introduction to ensemble prediction systems (eps)

Introduction to Ensemble Prediction Systems (EPS)Jos A. Garca-Moya & Carlos SantosAAPL-AEMETEumetcal NWP Applications CourseNovember 2009

Eumetcal NWP Applications Course - Chaos & Predictability

OutlineHow to introduce in operational forecasting.Uncertainties of deterministic models.Models to make probabilistic prediction, PDF.Predictability, spread and skill.Extreme events.What is an EPS?ECMWFs EPSSingular vectorsStochastic PhysicsConclusions


Ideal forecasterIncomplete knowledge of the atmospherePerfect forecastWhat is a perfect forecast?


Added value forecasterPartial knowledge of the state of the atmosphereForecast and Direct Model OutputHow to introduce in operational forecasting


Deterministic ModelPDEsSingle set of Initial ConditionsSingle PredictionDeterministic NWP Models


DA + more obsBetter model formulationBetter deterministic models++ Skill++ Increased forecast length 5 7d

Improvement of skill of NWP deterministic modelsCourtesy of B. Elvira


But there are still model failures D+3..D+7 (depending on weather regime)Why?Investigation of the sources of error in NWP modelsGetting deeper in theoryLorenzs ModelLimited performance of deterministic models


Sources of uncertainty


X = one point in the butterfly a single state of the atmosphere

All the X = the whole butterfly the ensemble of all possible states of the atmosphereLorenzs Strange Attractor Phase Space


Probabilistic Prediction ModelsSingle input = PDFSingle output = PDF


Time evolution of the PDF


PDF: Single place time evolution


The atmosphere is a nonlinear systemPredictability in function of timeSensitivity to the initial conditionsEvolution of PDF and spreadThe problem of predictability in a nonlinear systemCourtesy of T. Palmer


SpreadSkill

These factors are a key aspect of EPS.

There should be a linear relationship between them.

Calibration of the EPSSpread & skillCourtesy of T. Palmer


Pepe has a large orange ground in Valencia (Spain)If T2m falls down below 0 C deg oranges will frozen and Pepe will loose money.Pepe has several heating machines in the ground. If he switch on them oranges will not frozen but it costs some money.P is the probability of frost, C costs and L loses.Is C > Lp? If p > C/L then he must use the machines.See the butterflyPepes dilemmaCourtesy of T. Palmer


Forecaster based on the distribution of EPS members will make a frost-free weatherBut PDF had no spread enough and the verification wasThe large the spread the small the skillECMWF EPS forecastCourtesy of T. Palmer


To find an estimation of the weather PDF.To investigate the effects of the different sources of errors of the NWP models.To identify area with low predictabilityTo calibrate the flow dependent predictabilityTo compute the probability of different weather scenariosGoals of the EPS


What is an EPS?Ensemble Prediction System allows a probabilistic approach to weather forecasting

Using different methods an ensemble of forecasts for the same place is computed. Every of these forecasts is called member of the EPS. All together is:A PDF of the weather situation.It allows to see slightly different weather scenarios all of them with the same probability to be the best one.Give us back information about the predictability to the weather situation.


What is an EPS?


Perturbations in the initial conditionsHow to simulate uncertainty?Perturbations in the model physics


PDFSEFIClustersPlumesStampsTropical cyclone tracksPost-process


SummaryDirect EPS outputStampsPlumesControl vs OperationalSpaguetisPDFsEnsemble mean and SpreadProbability mapsEPSgramsSpecial post-processGroupsTubesEFITropical cyclonesWhat is new?Can we use EPS in a deterministic way?Quality and value of EPSConclusions


StampsHigh resolution operatinal1 Control50 members


PlumesPlots of variables vs. forecast length at one concrete place.There is a line for every ensemble member and colours for probabilities.It can be easily seen the dispersion of the EPS in that place.Predictability can be measured in function on that dispersion.


Control vs Operational (AEMET)Z500T500PsfcT850Pcp24h


SummaryDirect EPS outputStampsPlumesControl vs OperationalSpaguetisPDFsEnsemble mean and SpreadProbability mapsEPSgramsSpecial post-processGroupsTubesEFITropical cyclonesWhat is new?Can we use EPS in a deterministic way?Quality and value of EPSConclusions


Z1000 Martin StormEnsemble mean and spread


Probability maps


Interpolation of EPS in a placeVariables:CloudinessPrecipitation /6h10m wind speed2m temperatureTime evolution (D+1...D+10)/6h:Box-Plot (min-p10-p25-p50-p75-p90-max): PDF using EPS 50 membersRed: controlBlue: high resolution operational modelEPSgrams


To make information more understandable for the forecaster we can join members of EPS with similar evolution in a certain norm.We can choose cluster mean as a representative forecast of each clusterClustering (Wards algorithm):Similarity = diference RMS in Z500 para H+120..H+168 it guarantees synoptic continuity# clusters depends on three factors: Spread of the day, RMS threshold, multimodal degreeTubingFocusing on extreme more than in centroidTrying to get information on extreme weatherEvery method has advantages and disadvantages.Clustering


Grups: Centroid (I)


Productos derivados: ndice de prediccin extrema (EFI)


EPS is a probabilistic forecasting system.EPS spread can represent the uncertainty of the weather situation. This information cannot be get from a deterministic model.NOT newForecasters adjust the forecast according their experience on deterministic models errors (flow dependent errors, forecast length)The inconsistency of different model runs has been used as a sign of uncertainty of the forecast.NEWEPS gives an explicit representation of the uncertainty and they can represent the probability of extreme events.What is new?


EPS in Dutch TVCortesa de Robert Mureau, KNMI


[email protected]@inm.es


introduction to ensemble prediction systems (eps)

Documents

nwp modelsgetting

skillecmwf eps

c costs

dimprovement of skill

single state

model failures d

key aspect of eps

distribution of eps