Morgenstern et al., 2017) would be useful to identify the origins of biases.…”

Section: Discussionmentioning

confidence: 99%

Changes in Stratospheric Dynamics Simulated by the EC‐Earth Model From CMIP5 to CMIP6

Serva,

Christiansen,

Davini

et al. 2024

J Adv Model Earth Syst

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The simulated stratospheric dynamics have been improved compared to previous generations in many climate models taking part in the Coupled Model Intercomparison Project Phase 6 (CMIP6). This was achieved by going from low to high‐top configurations, that is, increasing the atmospheric vertical resolution, raising the model lid height and including parameterization schemes, such as non‐orographic gravity wave drag (GWD), to simulate small‐scale processes. This also applies to the EC‐Earth model, for which comprehensive analysis is now possible thanks to outputs archived within the Dynamics and Variability Model Intercomparison Project (DynVarMIP). Here we analyze atmosphere‐only simulations and evaluate the stratospheric dynamics of low‐top CMIP5 and high‐top CMIP6 configurations of EC‐Earth against the MERRA‐2 and ERA5 reanalyses. This allows us to identify and interpret model biases in the atmospheric component and to highlight deficiencies needed to be addressed in future model versions. We find substantial improvements in the simulation of the stratosphere in the CMIP6 configuration, both in the tropical and extratropical regions. The zonal wind momentum budget in the tropical stratosphere is much improved in the high‐top configuration. Seasonal variability of both models is qualitatively similar to the reanalysis, but for the low‐top model this is likely due to compensating errors, as its intraseasonal variability is less realistic. The non‐orographic GWD forcing is weak in the high‐top model and this limits the realism of the tropical stratosphere variability. We point to persisting issues in the stratosphere of EC‐Earth that should be tackled to improve hemispheric‐scale circulation and variability.

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Section: Discussionmentioning

confidence: 99%

Changes in Stratospheric Dynamics Simulated by the EC‐Earth Model From CMIP5 to CMIP6

Serva,

Christiansen,

Davini

et al. 2024

J Adv Model Earth Syst

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“…The settings for parameterized gravity wave drag are un-changed between the different vertical-resolution configurations. To verify whether changes in gravity wave drag are caused directly by changes in the vertical resolution as opposed to changes in the stratospheric zonal-mean basic state, we performed nudged simulations following the SNAPSI protocol (Hitchcock et al, 2022). Specifically, zonal-mean temperature, vorticity, and divergence above 90 hPa are relaxed towards the "observed" time series of ERA5 on a timescale of 6 h. Consequently, there are no significant differences in the zonal-mean zonal wind and polar-cap temperature anomaly between the different model configurations in the nudged simulations.…”

Section: Methodsmentioning

confidence: 99%

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

Wicker

Polichtchouk

Domeisen

2023

Weather Clim. Dynam.

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Abstract. Sudden stratospheric warmings (SSWs) 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 associated polar 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 low 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.

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“…Historical analysis suggest at least one third of SSWs immediately follow anomalous tropospheric wave activity, with the longer‐term accumulation of upward wave activity also important (Birner & Albers, 2017; de la Cámara et al., 2019; Polvani & Waugh, 2004). Understanding the roles of these different processes within subseasonal forecasts is an active area of research (Hitchcock et al., 2022).…”

Section: Introductionmentioning

confidence: 99%

“…Understanding the roles of these different processes within subseasonal forecasts is an active area of research (Hitchcock et al, 2022).…”

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confidence: 99%

Identifying Perturbations That Tipped the Stratosphere Into a Sudden Warming During January 2013

Kent,

Scaife,

Seviour

et al. 2023

Geophysical Research Letters

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We present a new methodology demonstrating that specific small‐magnitude regional perturbations can cause large systematic responses in subseasonal predictions. We show this with ensemble forecasts of the January 2013 Sudden Stratospheric Warming (SSW) from an operational long‐range global prediction system. In forecast members which predict the SSW, tropospheric ridging over the North Atlantic is strengthened 10 days prior to the event. This subsequently enhances planetary wave forcing and weakens the stratospheric polar vortex. Transplanting tropospheric conditions in this region from “correct” to “incorrect” forecasts (and vice versa) significantly alters the SSW forecast probability. The occurrence of this SSW is therefore strongly controlled by the troposphere several days prior. Tipping more members into a sudden warming also significantly affects surface predictions the following month. Despite chaotic behavior of the climate system, small‐magnitude synoptic‐scale perturbations can drive different dynamical states and systematically impact medium and long‐range predictions.

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Stratospheric Nudging And Predictable Surface Impacts (SNAPSI): a protocol for investigating the role of stratospheric polar vortex disturbances in subseasonal to seasonal forecasts

Cited by 18 publications

References 86 publications

Changes in Stratospheric Dynamics Simulated by the EC‐Earth Model From CMIP5 to CMIP6

Changes in Stratospheric Dynamics Simulated by the EC‐Earth Model From CMIP5 to CMIP6

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

Identifying Perturbations That Tipped the Stratosphere Into a Sudden Warming During January 2013

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