European windstorms: seasonal prediction and a signal to noise paradox - Prof. Adam Scaife
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European windstorms: seasonal prediction and a signal to noise paradox Prof. Adam Scaife Met Office Hadley Centre College of Engineering, Mathematics and Physical Sciences, Exeter University metoffice.gov.uk © Crown copyright
The NAO and why it is important to you
Walz et al, 2018 Mailier et al, 2018
The NAO is a key driver of year to year variations in European windstorms
This large scale interannual variability leads to apparent clustering of storms
Computer Models of Weather and Climate
5 fundamental equations + ideal gas law
Represented on a 3D grid of points
Small scale processes in ‘S’
The problem is well defined but these equations have
an unfortunate property…
Scaife et al, Physics World, 2007
‘One flap of a seagull’s wings may forever change the
course of the weather’
Ed Lorenz (1917-2008)
- Small changes now can grow into storms or dead calm a few months later
- Does this scupper long range forecasting?
- Can we use our computer models to investigate the possibilities?
Met Office Supercomputer
@sallybasker
Predictability of winter weather
from early November
Winter seasonal predictions
Skilful predictions of the North Atlantic Oscillation
Retrospective forecasts in red
Real time forecasts in blue
Real world observations in black
So where does this come from?
Correlation = 62%
Scaife et al, GRL, 2014
The importance of tropical rainfall
High skill in winter rainfall forecasts Tropical rainfall explains NAO forecast Tropical rainfall explains real NAO
Scaife et al., IJOC, 2018
Scaife et al., QJRMS, 2017
Predicted signals in the NAO come largely from tropics
Rainfall may even give a better prediction than the predicted NAO!
So how good are models at predicting tropical rainfall?
Seasonal forecasts do a decent job for rainfall
(despite what people say about computer models)
WINTER AVERAGE YEAR TO YEAR VARIATIONS
REAL
WORLD
MODEL
Scaife et al., IJOC, 2018
Prediction skill is near perfect in the
East Pacific
Scaife et al., IJOC, 2018
Prediction skill is very high in the
West Pacific
Scaife et al., IJOC, 2018Prediction skill is high in the Atlantic
Ocean
Scaife et al., IJOC, 2018Prediction skill is reasonable in the
Indian Ocean (but varies across models)
Scaife et al., IJOC, 2018ENSO explains ranking of interannual
variability and ranking of skill
Top row shows variability
Bottom row shows effect of ENSO
Note ranking:
TEP > TWP > TAT > TIO
Scaife et al., IJOC, 2018From tropical rainfall to the extratropics
Rossby Wave Sources Teleconnections as Rossby waves
A few common source regions Poleward and eastward propagation of waves
Fluctuate with forcing from tropical rainfall Introduces som symmetry about the equator
Scaife et al, QJRMS, 2017Tropical rainfall predictions allow
good predictions of the NAO
Observed rainfall => Observed NAO
Modelled rainfall => Observed NAO
Single models can beat a multi-model
Rainfall gives better prediction than forecast NAO
Scaife et al., IJOC, 2018A fly in the ointment: The signal to noise paradox metoffice.gov.uk © Crown copyright
Revisit the predictability of the NAO
Winter NAO seasonal predictions
Model
predicting real
Skilful predictions of the NAO world
But signals are small
Winter NAO predictions
Model
Skill rises slowly with ensemble size predicting itself
Model can predict the real world but not itself!
© Crown
Scaife et copyright
al 2014, Met Office
Eade et al 2014, Siegert et al 2015, Dunstone et al 2016, Scaife and Smith 2018A simple interpretation of
what’s going on
Real world Model
24 hPa2
Unpredictable
58 hPa2
64 hPa2 64 hPa2
40 hPa2
Predictable 6 hPa2
Forecasts appear to have about the right amount of variability
BUT
The proportion of variability that is predictable appears to be less in modelsAnother use of ensemble forecasts: UNprecedented Simulated Extremes using ENsembles metoffice.gov.uk © Crown copyright
January 2014: just a freak? What chance of recurrence?
www.theguardian.co.uk
January 2014 saw the greatest
monthly rainfall total on record
Could it have been even worse?
metoffice.gov.uk © Crown copyrightUNprecedented Simulated Extremes using ENsembles
SE England Rainfall Record SE England Rainfall Simulations
Observations
Model
Observations are inherently uncertain due to atmospheric chaos
Dynamical models now getting good enough in some cases to be indistinguishable from observations
=> Availability of many more samples & unprecedented events
metoffice.gov.uk Thompson et©al.,
Crown
Nat. copyright
Comm., 2017Unprecedented weather conditions…
Weather patterns for extreme rainfall in observations
Our physically based computer simulations have realistic but different patterns
Both the NAO and EAP patterns are visible in these extreme cases
metoffice.gov.uk Thompson et©al.,
Crown
Nat. copyright
Comm., 2017Global applications: heatwaves in SE China
We can use this method to estimate extremes in other regions
We can also track down remote global influences
Thompson et al., Clim. Dyn., 2018Summary
Skilful seasonal prediction of the winter NAO and hence European windstorm
activity is now possible and is driven largely from the tropics
It’s mainly the NAO that is predictable (rather than other important patterns)
Predicted signals are inexplicably small
Not just a simple matter of errors in spread
Models are unpredictable but they can predict the real world => ‘Signal to Noise Paradox’
Irrespective of any predictability, these ensemble simulations can also be used
to investigate the chances of extreme and even unprecedented events as well
as their meteorologyYou can also read
