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Use and Interpretation of ECMWF Products

Use and Interpretation of ECMWF Products. Use and Interpretation of ECMWF Products. Numerical weather prediction. The behaviour of the atmosphere is governed by a set of physical laws which express how the air moves, the process of heating and cooling, the role of moisture, and so on.

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Use and Interpretation of ECMWF Products

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  1. Use and Interpretation of ECMWF Products

  2. Use and Interpretation of ECMWF Products Numerical weather prediction • The behaviour of the atmosphere is governed by a set of physical laws which express how the air moves, the process of heating and cooling, the role of moisture, and so on. • Given a description of the current state of the atmosphere, numerical models can be used to propagate this information forwards to produce a forecast for future weather. • Resolution of the model is determined by available computing resources. It does not correspond to any natural scale separation. • Processes not resolved by the model must be ‘parametrized’. • Effective resolution is not same as model grid spacing. • Numerical algorithms are compromise between accuracy and speed; care needed to ensure numerical stability.

  3. Use and Interpretation of ECMWF Products • Observations • Acquisition/Pre-processing/Quality control/Bias correction • Data assimilation • Dynamical fit to observations • Forecasts • Product dissemination and archiving • Verification • operational/ pre-operational validation • Data Monitoring

  4. Use and Interpretation of ECMWF Products • Short-range weather forecast (0-2/3 days ahead) • Detailed prediction - regional forecasting system • Produce forecast few hours after observations are made • Medium-range weather forecast (2/3 days - 2 weeks ahead) • Less detailed prediction - global forecasting system • Produce forecast up to several hours afterobservations are made • Long-range weather forecast (more than 2 weeks ahead) • Predict statistics of weather for coming month or season • Climate prediction • Predicts the climate evolution on the basis of pre-defined scenarios (CO2, O3, …)

  5. Atmospheric model Atmospheric model Wave model Wave model Atmospheric model Ocean model Ocean model Wave model Use and Interpretation of ECMWF Products ECMWF forecasting systems Medium-range deterministic forecasts Medium-range EPS Monthly Forecasts Seasonal Forecasts

  6. Major assimilated data sets Use and Interpretation of ECMWF Products Polar, infrared Surface stations Polar, microwave Radiosonde balloons Aircraft Geostationary, IR Use and Interpretation of ECMWF Products --- January/February 2011

  7. Data assimilation system (4D-Var) Use and Interpretation of ECMWF Products • The observations are used to correct errors in the short forecast from the previous analysis time. • Every 12 hours we assimilate ~8,000,000 observations to correct the 100,000,000 variables that define the model’s virtual atmosphere. • This is done by a careful 4-dimensional interpolation in space and time of the available observations; this operation takes as much computer power as the 10-day forecast. Use and Interpretation of ECMWF Products --- January/February 2011

  8. Use and Interpretation of ECMWF Products The ECMWF IFS deterministic Model Physical processes with smaller scales than can be resolved by the model grid are represented by so-called “Parametrization Schemes” which represent the effect of the small-scale processes on the large-scale flow.

  9. Use and Interpretation of ECMWF Products Operational model grid Use and Interpretation of ECMWF Products --- January/February 2011

  10. Use and Interpretation of ECMWF Products Operational model levels High Resolution Deterministic forecast T1279:91 vertical levels First EPS vertical level Ensemble Prediction System (including Monthly) T639/T319: 62 vertical levels Below this arrow EPS levels and High Res Deterministic model vertical levels are identical Use and Interpretation of ECMWF Products --- January/February 2011

  11. Use and Interpretation of ECMWF Products ECMWF’S IFS web pages for WMO users… Username: wmo_tsms Password: v14DqK

  12. Use and Interpretation of ECMWF Products Forecasts are sometimes wrongHow do we deal with that?ECMWF’s Ensemble Prediction System

  13. Use and Interpretation of ECMWF Products ”The Blame Game” or ”The Passing of The Buck” The atmosphere is ”chaotic”! Atmosphere Erroneous observations misled the NWP! Scientists Computer models The NWP misled me! Forecaster The forecaster misled me! Customer/Public 2014-07-16

  14. Use and Interpretation of ECMWF Products MSLP 66-hour forecasts, VT: 16-Oct-1987, 6 UTC T399 EPS

  15. Use and Interpretation of ECMWF Products Why are forecasts sometimes wrong? • Initial condition uncertainties • Lack of observations • Observation error • Errors in the data assimilation • Model uncertainties • Limited resolution • Parameterisation of physical processes • The atmosphere is chaotic • small uncertainties grow to large errors (unstable flow) • small scale errors will affect the large scale (non-linear dynamics) • error-growth is flow dependant • Even very good analyses and forecast models are prone to errors

  16. Use and Interpretation of ECMWF Products What is an Ensemble Prediction System? • A set of forecasts run from slightly different initial conditions to account for initial uncertainties • At ECMWF perturbations are generated using singular vectors • The forecast model also contains approximations that can affect the forecast evolution • Model uncertainties are represented using “stochastic physics” • The ensemble of forecasts provides a range of future scenarios consistent with our knowledge of the initial state and model capability • Provides explicit indication of uncertainty in today’s forecast

  17. Initial Uncertainty Model Error Use and Interpretation of ECMWF Products Deterministic Forecasting Is this forecast “correct”? Temperature Forecast time Initial condition Forecast

  18. Use and Interpretation of ECMWF Products Ensemble Forecasting Temperature Forecast time Initial condition Forecast Complete description of weather prediction in terms of a Probability Density Function (PDF)

  19. Use and Interpretation of ECMWF Products Flow dependence of forecast errors 26th June 1995 26th June 1994 If the forecasts are coherent (small spread) the atmosphere is in a more predictable state than if the forecasts diverge (large spread)

  20. Use and Interpretation of ECMWF Products EPS grid

  21. Use and Interpretation of ECMWF Products EPS skill (RPSS-Ranked Probability Skill Score, temperature at 850hPA, NH)

  22. Use and Interpretation of ECMWF Products EPS skill compared to other centres T-2m, DJF 2008/09 NH (20°N - 90°N) BC vs. ERA-interim

  23. Use and Interpretation of ECMWF Products Early warnings of severe weather • ECMWF: early warnings (3-4 days ahead, or more) • Users are generally forecasters in Member States, not public • ECMWF products for severe weather: • Extreme Forecast Index (EFI) • Tropical cyclone tracks, strike probabilities

  24. Use and Interpretation of ECMWF Products EPS forecasts (field probabilities) Probability of precipitation more than 10mm in 24 hours Probability of 10m wind gusts more than 15 m/s Base Sun 12/10/08 12UTC, Valid Thu 16/10/08 12UTC

  25. Use and Interpretation of ECMWF Products EPS forecasts (Extreme forecast index) EFI for 24-hour 10m wind gusts EFI for 24 hours precipitation Base Sun 12/10/08 12UTC, Valid Thu 16/10/08 12UTC

  26. EPS forecasts: timeseries (EPSgram) Highest value of all members 90th centile 75th centile Median 25th centile 10th centile Lowest value of all members EPSgram for Reading Base Sun 11/10/09 00UTC

  27. EPS forecasts: timeseries (EPSgram) Highest value of all members 90th centile 75th centile Median 25th centile 10th centile Lowest value of all members EPSgram for Addis Abbaba Base Sun 11/10/09 00UTC

  28. Use and Interpretation of ECMWF Products Katrina forecasts (days from landfall) • 4 days • 3 days • 1.5 days

  29. Use and Interpretation of ECMWF Products EPS appearing on TV Netherlands Germany

  30. Use and Interpretation of ECMWF Products ECMWF’S EPS web pages for WMO users…

  31. Seasonal predictions at ECMWFCan the weather be predicted months in advance? Predictions may be possible a few months in advance based on the fact that irregular weather variations have been associated with El Niño - a warming of the Pacific Ocean near the equator- and La Niña, a similar event caused by the cooling of equatorial Pacific waters. The slow changes in the surface temperatures of the oceans are thought to impart a degree of predictability. Use and Interpretation of ECMWF Products

  32. THE EL NIÑO/SOUTHERN OSCILLATION (ENSO) CYCLE Use and Interpretation of ECMWF Products

  33. Chaotic nature of the atmosphere Use and Interpretation of ECMWF Products To deal with the chaotic processes in the atmosphere we use an ensemble of simulations: on the 1st of the month 40 forecasts are run for 7 months. They have initial conditions from 5-member ensemble of ocean analyses (wind and SST pert.) Seasonal forecasting does not give exact predictions, but it may allow us to describe the probability that a certain weather event can happen. 34

  34. Products Ocean Analysis Seasonal outlook: (up to 7 months ahead) - Forecasts for Nino3, Nino3.4 and Nino4 - Spatial plots (ens.mean anomaly, terciles ..) - Climagrams ( similar to Epsgrams) - Tropical storms Annual outlook: (up to 10 months ahead) - Forecasts for Nino3, Nino3.4 and Nino4 - Spatial plots (ens.mean anomaly, terciles ..) - Tropical storms Use and Interpretation of ECMWF Products

  35. Use and Interpretation of ECMWF Products • - 2m Temperature • Mean sea level pressure • Precipitation • Sea surface temperature • 850 hPa temperature • 500 hPa geopotential Forecast is made available on the 15th of eachmonth.

  36. Use and Interpretation of ECMWF Products • 2m Temperature • Mean sea level pressure • Precipitation • Sea surface temperature • 850 hPa temperature • 500 hPa geopotential Forecastismadeavailable on the 15th of each month.

  37. Use and Interpretation of ECMWF Products EUROSIP multi-model system • 3 Coupled Systems: ECMWF, Météo France, Met Office • Ensemble generation for the 3 systems is different • Met Office and Meteo-France systems are both running at ECMWF • Development of multi-model products is ongoing • EUROSIP products are available to WMO users

  38. Use and Interpretation of ECMWF Products EUROSIP

  39. Summary (1) Every month a wide range of products from the current seasonal forecast system is made available to the users. The ECMWF seasonal forecast is a good system for El Niño predictions. The skill of the past forecast should be used to assess the forecast uncertainties. Various skill estimates are available to the users. Checking the consistency of large-scale circulation anomalies is important to detect seasonal forecast signals, especially for regions with little skill Regional skill can be enhanced by defining a larger domain with a coherent climate. Predictive skill for seasonal averages is better than for monthly data In the multi-model EUROSIP system the uncertainties due to model errors are better represented so that the EUROSIP forecast is more reliable. ECMWF Newsletter N.110, N.111 and N.112 available at http://www.ecmwf.int/publications/newsletters/ contain articles describing the upgraded seasonal forecast system and its products Use and Interpretation of ECMWF Products

  40. Use and Interpretation of ECMWF Products Summary (2) A new oper. seasonal forecast system has been implemented in March 2007: • Improved predictions of tropical/summer variability than the previous system. • The range of products has been expanded and new products are made available to WMO users Products from the EUROsip Multi Model system are available on the web to the members states and will be soon made available to WMO users: • we are building up experience in the use of appropriated calibration/combination methods and verification thanks to the work done within the special projects. Various skill estimates conformed to the SVS are available to the users. • Statistics from the new system will be soon available as well as EUROsip verification

  41. Use and Interpretation of ECMWF Products ECMWF’S Seasonal Forecats web pages for WMO users…

  42. The Extreme Forecast Index (EFI)

  43. Use and Interpretation of ECMWF Products How can we forecast ‘extreme weather’ ? • What can be used to define ‘extreme’, right across the globe ? • Particular Thresholds ? NO • Return Periods ? Effectively YES • Therefore need reference threshold levels, for each weather parameter of interest, everywhere across the globe • Return Periods from observational data are not available everywhere • However global model forecasts provide output everywhere • Therefore global model forecasts can be used to provide the climatology from which to extract Return-Period-type information • Hindcasts are performed each time a model is upgraded, to provide the relevant climatology – some resolution-related imperfections, but overall a very useful dataset • This forms the basis for the EFI, or ‘Extreme Forecast Index’, whereby, for each gridpoint, the range of solutions (pdf) from the current ensemble, is compared with that gridpoint’s climatology from the hindcast data, to see how extreme these solutions are

  44. Use and Interpretation of ECMWF Products • Why is it good to have an EFI ? • The EPS system provides a HUGE amount of information; it is hard to synthesize. EFI provides a summary measure towards potentially extreme situations. • Definition of extreme weather is highly climate dependent, it varies from location to location. EFI delivers model-climate-related information, therefore it can be used as a generic “alarm bell” for extreme weather situations over any area. • Why is the probability product for example (or something similar) not enough ? • It is advantageous to take into consideration the whole of the EPS distribution, not just a section of it. EFI provides a picture with broader basis. • Consider the following two quite extreme 4-value EPS distributions: • Simple probabilities (eg > 35C) will not highlight the differences in the above. EFI will, by accounting for the distribution of all the ensemble members. 20 C° (5 members) 30 C° (25 members) 35 C° (15 members) 40 C° (5 members) 20 C° (20 members) 30 C° (10 members) 35 C° (15 members) 40 C° (5 members)

  45. model climate eps forecast Use and Interpretation of ECMWF Products Extreme Forecast Index (EFI) • The EFI is defined on the basis of the Cumulative Distribution Functions (CDF). The abnormality level in the EPS is determined based on the position and shape of the distributions. CDF PDF 1 1 Probability of not exceeding threshold Density per threshold Interval (normalised) 0 0 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25

  46. Use and Interpretation of ECMWF Products The reference EPS Model Climate • For climate related products like the EFI a reliable model climate is essential • Ideally the model climate is a large set of EPS re-forecasts with the latest model configuration (used operationally) for a long enough period (e.g. 30 years) • The current EPS model climate in use: • Running a full EPS re-forecast suite with 4 EPS members and the Control • Always for the most recent 18 years with initial conditions taken from ERA-Interim (the higher resolution successor to ERA-40). • Currently runs every Thursday (therefore climate files are available only for Thursdays. For days in between Thursdays the closest preceding Thursday’s files should be taken) • Integration up 32 days, post-processed fields as for EPS (data every 6 hours) • Use latest model cycle (resolution/ physics / etc.) • Allow an immediate adaptation of EFI and other model climate related products to any EPS model upgrade • For more information about the model climate please refer to: • http://www.ecmwf.int/products/changes/vareps-monthly/EFI_climate.html

  47. Use and Interpretation of ECMWF Products Model Climate • We now call this the ‘M-Climate’, which has a specific meaning… • M-Climate means that it is a function of 3 factors: • 1. Location, clearly • 2. Time of year, to take account of seasonal variations • 3. Forecast lead time

  48. Use and Interpretation of ECMWF Products ECMWF’S EFI web pages for WMO users…

  49. Use and Interpretation of ECMWF Products Acknowledgement: Thanks to various ECMWF presentations…

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