1 Introduction

“Behind good forecast practices are often hidden good theories; equally, good theories should provide a basis for good forecast practices.”     Professor Tor Bergeron, personal communication to Anders Persson, 1974

The aim of this User Guide is to help meteorologists make the best use of the forecast products from ECMWF - to increase understanding of the ensemble forecast process, to develop new products, to reach new sectors of society, to satisfy new demands.  The User Guide presents the Integrated Forecasting System (IFS) and advises on how best to use the output, not least on how to build up trust in the forecast information.  A good forecast that is not trusted is a worthless forecast.  The emphasis is on the medium-range forecast products, as this is ECMWF's primary goal, and because medium-range NWP output generally differs significantly from dealing with short-range or seasonal NWP.

This guide is intended to give an outline of structure and use of the ECMWF IFS and how the high-resolution forecast (HRES), ensemble forecast (ENS), extended range forecast and seasonal forecast models inter-depend and interact.  Links to more detailed descriptions of processes are given, mainly at the end of each section, but online ECMWF training resources are available to explain aspects of the ECMWF IFS more visually.  Education is a key element of the work at ECMWF and further educational material is available through the web site (e.g. Webinars (recordings), Slidecasts (slides and audio recordings), Tutorials, Training lectures (presentations in PDF))

The User Guide is broadly divided into two parts.  Sections 2 to 5 describe the structure of the ECMWF Integrated Forecasting System, while Sections 6 to 11 describe how the IFS may be used to best advantage by forecasters.

A glossary is included in the Annexe.

Section2: The ECMWF Integrated Forecasting System (IFS)

Section 2 describes in broad, non-technical terms the ECMWF IFS (the global atmospheric model, the wave and the oceanic dynamical models, and the data assimilation systems). It gives an overview of the way the atmospheric model uses sub-gridscale parameterisations for processes within the atmosphere and at the surface.  There are large differences in energy fluxes between land or sea and the atmosphere, and the definition of the model coastline by the land-sea mask is extremely important, not least in the way data is presented (e.g. for meteograms in coastal areas or on islands).

Numerical weather prediction (NWP) output is complicated by its often counter-intuitive, non-linear behaviour.   Understanding model processes enables forecasters to critically assess model output.

Section3: Availability and interpolation of NWP output

Section 3 gives an overview of the way ECMWF graphical forecast products are presented to the forecaster and gives some insights into ways the analysed and forecast data may be reduced in accuracy by the way it is presented.

Section4: NWP evolution versus reality

Section 4 discusses model error growth with time and the relationship between predictability and scale. An indication is given of how anomalies propagate downstream and gives some pointers towards recognition of these in the analysis.

Section5: Forecast ensemble (ENS) - rationale and construction

Section 5 describes the way the members of the ensemble are generated.  The use of ENS allows assessment of uncertainty in the model forecast by giving a range of results.  Each ensemble member starts from slightly perturbed initial data and evolves a little differently from the other members of the ensemble to give a range of possible forecast results.  The variation seen within the ensemble forecasts gives an indication of predictability of the atmosphere.

Section6: Using HRES and ENS forecasts

Section 6 discusses how high-resolution forecast (HRES) and ensemble forecast (ENS) may be interpreted, how best to combine their output,  and the reliance that can be placed upon each as forecast lead-time increases.  Each ENS member starts from slightly perturbed initial data and evolves a little differently from the other members of the ensemble to give a range of possible forecast results.  The variation seen within the ensemble forecasts gives an indication of predictability of the atmosphere.  The HRES should be considered as part of the ensemble during the initial 10 days or so and allows further information on possible outcomes through its use of a finer resolution.  However, it is important to note that the HRES does not necessarily provide more accurate predictions just because it presents more forecast detail.  The relative strengths of HRES and ENS products are discussed, and how best to use each model individually and together and the appropriate weighting to be used for each.  The use of probabilities or other risk assessments is an essential part of building forecasts useful to the customer.  This section emphasizes the benefit of using both models together to get the best description of evolution and uncertainty of the forecast state of the atmosphere. 

Section7: Dealing with uncertainty

Section 7 concentrates on methods that may be used to assess confidence in model results.   This section highlights gives guidance on interpretation of latest and previous ENS output to allow insight into the uncertainty of the forecast.  It also gives guidance on assessing the skill of a forecast and how to use run-to-run variability in the forecasts to best advantage.  The continuing role of the human forecaster is emphasized.

Section8: ENS products - what they are and how to use them

Section 8 concentrates on making best use of the extensive range of products available.  The IFS produces a very wide range of products.  Many forecast products regarding the structure of the atmospheric conditions and weather conditions can be viewed on the ECMWF web charts accessed through the ECMWF Forecaster page.   An important product produced within the IFS are model climates (M-climate for ENS, ER-M-climate for Extended Range ENS, S-M-climate for Seasonal forecasting) which are a wholly model-based assessment of worldwide climatology based on analyses and re-forecasts over a period of years (currently 20 years but 30 years for seasonal forecasting).  Model products may be deterministic, probabilistic, or in the form of anomalies from normal where normal is defined by  the model climates.  ENS output in the form of charts, plumes, meteograms (and wave meteograms), and charts showing the various evolutions of tropical cyclones and extratropical depressions all give an easy to use presentation of data.  Other charts give indication of the variability and uncertainty among the basic model forecasts or compare the latest model output with its predecessors.  The model climates are used extensively to highlight when weather conditions forecast by the ENS are locally extreme for that time of year and for the given forecast lead time and the Extreme Forecast Index (EFI), pioneered at ECMWF, compares the forecast probability distribution with the corresponding model climate distribution.  The Shift of Tails (SOT) index complements EFI by providing information about how extreme an event might be by comparing the tail of the ENS distribution with the tail of the M-climate.   The overall aim is to allow assessment of uncertainty to provide the customer with the best and most useful guidance possible. 

Section9: Physical considerations when interpreting model output

Section 9 gives pointers towards features which can have an impact on model output and allow users to modify and improve forecasts for issue to customers.  Some other short-comings of the models are noted which will be addressed in the future but which meanwhile need to be considered by the forecaster.  It is through forecaster user feedback that important points will be identified and addressed.  The importance of critical assessment of model output by human forecasters cannot be understated.

Section10: Interfaces for displaying model output

Section10 gives an outline of the way forecast data may be presented to the user using ECMWF Web Charts or through the more flexible ecCharts which allows users to pick-and-mix the data to be presented.

Section11: Conclusion

Section11 highlights the continuing importance of the forecaster in providing a consistent and useful product to the customer.

Section12: Appendices

Section12 contains additional detail on statistical concepts for verifying model forecasts, the current structure of IFS, a list of acronyms, and some references.

The forecaster is not a computer but is employed to add value to model forecasts, and to identify and quantify uncertainties.  Daily operational forecasting work is largely a matter of assessing, combining, correcting, and interpreting NWP information.  In the medium-range especially, the use of statistical know-how counts as much as synoptic experience. 

Throughout the User Guide forecasters are advised not to try to imitate or simply follow NWP, but to act quite differently by surveying and questioning results from many sources and to produce forecasts with fewer details, greater assessment of uncertainty, and ideally no sudden “U-turns”.  The philosophy is that all forecasts have uncertainty, that uncertainty increases with forecast lead-time, and that forecasters are employed to provide a balanced assessment of the probability of an event that is relevant to customer requirements.

The ECMWF model output is in the form of charts or GRIB code datasets and is readily available via imports into the forecasters' own workstation environments, or on the web using either the ecCharts application, or the more static "web charts".

This User Guide is an update of the "User Guide to ECMWF Forecast Products" written by Anders Persson and published in 2011 with minor additions in 2013 and 2015.   It continues to be based on advice and information from scientists, forecasters and meteorologists with an emphasis on use of statistical techniques.  The User Guide gives only an introduction to the forecast information provided by ECMWF.

Users are advised to keep themselves updated about changes and improvements to model processes and products through the ECMWF Newsletter and web site.