Recurrent Rossby wave packets and persistent extreme weather - EGU 2021 S. Mubashshir Ali, Olivia Martius, Matthias Röthlisberger, John Methven
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Recurrent Rossby wave packets and persistent extreme weather S. Mubashshir Ali, Olivia Martius, Matthias Röthlisberger, John Methven EGU 2021 mubashshir.ali@giub.unibe.ch
What are Recurring Rossby wave packets (RRWPs)? 2
RRWPs are a sequence of synoptic-scale wave packets amplifying in the same region in a short period Series of wave packets amplifying in the same phase in time quick succession (within a week) 3
Example events: 2017-18 DJF • Transient waves with fast phase propagation • Trough and ridges recur at the same location Notable events: Beast from the East, 95 casualties, damages: £ 1.2 billion time 42 fatalities in US, damages: approx. $ 5.7 billion 4
Example events: 2013-14 DJF • Transient waves with fast phase propagation • Trough and ridges recur at the same location Anomalous NH winter 2013-14 (Davies 2015) time 5
How to assess the impact of RRWPs on a climatological time-scale? 6
Quantifying RRWPs using a metric, “R” 14-day running mean wavenumber filter envelope V@250hPa, averaged between 35°N-65°N Röthlisberger et al. 2019 7
Using Weibull regression model • The impact on spells is assessed via a parametric regression model commonly used in survival analysis. (Zhang 2016) • Model outputs the factor of change in all quantiles of the predicted spell duration per unit increase in R. • exp 1 > 1 ∶ R Spell durations • exp 1 < 1 ∶ R Spell durations 8
Data • ERA-I reanalysis dataset on 1x1 spatial resolution • Period: 1980-2016 • 6 hourly T2M data for temperature spells • Aggregated daily precipitation data for wet and dry spells 9
Spell definition Hot: T2M (6 hourly) > 85th quantile One continuous spell Dry: Daily precipitation < 1 mm Gap of 1 day Wet: Daily precipitation > 1 mm mm Days Discard spells lasting less than 5 days 10
In the following slides we show areas affected by RRWPs, i.e. the output from the regression model 11
RRWPs increases hot spells across the globe Hot spells Röthlisberger et al. 2019 Red coloured areas are where RRWPs significantly increase hot spell duration. Ali et al. 2021 (in prep.) Stippling show grids where Weibull model assumption fails. 12
RRWPs can both increase or decrease dry spells Dry spells Ali et al. 2021 Brown coloured areas are where RRWPs significantly increase dry spell duration. 13
RRWPs can both increase or decrease wet spells Wet spells Ali et al. 2021 Turquoise coloured areas are where RRWPs significantly increase wet spell duration. 14
Implications and Outlook • RRWPs can foster weather spells resulting into extreme weather. • What causes RRWPs – role of planetary-scale flow? [on going work]. • Potential to improve sub-seasonal predictability. • Process-based diagnostic in Climate change simulations. 15
Thank You 16
• Ali, S.M., Martius, O. and Röthlisberger, M., 2021. Recurrent Rossby wave packets modulate the persistence of dry and wet spells across the globe. Geophysical Research Letters, 48(5), p.e2020GL091452. • Davies, Huw C. 2015. Weather chains during the 2013/2014 winter and their significance for seasonal References prediction. Nature Geoscience 8.11, 833. • Röthlisberger, Matthias, et al. 2019: Recurrent synoptic-scale Rossby wave patterns and their effect on the persistence of cold and hot spells. Journal of Climate 32.11, 3207-3226. • Zhang, Z., 2016: Parametric regression model for survival data: Weibull regression model as an example. Ann. Transl. Med., 4, 484. • Zimin, A. V. et al., 2003: Extracting envelopes of Rossby wave packets. Mon. Wea. Rev., 131, 1011– 1017. 17
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