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How do I obtain the uncertainty estimate data from MARS?
The uncertainty data is archived, as all ERA5 data, in ECMWF's data archive MARS. Uncertainty data is archived in stream 'Ensemble data assimilation' (enda), as Ensemble mean (type=em), Ensemble standard deviation (type=es) and Scaled ensemble standard deviation (type=ses).
Public catalogue entry point: http://apps..................ecmwf.int/data-catalogues/era5/?class=ea&stream=enda&expver=1
Restricted catalogue entry point (ECMWF member states only) : http://apps.ecmwf.int/data-catalogues/era5/?class=ea&stream=enda&expver=1
There are two ensemble spread values: ensemble
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standard deviation (es) and scaled ensemble
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standard deviation (ses). What is the difference between them and which one should be used?
In general we recommend using the raw ensemble spread standard deviation values ("es" fields in MARS) instead of the scaled ensemble spread scaled ensemble standard deviation ("ses" fields in MARS). In fact, the Climate Data Store (CDS) only contains the raw ensemble spread ensemble standard deviation fields. The scaled ensemble spread ensemble standard deviation is used to compute background error statistics for the higher resolution ERA5 model and contains much fewer variables and levels. The scaled ensemble spread ensemble standard deviation is only available at step=3h from the forecasts starting at 06 and 18 UTC on model levels (and at the "surface" for brightness temperature), while the ensemble spread ensemble standard deviation is available every 3h to step=18h, on model, pressure, potential temperature and potential vorticity levels and also at the surface. Users are encouraged to compute additional (beside ensemble mean and spreadstandard deviation) uncertainty parameters based on the raw ensemble members.
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Mostly, the resolution of ERA5 is not sufficient to properly describe tropical cyclones. Additionally, the EDA system is of lower resolution than that of ERA5, which means further limitations for describing such small-scale phenomenon. For the above mentioned tropical cyclone case the lowest pressure of the cyclone is 969.7 hPa (see figure below), which is higher than the real observed pressure, but the cyclone itself is reasonably well described. The larger spread area corresponds well with the shape of the cyclone and the largest value is 2.7 hPa. This gives an indication about the relative uncertainty of the event, though the spread is presumably smaller than the real analysis error.
Relevant user questions (translated from French) from CUS-4133.
Uncertainty information is available 3-hourly. How can I approximate uncertainty for the intermediate hours? Should I use interpolation, the temporally nearest, ...?
"Question 1: What methodology do you recommend to approximate these uncertainty parameters to HRES data: for the intermediate hours when the uncertainties are not available (interpolation, value temporally the nearest ...)? For the assignment of uncertainty deduced from the EDA for a grid point of an HRES parameter (take the value of the EDA grid point closest to the HRES grid point?)?"There is no general recipe provided in this matter. Common sense should prevail and of course the specific user need. Certainly, one solution is to have a temporal and spatial interpolation, though one has to understand the limitations using the interpolated uncertainty values. Please, note that errors in interpolation might be smaller than the effective errors in the uncertainty estimate itself."Question 2:
If
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I compute a climatology of a parameter derived from the HRES, do you have a methodology to assess its uncertainty using the information provided by the EDA?
It is We do not really recommended to compute a single mean from the ensemble spread values. The EDA system addresses mostly random errors (and not systematic ones), so one has to be careful computing and interpreting mean climatologies."Question 3:
Is there a forecast evolution of the indicators made available for the EDA (
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e.g. quartiles, percentiles ...)? If so, what horizon and what would these indicators be?
"Only ensemble spread and ensemble mean and the raw ensemble members will be provided to the users. So all the additional processed quantities should be computed from the ensemble members by the users. The ensemble forecasts are also available and can be used by the users."Question 4: Do you confirm that these
Are the uncertainty indicators
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available on the same 137 verticallevels
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as data from HRES?
"Both the EDA system and the ERA5 have 137 model levels (as does the high resolution operational IFS model) and the same pressure, potential temperature and potential vorticity levels. The raw ensemble data (all members) are available for the users to compute any processed information.
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