Difference between revisions of "Meteorological data from ECMWF models"
(→Extraction of data into files)
|Line 61:||Line 61:|
The and processing of the SEAS data is done in a separate setup: The input data is downloaded from the Copernicus Climate Data Store (CDS) as netcdf formats . The data is being processed with Python and R scripts.
Revision as of 10:16, 22 October 2021
The is one of the world's leading centres for numerical modelling. It runs a suite of global models and data assimilation systems for the dynamics, thermodynamics and composition of the Earth's fluid envelope and interacting parts of the Earth system. The data assimilation systems harmonise observations from ground stations, radiosondes, satellites and many other sources with the meteorological equations to form a physically valid state of the atmosphere. These data are used as initial conditions for the various forecast models.
To extend the analysis period and improve crop monitoring and yield forecasting, weather forecasts are integrated into the MCYFS. The data provide important information on the development of the most important meteorological phenomena on the mesoscale.
ECMWF model outputs are used to produce meteorological and derived agrometeorological parameters, which are then visualised in dynamic maps and diagrams in analyst viewers and in static maps quick-looks.
Data from the ECMWF's ensemble forecast system (ENS) and seasonal forecast model (SEAS) provide several forecast outputs. Since the atmosphere is a chaotic system, small differences in initial conditions can lead to large differences in the resulting forecasts. In 1992, ECMWF introduced an ensemble forecast system that provides information on the uncertainty of a weather forecast. Small perturbations of the initial condition are used to produce (nowadays) 50 different initial conditions. Together with the non-perturbed control run, this results in an ensemble of 51 model results.
Before the ECMWF predicted weather data can be fed into the MCYFS, the data must be pre-processed to obtain the required resolutions in time and space.
Data acquisition from ECMWF
Data from six products of the ECMWF model suite is ingested into the MCYFS:
|Model set with ECMWFs abbrevation||Abbreviation within MCYFS||Number of forecast days used for MCYFS||Number of ensemble members||Original ECMWF grid*||Corresponding original horizontal model resolution*||Acquired resolution in MCYFS**||Delivery of data files and maps|
|ERA5||ERA||1||1||N320 reduced Gaussian grid||~30 km||0.28125° x 0.28125°||in April for the previous year|
|Deterministic model as analysis HRES||OPE||1||1||O1280 octahedral grid***||~9 km ***||0.25° x 0.25°||Daily (10.30 hr)|
|Deterministic forecast HRES||OPE||10||1||O1280 octahedral grid||~9 km ***||0.25° x 0.25°||Daily (12.00 hr)|
|Ensemble Prediction System ENS||ENS||15||50+1||O640 octahedral grid ***||~18 km ***||0.5° x 0.5°||Daily (14.00 hr)|
|Seasonal forecast system SEAS||SEAS||183||50+1||0320 octahedral grid||~36 km||0.4° x 0.4°||February - June and November, 13th of the month|
* Grid in which the model simulates the weather indicators (state: 2021). Depending on the model subset, ECMWF uses for surface and pressure levels either a Octahedral grid or a . The octahedral grid names start with ‘O’ followed by the number of latitude lines between the pole and equator. Gaussian grid names start with 'N' followed by number of lines by which latitude is divided.
** Spatial resolution in which the simulated indicators are acquired and loaded into the MCYFS. The simulated indicators are distributed over the earth using a coordinate system.
*** HRES and ENS are run by ECMWF twice daily, based on 00 and 12 UTC observations. The SEAS is started by ECMWF each 01st of the month as 00 UTC-run. Depending on the forecast horizon it takes between 5.5 hours (+0 hours HRES) and nearly 9 days (last day SEAS) until the centre disseminates the results.
The short range results of the subsequent HRES model runs are processed as analysis of the previous day and added to the archive (as HIS), assuming this is the best estimator for weather indicators of that day. Details are described below.
The other data of the forecasting suite is replaced when a more recent forecast becomes available (OPE forecast, ENS, SEAS). As the delivery into the MCYFS databases needs to take place until 15.00 hours of each day in standard situation the 00 UTC model runs are used. In the rare case that the model dissemination is delayed as fallback the 12 UTC model result of the previous day is taken into account.
ECMWF’s reanalysis data set ERA5 is used to build a consistent archive of gridded model results from January 1979 onwards. Below, details are described.
The ECMWF model computes surface parameters of HRES and ENS on octahedral grids, with different resolutions. The central MCYFS database however requires the initial data in a specific grid resolution with regular latitudes and longitudes, see section Data acquisition from ECMWF. Therefore, conversion is needed.
The deterministic forecast model, within MCYFS addressed as OPE, including the short range forecast which is used as analysis, produces forecast weather for grid cells currently on a Octahedral O1280 grid (~9x~9km). This resolution is converted by ECMWF to a reduced Gaussian N640 grid (~16x~16km). Next a conversion of the N640 to a regular 0.25 x 0.25 degrees latitude longitude grid (OPE grid) is done.
Several height models at the regular 0.25 x 0.25 degrees latitude longitude exists:
- the height model for the OPE is calculated in the same way as the data itself: first the Octahedral grid is converted to a Gaussian N640 reduced grid and next to the regular 0.25° OPE grid (~25x~25km).
- in addition, the height model of a previous version of OPE model (prior to March 2016) is available. The previous OPE version was run on a Gaussian N640 reduced grid and the related height model was directly converted into the OPE grid.
- in addition, the height model of a previous version of OPE model (2008-2010) is available. This OPE version was run on a Gaussian N400 reduced grid and the related height model was directly converted into the OPE grid
For the grid conversion, original software from ECMWF (EMOS-lib) is applied. The grid description is stored in table GRID_HIS including the elevation models originating from two previous OPE versions: ~16km (N640) and ~25km (N400).
All surface parameters of the ENS forecast are calculated on a Octahedral O640 grid (~18x~18km). This resolution is converted by ECMWF to a reduced Gaussian N200 grid. Next a conversion of the N200 to a regular 0.5 x 0.5 degrees latitude longitude grid (ENS grid) is done. The height model for the ENS is calculated in the same way as the data sets: first the Octahedral grid is converted to a Gaussian N200 reduced grid and next to the regular 0.5° ENS grid. In addition, the height model of a previous version of ENS model (prior to March 2016), ran on a N320 reduced Gaussian grid, is available. For the grid conversion original software from ECMWF (EMOS-lib) is applied. The grid description is stored in table GRID_ENS, the elevation of the previous ENS model (prior to March 2016) is stored in column ALTITUDE.
The ERA5 data are calculated for a 0320 octahedral grid (~30x~30km). The acquisition and processing of the SEAS data is done in a separate setup: The input data is downloaded from the Copernicus Climate Data Store (CDS) as netcdf formats at a spatial resolution of 0.4 by 0.4 degree. The data is being processed with Python and R scripts.
The ERA5 data are calculated for a Gaussian N320 reduced grid (~30x~30km). The ERA5 data is fetched from the Copernicus CDS API, which offers the dataset already pre-interpolated to the target grid in a spatial resolution of 0.25° x 0.25. The official ECMWF MIR interpolation package is used for the regridding, ensuring consistent datasets.
Applied parameters from ECMWF grib deliveries
In total, analysis and forecast for 35 parameters of the ECMWF re-analysis and forecasting suite is used for the various applications in MCYFS and the production of the static maps.
|List of meteorological indicators from ECMWF as used within MCYFS|
ECMWF disseminates the model results for the surface layer in WMO FM 92 GRIB format, according WMO specifications, Manual on Codes in WMO Publication Nr 306. To extract the required parameters from the ECMWF data package(s) and to decode the binary GRIB formats the ECMWF GRIB API application program interface for C is used.
As a next step after acquisition and scaling to the regular lat-lon-grids, derived elements and daily indicators are calculated
Aggregation to daily data
First, aggregates of the 3- or 6-hourly data to daily means, extremes or sums are calculated. Total precipitation and global radiation are provided by ECMWF as accumulated values since the begin of the model runtime and therefore differences for the 24-hourly daily sums need to be computed. The box below summarizes the algorithms.
|Algorithms for aggregation to daily data|
|Abbreviations for the elements in the following table refer to the original ECMWF naming as summarized in section Applied parameters from ECMWF grib deliveries. Sub-scripted numbers behind the indicator abbreviations indicate the (UTC)-time of the day. The abbreviations for the model sets refer to the internal naming within MCYFS as defined in section Data acquisition from ECMWF
To consider the earth's different times zones, aggregation rules for 3 different areas (West, Central, East) have been defined. The aggregation rules for the model data align with the general report schedule of ground weather stations (e.g., maximum air temperature in Europe and Africa refers to the period between 06 and 18 UTC of the corresponding day).
The following table summarizes the rules for the different aggregation zones and data sets. Legend of characters used: p = previous day, f = following day. Temporal resolution of OPE is 3-hourly for the first 72 hours and 6-hourly afterwards. Thus algorithms for air temperature, dew point and wind speed of the OPE data set change when the aggregation includes forecast time step +72h. Temporal resolution of ENS and SEAS is 6-hourly. ERA-Interim is available every 3-hours.
* ”X” as representative abbreviation for the ECMWF elements as listed in the first cell of the line
* ”X” as representative abbreviation for the ECMWF elements as listed in the first cell of the line
Calculation of additional indicators
The following indicators are retrieved from other elements. These include:
- Snow depth (thickness snow cover)
- Climate water balance (for mapping purposes)
The snow depth (thickness of the snow layer, Dsn) is derived from snow depth water equivalent and snow density.
|Dsn equals (r_water/r_snow)*SD/c_snow|
The ECMWF provides snow depth water equivalent SD (m3/m2) and snow density RSN (kg/m3). In ECMWF's model documentation snow mass is referred as “snow water equivalent”, and leads to parameter SD, snow depth. Snow fraction is not provided by ECMWF (is not in the catalogue). ECMWF assumes c_snow to be 1 for snow depth > 15 cm (average of the grid box) and <1 for a thinner snow cover.
Climatic water balance
Climatic water balance is calculated based on evapotranspiration calculated through the equation of Penman-Monteith (ET0) and the total precipitation of a day. This calculated ET0 is only used for mapping purposes. Note that in the downstream processing, and after spatial scaling operations, ET0 is calculated again (see calculation of additional parameters after downscaling).
|CWB equals Rain – ET0|
Calculation of extreme weather events
For the static map production (quicklooks) it is necessary to derive additional parameters out of the raw ECMWF data set. This especially concerns probabilities and aggregated counts of number of days where a special condition is met. To compute the probability for the exceedance of thresholds (e.g. probability of freezing days) first the daily value for each separate ensemble member is computed and then the amount of members which fit the corresponding constraint (p.e. exceed 20 mm of daily precipitation sum) is counted. To compute the number of days where a parameter exceed a threshold first the numbers for each separate ensemble member is calculated (from the daily values of each ensemble member). Afterwards the median is derived for presentation on map. The deterministic run and the ensemble control run are treated like any other ensemble member. Probabilities for anomalies require comparison with ECMWF model climate and are therefore only visualized where available in the ECMWF catalogue.
|Derived probability and other threshold-dependent indicators|
Aggregation to 10-daily and weekly data
For the production of the maps, as well an aggregation to 10-daily and weekly aggregates of the daily data takes place. Therefore the average of mean temperature, maximum temperature, minimum temperature, snow depth and the sum of precipitation, ET0, climatic water balance and global radiation is computed.
Extraction of data into files
After processing data are exported as data files and static maps that can be distributed to users and other MCYFS processes.
A simple file naming scheme for the data files was adopted with the general format:
- ROI = region (GLD, EUR, ASI)
- model_code = ECMWF model (ERA, OPE, ENS)
- yyyy = the year (four digits),
- mm = the month number (two digits),
- dd = the day in the month (two digits)
- member = the member number (two digits)
The date in the filename links to the forecast day = 0 (FORECAST_OFFSET = 0). In case of OPE and ERA only member 00 is allowed; in case of the ENS, the ENSEXT and the SEAS the member number runs from 0 to 50.
An example of a file name for each of the 4 models is:
- GLO_OPE_20160715_00.dat OPE data issued July 15, 2016 (only member 00 allowed)
- GLO_ENS_20160704_35.dat ENS data issued July 4, 2016, member 35
An input file basically contains the following structure:
- A header providing geo referencing information
- Blocks of data for the first forecast date (for each variable)
- Blocks of data for the second forecast date (for each variable)
For simplification purposes, below a simple example is given with a detailed explanation.
|Explanation of file format|
| The example contains just rainfall and daily mean temperature for two forecast days for a grid ranging from 20 to 40 degrees longitude and from 50 to 60 degrees latitude, with a grid size of 5 degrees. The forecast is issued on 23 January 2009 and first day in the forecast (FORECAST_OFFSET=0) is linked to this date.
The meaning of each of the lines is given in the following table:
The possible forecast offsets are given in the following table:
* = Day of ECMWF model initialization (“model run”). For instance if the model is initialized on the first October 2014 00 UTC the FORECAST OFFSET = 0 refers to Oct 01 2014. Data is always aggregated to daily values (see rules for aggregation from 3- resp. 6-hourly data to daily values).
The provided grids are summarized in the following table:
During preparing the files some unit conversion is done:
The variable abbreviations and their explanation are given in the following table:
All elements refer to the model surface. The temperatures refer to a level 2 meters above (model) ground, wind speed refers to a level 10 meters above (model) ground.
The data files are loaded in the MCYFS database, tables WEATHER_<MODEL>_GRID_RAW where <MODEL> is to be replaced by the abbreviation of one of the five ECMWF products (OPE, ENS,). During loading two actions are executed:
- additional unit conversion
- plausible range checks
|Unit conversion and range checking|
The SEAS and the ERA5 data is being processed with separate lines, respectively. The resulting daily data is delivered as NetCDF.
Extraction of data into maps
The static maps are exported as flat images and animated images with full layout and directly made available to analysts that use them during analysis of weather indicators. The geographic extent of the static maps for Europe is defined by the upper-left corner at 75° North/25° West and the lower-right corner 20° North/70° East, the maps for the RUK domain cover a domain 65° North/25° West to 40° North/105° East. This production line includes GrADS mapping software which is able to create maps directly from GRIB files.
|Overview: Produced maps|
The abbreviations for the various maps (including the animated) in the following tables are used for the directory structure on the ftp-server and the transmitted zip. The zip-name is extended by the day of the underlying ECMWF model set run.
The following table summarizes the map production as set up for the OPE and the ENS:
Messages to the Project Management Board
Information on successfull completion of the various processing steps is sent to the Project Management Board (PMB).