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Predictability of weather and climate, seasonal prediction, seamless prediction
Reto Knutti, IAC ETH(with material from Andreas Weigel, Meteoswiss/SwissRE)
ETH Zurich | Reto Knutti
Reto Knutti / David Bresch, ETH Zurich
Weather prediction has a value
Prediction of storm Joachim 16.12.2011
«COSMO» Prediction of probability for wind gusts >90km/h (Forecast lead time 36h).
(Blic
k am
Abe
nd 1
6.12
.11)
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Reto Knutti / David Bresch, ETH Zurich
Timescales of forecasts
Question 1: Why do we have weather forecast and climateprojections but nothing in between?
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Reto Knutti / David Bresch, ETH Zurich
Quality of forecasts
Question 2: Why is the forecast sometimes completelyunclear, and sometimes almost certain for many days?
Early March 2016 Early July 2015
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Reto Knutti / David Bresch, ETH Zurich
Predictability of the first kind The sensitivity to initial
conditions can be shown with the conceptual three component Lorenz model (Lorenz 1963)
3 coupled differential equations
Sensitivity to initial conditions, i.e. predictability depends on the state of the system.
Adapted from M. Liniger & T. Palmer
x
z
y
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Reto Knutti / David Bresch, ETH Zurich
Predictability of the first kind
high predictability
medium predictability
low predictability
Adapted from M. Liniger & T. Palmer
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Reto Knutti / David Bresch, ETH Zurich
Predictability of the first kind
Adapted from M. Liniger & T. Palmer
The memory of the atmosphere to initial conditions is limited to approximately 10 days
The memory of the oceans to initial conditions can range from months to years
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Reto Knutti / David Bresch, ETH Zurich
Weather prediction works, and improves
Bauer et al., Nature 2015
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Reto Knutti / David Bresch, ETH Zurich
Summary
Two kinds of predictability:Predictability of first kind (initial conditions) and predictability of second kind (boundary conditions)
Weather forecasting relies on initial conditions and exploits predictability of first kind
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Reto Knutti / David Bresch, ETH Zurich
Predictability of the second kind
(Palmer, 1998)
Experiment 1: coincidence
Experiment 2: With boundary cond.
Even though individual weather events are not predictable beyond 10 days, the average weather behavior (=climate) may be influenced by predictable boundary conditions for several months and longer.
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Reto Knutti / David Bresch, ETH Zurich
Sectors affected by seasonal climate variability
Tourism Water resources
management Energy Agriculture Infrastructure Consumer goods industry Insurance …
Switzerland, winter 2001/02
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Reto Knutti / David Bresch, ETH Zurich
The Böögg
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Reto Knutti / David Bresch, ETH Zurich
The Böögg
Böögg
Time until head explodes (min)
Mea
n JJ
A te
mpe
ratu
re
R2 = 0.007p = 0.60
heat summer 2003
Schmuki & Weigel, 2006
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Reto Knutti / David Bresch, ETH Zurich
El Nino
SST anomalies April 2016Source: NOAA
normal
El Niño
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Reto Knutti / David Bresch, ETH Zurich
El Nino El Niño is felt worldwide Being able to predict El Niño
implies being able to predict climate anomalies around the globe (in certain regions and certain seasons)
Other sources of seasonal predictability: SST anomalies in Indian and
Atlantic oceans Anomalies in soil moisture (e.g.
European summer) Anomalies in continental snow
cover (e.g. European spring)
El Niño
La Niña
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Reto Knutti / David Bresch, ETH Zurich
El Nino provides seasonal predictability
http://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php?lead=2
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Reto Knutti / David Bresch, ETH Zurich
El Nino links to weather
Weather Channel forecasts on March 6, 2006; https://weather.com/storms/tornado/news/severe-flood-forecast-march-7-12-2016
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Reto Knutti / David Bresch, ETH Zurich
El Nino
Dec 27, 2015
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Reto Knutti / David Bresch, ETH Zurich
2016 Forecast and verification
https://www.ncdc.noaa.gov/temp-and-precip/us-maps/3/201605#us-maps-select
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Reto Knutti / David Bresch, ETH Zurich
Ensembles
Look at distribution rather than
single values:
PROBABILITYFORECASTS
To account for initial condition uncertainty, the analysis is perturbed and ensembles are generated which sample the distribution of possible initial conditions, given the observations available.
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Reto Knutti / David Bresch, ETH Zurich
Calibration
Observed climatology
Prob
abilit
y
Temperature
ModelClimatology
Prob
abilit
y
Temperature
How can we obtain these transfer functions?
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Reto Knutti / David Bresch, ETH Zurich
Calibration
Hindcast 1 Observation 1
Hindcast 2 Observation 2
Hindcast N Observation N
2009
2008
1981
Mod
el c
limat
olog
y
Obs
erve
d cl
imat
olog
y Forecasts of past cases (so-called hindcasts) are used to derive
correction terms for systematic biases in mean and variance. This procedure is called calibration.
At MeteoSwiss, hindcasts are made back to 1981.
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Reto Knutti / David Bresch, ETH Zurich
Summary
Using all observations available, a best guess of the initial conditions of the ocean is obtained by data assimilation (the analysis).
To sample the uncertainty distribution of possible initial conditions, given the observations, ensembles are generated by perturbing the analysis.
Dynamical coupled atmosphere-ocean general circulation models are applied to calculate the evolution of each ensemble member.
Typically 20 to 40 years of hindcasts are calculated to derive correction functions to remove systematic biases in mean and variance (calibration).
Forecast skill depends on the lead time, variable, the region, the initial state, etc. Poor skill may be due to a poor model, but can also simply be a consequence of limited predictability.
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Reto Knutti / David Bresch, ETH Zurich
Ranked probability score
The RPS is defined as the squared area enclosed by the forecast CDF and the observation CDF.
Width and location matter.
Perfect forecast: RPS = 0
Imperfect forecast:RPS > 0
Cum
ulat
ive
prob
abilit
y
CDF variable
66%
100%
33%
variable
Prob
abilit
y de
nsity
Cum
ulat
ive
prob
abilit
y
CDF variable
66%
100%
33%
variable
Prob
abilit
y de
nsity
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Reto Knutti / David Bresch, ETH Zurich
Ranked probability skill score (RPSS)
Often, one wants to know how much added value a forecast provides with respect to climatology: Ranked Probability Skill Score (RPSS):
Quantifies deviation of climatologic forecasts from observation
Quantifies deviation of ensemble forecasts from observation
Perfect: RPSS = 1Skill: RPSS > 0No skill: RPSS ≤ 0
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Reto Knutti / David Bresch, ETH Zurich
Skill of System 3 for temperature
prediction climate prediction
climate
Winter1981-2007
RPSS
0.4
-0.4
0
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Reto Knutti / David Bresch, ETH Zurich
Overconfidence in real forecasts
Fcst 1 Nov 2007 Fcst 1 Aug 2008
Overconfidence can imply negative skill.27
Reto Knutti / David Bresch, ETH Zurich
Skill of System 3 for temperaturespring summer
autumn winter
?
?
0.4-0.4 0RPSS
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Reto Knutti / David Bresch, ETH Zurich
Single model vs. multi-modelFcst 1 Aug 2008
ECMWFECMWF
+ UK Met Office+ Météo-France
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Reto Knutti / David Bresch, ETH Zurich
Skill of multi-models (summer predictions)
ECMWF
UKMO
UKMO + ECMWF
Further improvement by weighting ?
0.4-0.4 0
RPSS
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Reto Knutti / David Bresch, ETH Zurich
Recalibration: rescale and inflate
Rescale ensemble mean + inflate ensemble spread
r
s
(Weigel et al, 2009, Mon. Wea. Rev.)
Values of r and s can – like model weights - be estimated from verification data
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Reto Knutti / David Bresch, ETH Zurich
Conventional and recalibrated forecastsJJA forecasts of T2m, Initialization 1 May,1960-2001
0.4
-0.4
0Conventional
Recalibrated RPSS
Weigel et al, 2009, Mon. Wea. Rev.32
Reto Knutti / David Bresch, ETH Zurich
Seasonal forecast for Switzerland
http://www.meteoswiss.admin.ch/home/climate/future/seasonal-outlook.htmlhttp://www.meteoswiss.admin.ch/home/climate/present-day/climate-development.html
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Reto Knutti / David Bresch, ETH Zurich
Seasonal forecast for Switzerland
(http://www.meteoswiss.admin.ch/home/climate/future/seasonal-outlook/background-information-on-seasonal-climate-outlook.html)
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Reto Knutti / David Bresch, ETH Zurich
Summary
The verification of ensemble forecasts requires a sufficient number of verification samples and involves the application of probabilistic skill metrics.
One of the most widely used skill scores is the ranked probability skill score (RPSS), a probabilistic generalization of the mean squared error.
Seasonal forecasts show high prediction skill in the tropics, particularly the ENSO region. Predictability is low in the extratropics.
In central Europe, seasonal forecasts currently are at best only slightly better than climatology.
In some regions, negative skill is observed.
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Reto Knutti / David Bresch, ETH Zurich
Applications: Malaria
Red dots: Estimate of malaria incidence
Epidemics with high death rates occur in wetter and/or warmer-than-average years (Thomson et al., 2006, Nature)
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Reto Knutti / David Bresch, ETH Zurich
Energy trading
Nov. 2002 – Feb. 2003
EEX
pric
es(B
lock
Bas
e)
HD
D a
nom
alie
sov
er W
este
rn E
urop
e
HDD = heating degree dayEEX = European energy exchange
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Reto Knutti / David Bresch, ETH Zurich
Predictability in climate change
Scenarios Model structureParameters
VariabilityInitial conditions
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Reto Knutti / David Bresch, ETH Zurich
Sources of uncertainty
(Hawkins and Sutton, 2009)39
Reto Knutti / David Bresch, ETH Zurich
Model agreement in CMIPNew (CMIP5) Old (CMIP3)
Stippling: high model agreement, hatching: no significant change,white: inconsistent model projections (Knutti and Sedlacek, 2012)
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Reto Knutti / David Bresch, ETH Zurich
Model agreement in CMIPNew (CMIP5) Old (CMIP3)
Stippling: high model agreement, hatching: no significant change,white: inconsistent model projections (Knutti and Sedlacek, 2012)
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Reto Knutti / David Bresch, ETH Zurich
Lack of precipitation change explains much of the lack of model agreement
(Knutti and Sedlacek, 2012)42
Reto Knutti / David Bresch, ETH Zurich
Kerr Science 2011But ask researchers what’s in store for the Seattle area, the Pacific Northwest, or even the western half of the United States, and they’ll often demur. As Mass notes, “there’s tremendous uncertainty here,” and he’s not just talking about the Pacific Northwest. Switching from global models to models focusing on a single region creates a more detailed forecast, but it also “piles uncertainty on top of uncertainty,” says meteorologist David Battisti of UW Seattle. First of all, there are the uncertainties inherent in the regional model itself. Then there are the global model’s uncertainties at the regional scale, which it feeds into the regional model. As the saying goes, if the global model gives you garbage, regional modeling will only give you more detailedgarbage. And still more uncertainties are created as data are transferred from the global to the regional model.
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Reto Knutti / David Bresch, ETH Zurich
Variability in a the 40 member CCSM ensemble
(Deser et al., 2012)44
Reto Knutti / David Bresch, ETH Zurich
Variability in a the 40 member CCSM ensemble
(Deser et al., 2012)45
Reto Knutti / David Bresch, ETH Zurich
Seamless prediction“[…] Advances in climate prediction will require closecollaboration between the weather and climate predictionresearch communities. It is essential that decadal and multi-decadal climateprediction models accurately simulate the key modes ofnatural variability on the seasonal and sub-seasonal time scales. […] This synergy between the weather and climate predictionefforts will motivate further the development of seamlessprediction systems. […]”
Summit Statement from World Modelling Summit for Climate Prediction, 6-9 May 2008, ECMWF, Reading (UK)
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Reto Knutti / David Bresch, ETH Zurich
Making predictions
Damage threshold What will the nextdecadesbring?
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Reto Knutti / David Bresch, ETH Zurich
Making predictions Understand the system Characterize trend, mean, variability, memory Build a model, make predictions, to estimate the probability of exceeding
the damage threshold, quantify damage Quantify uncertainty
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Reto Knutti / David Bresch, ETH Zurich
Forced response vs. variability
Damage threshold
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Reto Knutti / David Bresch, ETH Zurich
Periodic forced response?
Damage threshold
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Reto Knutti / David Bresch, ETH Zurich
Initial condition predictability
Damage threshold
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Reto Knutti / David Bresch, ETH Zurich
Model error, verification
Damage threshold
Linear trend plus initialized red noise
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Reto Knutti / David Bresch, ETH Zurich
Making predictions Understand the system Characterize trend, mean, variability, memory Build a model, make predictions, to estimate the probability of exceeding
the damage threshold, quantify damage Quantify uncertainty and model error
53