the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Tropospheric Links to Uncertainty in Stratospheric Subseasonal Predictions
Abstract. Variability in the stratosphere, especially extreme events such as Sudden Stratospheric Warmings (SSWs), can impact surface weather. Understanding stratospheric prediction uncertainty is therefore crucial for skillful surface weather forecasts on weekly to monthly timescales. Using ECMWF subseasonal hindcasts, this study finds that stratospheric uncertainty is most strongly linked to tropospheric uncertainty over the North Pacific and Northern Europe, regions that can modulate but also respond to stratospheric variability, suggesting a two-way propagation of uncertainty. A case study of the 2018 SSW event shows an initial poleward and upward propagation of uncertainty from tropical convection, followed by a downward propagation where ensemble members that accurately predict the SSW also better at predicting its downward impacts. These findings highlight the locations in the troposphere that are linked to stratospheric uncertainty and suggest that improved model representation of tropospheric mechanisms linked to polar vortex variability could enhance both stratospheric and extratropical surface prediction.
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Status: open (until 17 Jul 2024)
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RC1: 'Comment on egusphere-2024-1652', Anonymous Referee #1, 20 Jun 2024
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General comments
The paper is well written and relevant for the community. Its aim is to evidence the link between tropospheric and stratospheric uncertainty in a climatological context, using subseasonal ensemble hindcasts issued by ECMWF. The focus on the example of the 2018 SSW prediction is useful to make a link with individual subseasonal forecast cases. After the comments below have been addressed, I will be happy to advise pubblication of the manuscript in Weather and Climate Dynamics.
Specific comments
Lines 23-24 : From this sentence it seems as though the only sources of uncertainty for the stratospheric variability are the mean state of the stratosphere and the vertical wave propagation from the troposphere. Could you make clear that vortex preconditioning, also regulated by internal stratospheric oscillations, is an important factor modulating vertical wave propagation from the troposphere (hence stratospheric variability) ? For discussing this, I would advise to check the references in the second and third paragraph of the introduction of De la Cámara et al. 2019.
Line 78 : Your measure of (average) spread, used to separate hindcasts in large and small uncertainty groups, mixes short-term and subseasonal time ranges. Would the hindcast selection change if the spread was averaged starting from time +1 week (or +10 days) until the end of the hindcast period? Can you re-compute Figure 1 with this new selection as a sensitivity test ?
Figure 2 : It would be useful to show significance from the hindcast climatology (in a,b,c,d) and between the two hindcasts groups (in c,f).
Lines 218-232 : While the effect of mid-latitude precursors on the wave-2 propagation and the SSW development is quite clear and interpretable, the impact of the tropics is more subtle, and is suggested with a certain caution also in Statnaia et al. 2020. In Figure 5 a,b you show the tropical OLR before the event. How do you connect this directly to the anomalous propagation of wave-2 ? Does the mean error of the two ensemble means from the reanalysis in terms of OLR provide evidence that the tropics are better captured by the SSW ensemble ? In my opinion, in the Conclusions you stress a lot the impacts of the Tropics although the evidence supporting this is rather weak. The same also applies to a sentence in the Abstract.
Line 207 : It would be interesting to mention the reason for the large-spread ensamble having a stronger vortex at initial times.
Technical corrections
Line 7 : missing verb.
Lines 18-22 : Break up in two sentences.
Lines 30-31 : Check incorrect wording.
Lines 37-41 : Break up in two sentences.
Line 48 : « To » or « with » instead of « into ».
Lines 69-70 : Why do you mention the computation date ? I find this quite confusing when talking about hindcasts for a different date in 2018.
Line 72 : I would suggest to change u1060 to something more clear like U10hPa60N
Line 124 : Uncertainty is large also where the ensemble mean flux is anomalously negative.
Line 240 : Repetition of « in this region ».
Cámara, A. d. l., T. Birner, and J. R. Albers, 2019: Are Sudden Stratospheric Warmings Preceded by Anomalous Tropospheric Wave Activity?. J. Climate, 32, 7173–7189, https://doi.org/10.1175/JCLI-D-19-0269.1.
Citation: https://doi.org/10.5194/egusphere-2024-1652-RC1
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