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CEMS-Flood data is associated with a certain terminology that can be difficult to grasp. This page summarises some of the most important terms that can be useful when dealing with the data.
| Term | Definition |
|---|---|
| Analysis | An analysis is our best estimate of reality, used as a proxy for observations which do not exist everywhere. It is the archived lead time 0 of a forecast. For example, the archived water balance of EFAS is an analysis. |
| Archived forecast | Archived forecasts are past forecast runs; these differ in that they are not re-run when the model changes, so the model version is not consistent throughout the time period. |
Class | Defines different groups of data in MARS. Class ce stands for Copernicus Emergency Management Services. This is important for accessing data through the Climate Data Store. |
| Climatology | Set of reference simulations, including historical simulations forced with observations, reanalysis or forecasts re-runs. |
| Control forecast (cf) | Ensemble forecast sub-set only including simulation obtained from the control member of the corresponding NWP. The control member is the unperturbed forecast run at the resolution of the ensemble forecast. Recommended to be used in combination with the perturbed forecasts. |
| Ensemble forecast (ENS, pf) | A number of simulations (members) using the same Numerical Weather Prediction model, but varying the initial conditions and/or the tendencies. The ensemble members are perturbed (pushed away from the observed state) with regards to the best estimate of the initial conditions, and the ensemble spread should ideally reflect the model error. Ensemble forecasts are often used to express the uncertainty of a forecast by associating a probability with different forecast values. The mean of an ensemble forecast has been shown, on average, to have higher predictive skill than the corresponding high-resolution forecast. |
| Ensemble Streamflow Prediction (ESP) | A forecasting method developed in the mid-1970s for reservoir operations in the US. It consists of starting a hydrological model with the latest initial hydrological conditions (e.g., soil moisture, river level, snow cover, etc) and forcing it with historical meteorological observations (e.g., precipitation, temperature, evaporation, etc). Each ensemble member will thus correspond to a year of historical meteorological observations. For example, to generate hydrological forecasts on the 1st of February 2016 for the next 7 months of lead time, the hydrological model would be started with the hydrological conditions observed on this day. Meteorological forcing would then be taken from each available year of meteorological observations for the 1st of February - 31st August (+7 months). |
| Experiment version (expver) | Defines the type of simulation. Expver 1 stands for the operational data and is the official release of the data. Other numbers define test versions and experimental data, and should only be used with caution. |
| Extended-range forecast | In the ECMWF context it is the ensemble forecast with a lead time of 46 days. It is issued bi-weekly (Mondays and Thursdays) at 00UTC. |
| Fill-up (fu) | A period used to bridge the gap between the latest proxy-observations (e.g. reanalysis forcing such as ERA5 is used as proxy observation, updated every 24-hours at 00UTC with a latency of 2-3 days) and the start of the forecast (00UTC or 12UTC, several days after the latest observations are available). It is generated using the most recent high-resolution Numerical Weather Prediction forecast as forcing, so not as accurate as the operational simulations forced by observations. Associated data are not recommended for use except for very specialised modelling exercise for warm-start of past forecasts. |
| Forced simulation (sfo) | Hydrological time series simulation forced with observations, reanalysis or other proxy for observations. Typically used to generate initial conditions for a hydrological forecast and reference long-term run. the latter can be used to calculate return periods and for validation of forecasts. |
| High-resolution forecast (HIRES, fc) | Single simulation with one meteorological forecast input. The high-resolution NWP input is usually better than ensemble forecasts in resolving local features, such as orographic precipitation, but small deviations in the model simulation, for example, geographical displacement, can cause large deviations in the hydrological output in comparison with observations. |
| Hindcast | Hindcast is another commonly used term for reforecast; while these mean the same thing, hindcast is used less and less to be replaced with reforecast. |
| Historical simulation forced with observations | Long (> 20year) hydrological time series simulation forced with proxy-observed meteorological fields (e.g. reanalysis such as ERA5). Typically used to generate reference statistics, for example, return period discharge magnitude against which to compared forecast simulations to generate warnings, and to validate hindcasts. |
| Lead time | Maximum forecast horizon, meaning how far into the future the forecast stretches. |
| Medium-range forecast | Forecasts produced for a lead time of several days (typically 3-15 days). In the ECMWF context, the HIRES and the ENS constitutes the constitute medium-range forecasts. |
| Numerical weather prediction model (NWP) | Mathematical models of the atmosphere, oceans, and land processes that predicts predict the weather based on current weather conditions. |
| Perturbed forecast (pf) | Ensemble forecast sub-set excluding its control member. Generally used in combination with the control forecast. The order of the numbering of the perturbed forecasts are random.is random. |
| Reanalysis | A reanalysis is the same as analysis but produced using a frozen model version giving the benefit that the model has not changed over the time period of the reanalysis (note that the observations assimilated will still change over time depending on what was available). |
| ReforecastReforecasts | Forecast simulations performed for a set of past dates, based on a configuration as close as possible to the operational setting. They are used to evaluate the forecast skills against a benchmark or reference forecast. |
| Season-range forecast | Forecasts produced for a lead time of several months (typically 30 days to 7 months) |
| Time step | Time increments for each forecast. A time step of 6 hour hours means that the forecast as a value every 6 hours until the maximum forecast horizon. |
| Upstream area | The total area that contributes with water to a specific point on the river network. Also known as a catchment area. |