Contributors: Jan Wuite, Thomas Nagler (ENVEO IT GmbH)
Issued by: ENVEO IT GmbH / Jan Wuite
Date: 24/08/2023
Ref: C3S2_312a_Lot4.WP2-FDDP-IS-v1_202212_IV_PUGS-v4_i1.1
Official reference number service contract: 2021/C3S2_312a_Lot4_EODC/SC1
History of modifications
List of datasets covered by this document
Related documents
Acronyms
General definitions
Interferometric Wide (IW): The Interferometric Wide (IW) swath mode is the main acquisition mode of Sentinel-1 over land, including ice sheets. It acquires data with a 250 km swath at 5 m by 20 m spatial resolution. IW mode captures three sub-swaths using Terrain Observation with Progressive Scans SAR (TOPS). (Adapted from: https://sentinels.copernicus.eu/)
Offset Tracking (OT): OT refers to several related methods that include speckle tracking, coherence tracking and amplitude tracking or feature tracking. Feature tracking uses cross-correlation of image patches to find the displacement of surface features such as crevasses or rifts and edges, that move with the same speed as the ice and are identifiable on two co-registered amplitude images, to derive ice flow velocity. In coherence tracking, the offset which maximises the interferometric coherence within a certain window size is determined and used to derive the ice velocity. Speckle tracking uses the cross-correlation function of radar speckle patterns, rather than visible features, to derive ice flow velocity.
Single Look Complex (SLC): Level-1 Single Look Complex (SLC) products are Synthetic Aperture Radar (SAR) images in the slant range by azimuth imaging plane, in the image plane of satellite data acquisition. Each image pixel is represented by a complex magnitude value and therefore contains both amplitude and phase information. The imagery is geo-referenced using orbit and attitude data from the satellite. SLC images are produced in a zero Doppler geometry. (Adapted from: https://sentinels.copernicus.eu/)
Scope of the document
This document is the Product User Guide and Specification for Ice Velocity (IV) as part of the Copernicus Ice Sheets and Ice Shelves service. It describes the datasets produced, and their specifications.
The service addresses three essential climate variables (ECVs) by providing the following products:
- Ice velocity is given for Greenland and Antarctica in product WP2-FDDP-IV-CDR
- Gravimetric mass balance is given for Greenland and Antarctica in product WP2-FDDP-GMB-CDR5
- Surface elevation change is given for Greenland in product WP2-FDDP-SEC-CDR-GrIS
Executive summary
The dataset contains annual ice velocity maps of the Greenland Ice Sheet derived from Sentinel-1 data. The data represent the current state-of-the-art in Europe for production of satellite-based ice velocity data records. It follows on from the ESA Greenland Ice Sheet Climate Change Initiative and is guided by the GCOS (Global Climate Observing System) targets for the Ice Sheets Essential Climate Variable.
Mapping glacier flow velocity and its temporal changes provides key information for investigating the dynamic response of glaciers and ice sheets to changing boundary environmental conditions. Remote sensing techniques that utilise satellite data are the only feasible manner to derive accurate surface velocities of the remote Greenland glaciers on a regular basis.
The surface velocity is derived by applying offset tracking techniques using Sentinel-1 synthetic aperture radar (SAR) data acquired in the Interferometric Wide (IW) swath mode. Ice velocity is provided at 250m grid spacing in North Polar Stereographic projection. The horizontal velocity components are provided in true meters per day, towards easting and northing direction of the grid. The vertical displacement is derived from a digital elevation model. Provided is a NetCDF file with the velocity components: vx, vy, vz, along with maps showing the magnitude of the horizontal components, the valid pixel count and uncertainty (based on the standard deviation). The product combines all ice velocity maps, based on 6- and 12-day repeats, acquired over a full year in an annually averaged product running from October 1st to September 30th, mimicking a glaciological year. The dataset is extended on an annual basis.
This dataset is produced on behalf of the Copernicus Climate Change Service (C3S) and available through the Climate Data Store.
Product Change Log
Version | Product Changes |
v201901 | First release of the dataset: CDR v1.1. Ice velocity map produced for 2017-10-01 to 2018-10-31. |
v202001 | Release of data set CDR v1.2. Product algorithm same as v201901. CDR v1.1 product extended to include map for 2018-10-01 to 2019-09-31, including also reprocessed CDR v1.1 dataset with adjusted end month: 2018-09-31 instead of 2018-10-31. |
v202101 | Release of data set CDR v1.3. Product algorithm updated from v202001, providing maps at higher resolution (250m instead of 500m) and uncertainty/error is provided component wise for easting and northing. CDR v1.2 product extended to 2020-09-31, including also reprocessed CDR v1.2 dataset at 250m. |
v202212 | Release of data set CDR v1.4. Product algorithm updated from v202101, including dynamic ice/ocean masking for outlet glaciers based on updated calving fronts. CDR v1.4 product extended to 2021-09-31. |
1. Greenland ice sheet velocity
1.1. Product description
The product contains annually averaged ice velocity (IV) for Greenland derived from Sentinel-1 (S1) SAR data (Figure 1). The surface velocity is derived by applying advanced iterative offset tracking techniques (Nagler et al., 2015). The primary processor for IV generation is the ENVEO software package (ESP v2.1), see the related ATBD [D1]. The system’s core module performs coherent and incoherent offset tracking utilizing long stripes of S1 single look complex (SLC) data acquired in interferometric wide (IW) swath mode. The ESP v2.1 is a state-of-the-art IV retrieval algorithm designed for SAR sensors (e.g. Sentinel-1, TerraSAR-X, European Remote-sensing Satellite (ERS)-1, ERS-2, Advanced Land Observation Satellite (ALOS) Phased Array type L-band SAR (PALSAR), Cosmo-SkyMed), and has been tested rigorously through intercomparisons with other packages and extensive validation efforts.
Feature tracking is a technique capable of acquiring ice flow velocity data over short (days) and longer time spans (years) and in regions with fast flow, as no coherence is required. The method uses the displacement of surface features such as crevasses or rifts and edges that move approximately with the same speed as the ice and are identifiable on two co-registered satellite images to derive velocity. The ice velocity product is derived using feature tracking on repeat pass SAR images. The velocity grid for a given file represents the average ice surface velocity over the repeat pass period. For Sentinel-1 acquisitions in Greenland the repeat pass period is 6 to 12 days. The C3S Greenland Ice Sheet Velocity combines all ice velocity maps acquired over a full year in an annually averaged product provided at 250 m grid spacing. The averaging year runs from Oct 1st until September 30th, roughly mimicking a glaciological mass balance year.
Figure 1: C3S ice velocity map of the Greenland Ice Sheet based on Sentinel-1 data acquired from October 2020 to September 2021.
1.2. Target requirements
Table 1 shows the C3S IV product achievement versus the 2022 GCOS requirements for measurement uncertainty and spatial/temporal resolution of ice sheet velocity. The C3S product fulfills all threshold requirements for spatial and temporal resolution and falls well within the goal accuracy range. Further details on the the target requirements for each product within the service are provided in the Target Requirements and Gap Analysis Document (TRGAD, [D3])
Table 1: C3S IV product achievement versus 2022 GCOS requirements.
Requirement | C3S GrIS IV Product | GCOS Threshold Req | GCOS Breakthrough Req | GCOS Goal Req |
Spatial resolution | 250m | 1000m | 100m | 50m |
Temporal resolution | Annual | Annual | Month | |
Accuracy | 0.01 cm/d | 0.27 m/d | 0.08 m/d | 0.03 m/d |
1.3. Data usage information
1.3.1. Product Data Format and Content
The product is provided as a NetCDF file with separate layers for the velocity components: vx, vy, vz and vv (magnitude of the horizontal components), and maps showing the valid pixel count and uncertainty (std) (Table 2). The ice velocity map is annually averaged and provided at 250m grid spacing in North Polar Stereographic projection (EPSG: 3413). The horizontal velocity is provided in true meters per day, towards easting (vx) and northing (vy) direction of the grid, and the vertical displacement (vz), is derived from a digital elevation model (TanDEM-X 90m Digital Elevation Model (DEM); Rizzoli et al., 2017). For all maps a nodata value of 3.4028235e+38 is used.
Table 2: IV main data variables
Variable name | Variable description | Type |
land_ice_surface_easting_velocity | Ice velocity East component [m/day] | 32-bit floating-point |
land_ice_surface_northing_velocity | Ice velocity North component [m/day] | 32-bit floating-point |
land_ice_surface_vertical_velocity | Ice velocity Vertical component [m/day] | 32-bit floating-point |
land_ice_surface_velocity_magnitude | Ice velocity magnitude [m/day] | 32-bit floating-point |
land_ice_surface_measurement_count | Valid pixel count [#] | 32-bit integer |
land_ice_surface_easting_stddev | Standard deviation [m/day] (Easting) | 32-bit floating-point |
land_ice_surface_northing_stddev | Standard deviation [m/day] (Northing) | 32-bit floating-point |
1.3.2. Product Known Limitations
The following lists some known product limitations:
1) The IV products contain separate layers for the horizontal (Easting, Northing) and the vertical components of velocity. This is, however, not the true 3D velocity, which requires both ascending and descending image pairs acquired close in time. The vertical component is derived from the difference in height of start and end position of the displacement vector taken from a DEM.
2) The IV products do not have a time stamp for a single date, but give the average velocity over the time-period covered (1 year).
3) For various reasons, the tracking software sometimes fails to find matching features leading to gaps in the velocity fields. This can be caused by a lack of surface features or when features, for example crevasses, rapidly change due to shearing leading to low correlation. Other reasons for gaps in the IV maps can be areas affected by radar shadow or anomalous pixels that are filtered out. A simple distance-weighted averaging filter is applied to get rid of outliers and to fill small gaps in the data (<5 pixels), further filtering/gap filling is left to the user if required.
4) Due to different acquisition modes, sensor type, resolution and processing strategy there can be differences between S1 IV products and IV products derived from other sensors that complicate a direct comparison between the data sets. Because of differences in resolution, the image patches used for feature tracking have different dimensions impacting the type of features that can be resolved. S1 can for instance not capture the high velocity gradients that may be found in shear zones with the same detail as for example TerraSAR-X (TSX). On the other hand, due to the regular repeat acquisition the temporal sequence of S-1 is much higher than that of TSX and the covered area of the IV maps is much larger.
5) In-situ Global Positioning System (GPS) data for validation of ice velocity are only sparsely available. Velocity products are therefore also compared with ice velocity maps retrieved from other sensors (e.g. S1 vs TSX) to estimate product performance and uncertainty. As an additional quality test, velocity results on stable terrain (rock outcrops), where no movement is expected, are analysed.
1.3.3. Product Nomenclature
The Filename of the IV Products consists of:
C3S_GrIS_IV_250m_S1_<Startdate><Enddate><Version>.<Format>
Table 3: Product nomenclature
C3S | Program |
GrIS | Region (Greenland Ice Sheet) |
IV | Parameter (Ice Velocity) |
250m | Grid Spacing |
S1 | Platform |
<Startdate> | Date of first acquisition |
<Enddate> | Date of last acquisition |
<Version> | Version of the product in the format vMAJOR_MINOR |
<Format> | Fileformat (nc: NetCDF) |
Example: C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc, Explanation: Annually averaged Greenland Ice Sheet surface velocity derived from Sentinel-1 for Oct 1st 2020 – Sept 30th 2021 provided at 250 m grid spacing, processing version 1_4 for C3S.
1.3.4. Structure of NetCDF files
The product metadata is included in the NetCDF file and is shown in Table 4. The NetCDF file contains 7 sub-datasets. Table 5 shows as an example the metadata for the "land_ice_surface_easting-_velocity" sub-dataset.
Table 4: Example metadata for Greenland Ice Sheet Velocity product
NC_GLOBAL#comment=Ice velocity map of Greenland derived from Sentinel-1 SAR data acquired from 2020-10-01 to 2021-09-30. The surface velocity is derived applying feature tracking techniques. The ice velocity map is provided at 250m grid spacing in North Polar Stereographic projection (EPSG: 3413). The horizontal velocity is provided in true meters per day, towards EASTING(vx) and NORTHING(vy) direction of the grid, and the vertical displacement (vz), is derived from a digital elevation model. Provided is a NetCDF file with the velocity components: vx, vy, vz and vv (magnitude of the horizontal components), along with maps showing valid pixel count and uncertainty (stdx, stdy). The product was generated by ENVEO. NC_GLOBAL#contact=copernicus-support@ecmwf.int NC_GLOBAL#Conventions=CF-1.7 NC_GLOBAL#creation_date=2022-12-12 NC_GLOBAL#history=product version 1.4 NC_GLOBAL#institution=Copernicus Climate Change Service NC_GLOBAL#keywords=EARTH SCIENCE CLIMATE INDICATORS CRYOSPHERIC INDICATORS GLACIAL MEASUREMENTS ICE SHEET VELOCITY CRYOSPHERE GLACIERS/ICE SHEETS NC_GLOBAL#license=C3S general license NC_GLOBAL#project=C3S_312b_Lot4_ice_sheets_and_shelves NC_GLOBAL#reference=Main: Nagler, T.; Rott, H.; Hetzenecker, M.; Wuite, J.; Potin, P. The Sentinel-1 Mission: New Opportunities for Ice Sheet Observations. Remote Sens. 2015, 7, 9371-9389. https://doi.org/10.3390/rs70709371 NC_GLOBAL#source=Copernicus Sentinel-1A and Sentinel-1B NC_GLOBAL#summary=Ice velocity derived for Greenland Ice Sheet gridded at 250m averaged from 2020-10-01 to 2021-09-30. NC_GLOBAL#title=Ice Velocity of the Greenland Ice Sheet Subdatasets: SUBDATASET_1_NAME=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":land_ice_surface_easting_velocity SUBDATASET_1_DESC=[10801x5984] land_ice_surface_easting_velocity (32-bit floating-point) SUBDATASET_2_NAME=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":land_ice_surface_northing_velocity SUBDATASET_2_DESC=[10801x5984] land_ice_surface_northing_velocity (32-bit floating-point) SUBDATASET_3_NAME=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":land_ice_surface_vertical_velocity SUBDATASET_3_DESC=[10801x5984] land_ice_surface_vertical_velocity (32-bit floating-point) SUBDATASET_4_NAME=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":land_ice_surface_velocity_magnitude SUBDATASET_4_DESC=[10801x5984] land_ice_surface_velocity_magnitude (32-bit floating-point) SUBDATASET_5_NAME=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":land_ice_surface_measurement_count SUBDATASET_5_DESC=[10801x5984] land_ice_surface_measurement_count (32-bit integer) SUBDATASET_6_NAME=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":land_ice_surface_easting_stddev SUBDATASET_6_DESC=[10801x5984] land_ice_surface_easting_stddev (32-bit floating-point) SUBDATASET_7_NAME=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":land_ice_surface_northing_stddev SUBDATASET_7_DESC=[10801x5984] land_ice_surface_northing_stddev (32-bit floating-point)
Table 5:Example metadata for land_ice_surface_velocity_magnitude.
Driver: netCDF/Network Common Data Format
Files: C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc
C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc.aux.xml
Size is 5984, 10801
Coordinate System is:
PROJCRS["WGS 84 / NSIDC Sea Ice Polar Stereographic North",
BASEGEOGCRS["WGS 84",
DATUM["World Geodetic System 1984",
ELLIPSOID["WGS 84",6378137,298.257223563,
LENGTHUNIT["metre",1]]],
PRIMEM["Greenwich",0,
ANGLEUNIT["degree",0.0174532925199433]],
ID["EPSG",4326]],
CONVERSION["unnamed",
METHOD["Polar Stereographic (variant B)",
ID["EPSG",9829]],
PARAMETER["Latitude of standard parallel",70,
ANGLEUNIT["degree",0.0174532925199433],
ID["EPSG",8832]],
PARAMETER["Longitude of origin",-45,
ANGLEUNIT["degree",0.0174532925199433],
ID["EPSG",8833]],
PARAMETER["False easting",0,
LENGTHUNIT["metre",1],
ID["EPSG",8806]],
PARAMETER["False northing",0,
LENGTHUNIT["metre",1],
ID["EPSG",8807]]],
CS[Cartesian,2],
AXIS["easting",south,
ORDER[1],
LENGTHUNIT["metre",1]],
AXIS["northing",south,
ORDER[2],
LENGTHUNIT["metre",1]],
ID["EPSG",3413]]
Data axis to CRS axis mapping: 1,2
Origin = (-640000.000000000000000,-655500.000000000000000)
Pixel Size = (250.000000000000000,-250.000000000000000)
Metadata:
crs#false_easting=0
crs#false_northing=0
crs#grid_mapping_name=polar_stereographic
crs#latitude_of_projection_origin=90
crs#spatial_ref=PROJCS["WGS 84 / NSIDC Sea Ice Polar Stereographic North",
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]],
PROJECTION["Polar_Stereographic"],
PARAMETER["latitude_of_origin",70],
PARAMETER["central_meridian",-45],
PARAMETER["false_easting",0],
PARAMETER["false_northing",0],
UNIT["metre",1,
AUTHORITY["EPSG","9001"]],
AXIS["Easting",SOUTH],
AXIS["Northing",SOUTH],
AUTHORITY["EPSG","3413"]]
crs#standard_parallel=70
crs#straight_vertical_longitude_from_pole=-45
crs#unit=metre
land_ice_surface_velocity_magnitude#coordinates=y x
land_ice_surface_velocity_magnitude#description=magnitude of horizontal ice velocity
land_ice_surface_velocity_magnitude#grid_mapping=crs
land_ice_surface_velocity_magnitude#units=m/day
land_ice_surface_velocity_magnitude#_FillValue=3.4028235e+38
NC_GLOBAL#comment=Ice velocity map of Greenland derived from Sentinel-1 SAR data acquired from 2020-10-01 to 2021-09-30. The surface velocity is derived applying feature tracking techniques. The ice velocity map is provided at 250m grid spacing in North Polar Stereographic projection (EPSG: 3413). The horizontal velocity is provided in true meters per day, towards EASTING(vx) and NORTHING(vy) direction of the grid, and the vertical displacement (vz), is derived from a digital elevation model. Provided is a NetCDF file with the velocity components: vx, vy, vz and vv (magnitude of the horizontal components), along with maps showing valid pixel count and uncertainty (stdx, stdy). The product was generated by ENVEO.
NC_GLOBAL#contact=copernicus-support@ecmwf.int
NC_GLOBAL#Conventions=CF-1.7
NC_GLOBAL#creation_date=2022-12-12
NC_GLOBAL#history=product version 1.4
NC_GLOBAL#institution=Copernicus Climate Change Service
NC_GLOBAL#keywords=EARTH SCIENCE CLIMATE INDICATORS CRYOSPHERIC INDICATORS GLACIAL MEASUREMENTS ICE SHEET VELOCITY CRYOSPHERE GLACIERS/ICE SHEETS
NC_GLOBAL#license=C3S general license
NC_GLOBAL#project=C3S_312b_Lot4_ice_sheets_and_shelves
NC_GLOBAL#reference=Main: Nagler, T.; Rott, H.; Hetzenecker, M.; Wuite, J.; Potin, P. The Sentinel-1 Mission: New Opportunities for Ice Sheet Observations. Remote Sens. 2015, 7, 9371-9389. https://doi.org/10.3390/rs70709371
NC_GLOBAL#source=Copernicus Sentinel-1A and Sentinel-1B
NC_GLOBAL#summary=Ice velocity derived for Greenland Ice Sheet gridded at 250m averaged from 2020-10-01 to 2021-09-30.
NC_GLOBAL#title=Ice Velocity of the Greenland Ice Sheet
x#axis=X
x#long_name=x coordinate of projection
x#standard_name=projection_x_coordinate
x#units=m
y#axis=Y
y#long_name=y coordinate of projection
y#standard_name=projection_y_coordinate
y#units=m
Geolocation:
LINE_OFFSET=0
LINE_STEP=1
PIXEL_OFFSET=0
PIXEL_STEP=1
SRS=GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AXIS["Latitude",NORTH],AXIS["Longitude",EAST],AUTHORITY["EPSG","4326"]]
X_BAND=1
X_DATASET=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":x
Y_BAND=1
Y_DATASET=NETCDF:"C3S_GrIS_IV_250m_S1_20201001_20210930_v1_4.nc":y
Corner Coordinates:
Upper Left ( -640000.000, -655500.000) ( 89d18'52.26"W, 81d33'27.74"N)
Lower Left ( -640000.000,-3355750.000) ( 55d47'51.49"W, 59d11'51.92"N)
Upper Right ( 856000.000, -655500.000) ( 7d33'22.15"E, 80d 4'16.22"N)
Lower Right ( 856000.000,-3355750.000) ( 30d41'23.64"W, 58d47'38.46"N)
Center ( 108000.000,-2005625.000) ( 41d55' 3.66"W, 71d36'42.42"N)
Band 1 Block=748x1351 Type=Float32, ColorInterp=Undefined
Min=0.000 Max=23.788
Minimum=0.000, Maximum=23.788, Mean=0.083, StdDev=0.396
NoData Value=3.4028234663852886e+38
Unit Type: m/day
Metadata:
coordinates=y x
description=magnitude of horizontal ice velocity
grid_mapping=crs
NETCDF_VARNAME=land_ice_surface_velocity_magnitude
STATISTICS_MAXIMUM=23.78809928894
STATISTICS_MEAN=0.082814344301973
STATISTICS_MINIMUM=3.1557854072162e-06
STATISTICS_STDDEV=0.39557537806555
units=m/day
_FillValue=3.4028235e+38
2. Data access information
2.1. All products
Data will be made available through the Copernicus Climate Data Store (CDS), which is the sole data distributor. Registration (free) is required to access the CDS and its toolbox software suite. Data can be downloaded from the website and used under the License to Use Copernicus Products (included on download page). Data may also be viewed online. All requests for information or further data should be channelled through the CDS Knowledge Base.
References
GCOS, 2022. The 2022 GCOS Implementation Plan. Geneva: World Meteorological Organization, 85. https://gcos.wmo.int/en/publications/gcos-implementation-plan2022
Nagler, T.; Rott, H.; Hetzenecker, M.; Wuite, J.; Potin, P. The Sentinel-1 Mission: New Opportunities for Ice Sheet Observations. Remote Sens. 2015, 7, 9371-9389. https://doi.org/10.3390/rs70709371
Rizzoli, P., Martone, M., Gonzalez, C., Wecklich, C., Tridon, D.B., Bräutigam, B., Bachmann, M., Schulze, D., Fritz, T., Huber, M. and Wessel, B., (2017). Generation and performance assessment of the global TanDEM-X digital elevation model. ISPRS Journal of Photogrammetry and Remote Sensing, 132, pp.119-139. https://doi.org/10.1016/j.isprsjprs.2017.08.008
This document has been produced in the context of the Copernicus Climate Change Service (C3S).
The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Contribution agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.
The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.
