Predictability and Chaos EPS and Probability Forecasting
Dec 17, 2015
Objectives of this session
Appreciate that NWP is not the complete answer
State reasons for uncertainties in weather prediction
Understand how the principle of chaos effects predictability of the atmosphere
Appreciate how ensemble forecasts help to account for chaos
Deterministic versus Probabilistic forecasts
Deterministic forecast - A forecast in which a single answer is given – It will snow this afternoon
– Temperatures will reach 4C today
Probabilistic forecast – A forecast in which a numerical estimate of the certainty of the forecast is given– 30% chance of a shower
Why is there uncertainty in weather forecasting?
The variability of ‘local’ weather– Exactly where will a shower fall?
Analysis errors – NWP models sensitive to errors in initial
state
Systematic errors in NWP models– Assimilation, parametrization
Predictability
Errors in initial conditions have different effects
Why is the atmosphere predictable
on some occasions, not on others?
Chaos Theory !
Definition of Chaos
Dictionary Definition:Lack of form or systematic arrangement
Scientific Definition: Processes that are not random but look
random
– Random - toss a coin
– Chaotic - a pin ball machine
Edward Lorenz
1963 Massachusetts Institute of Technology
Used 3 equations in a simple model Truncating numbers produced
different results Introduced concept of “attractors” to
describe the state of dynamical systems
– certain states will never occur
Why is the atmosphere chaotic?
Weather patterns are not totally random– e.g. seasonal variation is regular
… but they can appear so. Climate is the Attractor
– Set of patterns that have at least some chance of occurring
– Heat-wave in Arctic, snow in Sahara do not occur
Implications of chaos theory
There is no one single solution to find There is a time limit beyond which
deterministic forecasts of daily weather become unpredictable
The outcome of all forecasts could be a set of probabilities
The predictability of the atmosphere will vary depending upon its initial state
NWP SYSTEM
ClimatologyClimatology
Predictability Predictability rangerange
The forecastThe forecast
Deterministic Deterministic solutionsolution
NWP SYSTEM
ClimatologyClimatology
Better model:Reduce the errorPredictability Predictability
rangerange
Deterministic Deterministic solutionsolution
Coping with chaos: EPS
Ensemble forecasting at ECMWF: 51 forecasts run from similar initial
conditions
Use a lower resolution model (T399)
Used for guidance beyond 3-4 days
Generates a lot of data!
Interpreting Ensemble Data
The presentation of results is important Need to reduce the different solutions to something manageable
Clustering - grouping solutions that are similar
Probability forecasting
Deterministic versusprobabilistic forecasts
Deterministic forecast - A forecast in which a single answer is given – It will rain this afternoon
– Temperatures will reach 11 C today
Probabilistic forecast - Forecasts in which a numerical estimate of the certainty of the forecast is given– 30% chance of a shower