Wave Height Data from
Open-Ocean Altimetry
Issue 1.0, 22 January 2013
Edited by the Coastal & Marine Research Centre, University College Cork
DOCUMENT DETAILS
Author
Phillip Harwood, Logica,
Rory Scarrott, CRMC
Document Title
Product Data Handbook: Wave products from Open-Ocean
Altimetry
Document Reference
D180N_HB_WA1a
Product Reference
WA1a
Issue
1.0
Date of Issue
22 January 2013
CHANGE RECORD
Version
Date
Change Description
Author
0.9
3/12/12
First issue
1.0
22/12/13
Final edits and release of Version RS
1.0
PH. RS
Front cover image credit: ESA, NASA, Staff Sergeant Val Gempis (USAF), ABC Action News, GLOBWAVE
2
Contents
Summary .......................................................................................................................... 4
Abbreviations and Acronyms .................................................................................................... 5
1.
2.
3.
4.
THE OPEN-OCEAN WAVE HEIGHT PRODUCT (WA1a)................................................... 6
1.1
Introduction .................................................................................................................. 6
1.2
Source Data .................................................................................................................. 6
1.3
Variables Calculation ..................................................................................................... 7
1.3.1 Calibration variables .............................................................................................................. 7
1.3.2 Quality variables .................................................................................................................... 8
1.3.3 HS standard error variables ................................................................................................... 9
1.3.4 Rejection flag variables .......................................................................................................... 9
1.3.5 Ancillary variables ................................................................................................................ 10
1.4
Product Validation ...................................................................................................... 11
THE NRT WAVE HEIGHT PRODUCT (NRT3) ................................................................ 13
2.1
Introduction ................................................................................................................ 13
2.2
Data origins................................................................................................................. 13
2.3
Variable Calculation .................................................................................................... 13
2.3.1 Altimetry Correction ............................................................................................................ 13
2.3.2 Ancillary Variables ................................................................................................................ 13
PRODUCT DETAILS ................................................................................................... 14
3.1
Technical Description .................................................................................................. 14
3.2
File contents ............................................................................................................... 15
3.2.1 Dimensions........................................................................................................................... 15
3.2.2 Variables............................................................................................................................... 15
3.2.3 Variable attributes ............................................................................................................... 18
3.2.4 Global attributes .................................................................................................................. 19
3.2.5 Data ...................................................................................................................................... 19
3.3
Known Issues .............................................................................................................. 20
3.3.1 Historic data (GDR)............................................................................................................... 20
3.3.2 NRT ....................................................................................................................................... 20
ACQUIRING THE PRODUCT ....................................................................................... 21
4.1
4.2
4.3
4.4
The eSurge Database ................................................................................................... 21
Accessing the Product ................................................................................................. 21
Additional sources of data ........................................................................................... 21
Conditions of Use ........................................................................................................ 21
5.
Frequently Asked Questions .................................................................................... 22
6.
Further Information and Contacts ............................................................................ 23
7.
REFERENCES ............................................................................................................ 24
ANNEX A: THE ESURGE PROJECT ..................................................................................... 25
8.
9.
B.1 About eSurge ............................................................................................................ 25
B.2 The eSurge Consortium ............................................................................................. 26
3
Summary
This Product Handbook covers the along-track wave heights from open-ocean altimetry product
(eSurge product ID WA1a). This product, originally generated by the ESA DUE GlobWave 1 project,
gives a consistent set of satellite wave data from available satellite altimeter data.
The Level 2 Pre-processed (L2P) satellite wave data set is one of the fundamental outputs of the
GlobWave project. It was defined following the approach pioneered for satellite sea surface
temperature data by ESA’s Medspiration project. The L2P data consists of Level-2 data from multiple
altimetry sensors which has been transcribed into a common netCDF format, and which has been
augmented with error estimates for each wave measurement.
The GlobWave historical archive contains altimeter data from 8 satellites, ranging from Geosat
(operating between 1985 and 1989) through to Envisat, Jason-1 and Jason-2, still operating in 2012.
The historical data was continuous in time from 1991 to 2012, with a NRT service operating after
December 2010.
This product and associated handbook were compiled for the eSurge Project using the outputs of
the GlobWave project. Both of these projects were funded by the European Space Agency under the
ESA Data User Element (DUE) Programme.
A separate product (eSurge ID WA1b) has been generated using a modified coastal altimetry
processor and is described in product handbook D180G. eSurge also provides access to SAR wave
spectra from the GlobWave project, described in product handbook D180O.
For more information on the eSurge project go to http://www.storm-surge.info/.
The eSurge project would like to gratefully acknowledge the GlobWave project consortium (Logica,
Ifremer, CLS, NOC and SatOC) both for allowing reuse of their products and for supplying text for this
product handbook.
Also please note that this is considered the initial draft version of the open-ocean altimetry wave
dataset handbook and subject to review by users. To this end, we have made available a quick online
survey to gather your views on our handbooks, and shape them further into what you need as a
user. The simple Survey Monkey ® questionnaire can be found at
http://www.surveymonkey.com/s/XX9VH5S, and will take a maximum of five minutes.
1
www.globwave.org
4
Abbreviations and Acronyms
The acronyms and abbreviations used within this document are listed below.
Acronym
AOI
AVISO
BODC
BOOS
CCI
CMRC
DMI
DUE
DUP
ENVISAT
EO
EOEP
ERS
ESA
INSPIRE
KNMI
NERC
NOC
NOOS
NRT
RADS
SAF
SAR
STFS
UCC
WOND
Meaning
Area of Interest
Archiving, Validation and Interpretation of Satellite Oceanographic Data
British Oceanographic Data Centre
Baltic Sea Operational Oceanographic System
Climate Change initiative
Coastal and Marine Research Centre
Danmarks Meteorologiske Institut (Danish Meteorological Institute).
Data User Elements
Data User Programme
ESA’s Environmental Satellite
Earth Observation
Earth Observation Envelope Programme
European Remote-Sensing Satellite
European Space Agency
Infrastructure for Spatial Information in the European Community
Koninklijk Nederlands Meterologisch Instituut (Royal Netherlands Meteorological
Institute)
Natural Environment Research Council (UK)
National Oceanography Centre
North West Shelf Operational Oceanographic System
Near Real Time
Radar Altimeter Database System
Satellite Application Facility
Synthetic Aperture Radar
Storm Tide Forecasting System
University College Cork
The research section of the Royal Netherlands Meteorological Institute
5
1. THE OPEN-OCEAN WAVE HEIGHT PRODUCT (WA1a)
1.1
Introduction
This handbook covers the altimeter derived along-track wave height data product (eSurge product ID
WA1). This product, originally generated by the ESA DUE GlobWave project, gives a consistent set of
satellite wave data from all available satellite altimeter data.
The Level 2 Pre-processed (L2P) satellite wave data set is one of the fundamental outputs of the
GlobWave project. It was defined following the approach pioneered for satellite sea surface
temperature data by ESA’s Medspiration project. The L2P data consists of Level-2 data from multiple
altimetry sensors which has been transcribed into a common netCDF format, and which has been
augmented with error estimates for each wave measurement.
The GlobWave historical archive contains altimeter data from 8 satellites, ranging from Geosat
(operating between 1985 and 1989) through to Envisat, Jason-1 and Jason-2, still operating in 2012.
The historical data was continuous in time from 1991 to 2012, with a NRT service operating after
December 2010.
The GlobWave project was due to end at the end of 2012, but at time of writing of this handbook, it
was expected that on-going access to historical data and a NRT service would continue to be
provided by Ifremer.
1.2
Source Data
The following archive data sources have been used to generate the L2P open ocean altimetry wave
data products:
Table 1-1: L2 Archived Satellite Altimetry Data for GlobWave
Source
L2 Product
Type
Dates
Official Source
ESA ERS-1
ERS.ALT.OPR
Altimeter 01/02/1992 –
02/06/1996
CERSAT
http://cersat.ifremer.fr
ESA ERS-2
ERS.ALT.OPR
Altimeter 15/05/1995 –
11/05/2009
CERSAT
http://cersat.ifremer.fr
ESA Envisat
RA2_GDR_2P
Altimeter 26/08/2002
08/04/2012
ESA
www.esa.int
CNES/NASA
Topex/POSEIDON
MGDR
Altimeter 25/09/199208/10/2005
AVISO
www.aviso.oceanobs.com,
PODAAC
http://podaac.jpl.nasa.gov
CNES/NASA Jason- GDR
1
Altimeter 15/01/2002
onwards
AVISO
www.aviso.oceanobs.com,
PODAAC
http://podaac.jpl.nasa.gov
CNES/NASA Jason- GDR
2
Altimeter /12/2008
onwards
NOAA
www.noaa.gov,
EUMETSAT
www.eumetsat.int
US Navy
Altimeter 31/03/1985-
NOAA
GDR
6
Source
L2 Product
Type
GEOSAT
Dates
Official Source
30/12/1989
www.noaa.gov
US Navy/NOAA
GEOSAT Follow
On
GDR and I-GDR Altimeter 08/01/2000 –
23/07/2008
NOAA
www.noaa.gov
ESA/NOAA
CryoSat
IGDR
ESA
www.esa.int
NOAA
http://ibis.grdl.noaa.gov
1.3
Altimeter 28/01/2011
onwards
Variables Calculation
The full set of variables within this product is listed in section 3. Many of these variables are
transferred directly from the source product. This section describes the changes which have been
made to the source products in order to reach the L2P product.
1.3.1 Calibration variables
Recommended corrections have been applied to the altimeter 1Hz estimates of significant wave
height (swh) to derive the calibrated values (swh_calibrated).
For historical data (GDR) calibrations are taken from Queffeulou & Croizé-Fillon (2009), except for
Geosat, Envisat and Jason-2. Geosat has the swh values from the revised data set Geosat Altimeter
JGM-3 GDRs on CD-ROM already increased by 13% in accordance with Carter et al. (1992). Envisat
has new calibrations since the update to L2 source processing in January 2010 (and for all
reprocessed data before then). These calibrations are taken from Queffeulou et al. (2011). For Jason2 calibrations are taken from the GlobWave error analysis.
The calibration corrections for historical data are given in Table 1-2.
Table 1-2: Calibration correction values for historical altimeter data
Satellite (satid)
Correction
Comments
Geosat (GEOS)
swh_calibrated = swh
ERS-1 (ERS1)
swh_calibrated =
1.1259 x swh + 0.1854
ERS-2 (ERS2)
swh_calibrated =
1.0642 x swh + 0.0006
Envisat (ENVI)
swh_calibrated =
1.0585 x swh – 0.1935
Version 1, used for initial L2P release.
[swh > 3.41m]
swh_calibrated =
1.0095 x swh + 0.0192
[swh < 3.41m]
swh_calibrated =
-0.021 x swh3 +
0.1650 x swh2 +
0.5693 x swh + 0.4358
Version 2, applies to all data since Jan 15th
2010 and all reprocessed data before then.
See L2P release notes.
Side A:
swh_calibrated =
Side A operated up to cycle 235. There was
a drift in the measured swh values
Topex (TOPX)
JGM-3 version has calibrations applied
7
Satellite (satid)
Poseidon (TOPX)
Correction
Comments
1.0539 x s–h - 0.0766 + beginning in cycle 98, which is corrected for
dh
in the calibration.
with:
dh = 0 for cycle < 98
dh = poly3(9–) poly3(cycle) for
98 <= cycle <= 235
with a0 = 0.0864 ;
a1= -6.0426 x 10-4 ;
a2 = -7.7894 x 10-6 ;
a3 = 6.9624 x 10-8
Side B:
swh_calibrated =
1.0237 x s–h - 0.0476
swh_calibrated =
0.9914 x swh – 0.0103
Side B operated after cycle 235
Poseidon operated throughout the mission
approximately 5% of the time.
Geosat Follow-On
(GFO_)
swh_calibrated =
1.0625 x swh + 0.0754
Jason-1
swh_ calibrated =
1.0250 x swh + 0.0588
Jason-2
swh_ calibrated = 1.041 Calibration derived from GlobWave error
x swh - 0.042
analysis
CryoSat-2
L2 data version b.
Not yet calibrated
In the version 1.3 of the GlobWave L2P (October 2010), calibrations are only provided for primary
frequency significant wave height. Variables for calibrated values are also provided for second
frequency significant wave height, primary and secondary frequency sigma0 and altimeter wind
speed. These may be filled in future releases of the l2p, or can be used by Users to include their own
calibrated values of these quantities
1.3.2 Quality variables
Quality variables are included for significant wave height (Hs) and backscatter coefficient (sigma0)
for both main and second altimeter frequencies where applicable. The quality variables are:
swh_quality
sigma0_quality
swh_2nd_quality
swh_2nd_quality
In the current release of the GlobWave L2P product (April 2010) quality variables are only calculated
for the main altimeter frequency, and have the same value for Hs and sigma0 at a data point. The
values and corresponding meanings of the quality variables are given in Table 1-3.
8
Table 1-3: Quality variables values
Value (decimal)
Meaning
0
Probably good measurement
1
Suspect, probably okay for some applications. For
example this is set when rain is detected for an otherwise
good measurement.
2
Probably bad measurement
127
Not evaluated
The calculation of the quality variables varies by satellite and sensor. For full details refer to Annex B
of the GlobWave PUG, available at www.globwave.org.
1.3.3 HS standard error variables
The Hs standard error variable gives a measure of the standard error for significant wave height and
is calculated using collocations with in situ measurements. The Hs standard error variable is:
swh_standard_error
The error is derived from the spread of the regression of collocation measurements, and varies with
altimeter and with significant wave height. The method of calculating the swh_standard_error is
described in full in annex C of the GlobWave PUG, available at www.globwave.org.
1.3.4 Rejection flag variables
The rejections flag variable contains consolidated flags from the L2 source data together with an
externally derived ice flag. The rejection flags variable is:
rejection_flags
The content of the rejections flag is based loosely on the ‘flags’ variable used in the RADS database,
and is summarised in Table 1-4.
Table 1-4: Rejection Flags
Bit
Description
Satellites
0
Hardware status
TOPX JAS1 JAS2 ENVI
1
Satellite on track
Attitude status
ERS1
ERS2 GEOS TOPX GFO_ JAS1
JAS2
2
spare
3
spare
4
Altimeter land flag
all
5
Altimeter ocean / non-ocean flag
all
6
Radiometer land flag
all except GEOS
7
Corruption of Altimeter measurement
TOPX JAS1 JAS2 GFO_ ENVI
CRYO
8
Corruption of radiometer measurement
ERS1 ERS2 TOPX JAS1 JAS2
ENVI
9
Bit
Description
Satellites
9
spare
10
spare
11
Quality of the range estimate
all
12
Quality of the wave height estimate
Sea state bias
ERS1 ERS2 TOPX GFO_ JAS1
JAS2 CRYO
GEOS
13
Quality of sigma0 estimate
Quality of windspeed measurement
all except GEOS
GEOS
14
spare
15
Quality of the orbit
ERS1 ERS2 TOPX JAS1 JAS2
ENVI
16
Quality of the wave height estimate 2nd
frequency
JAS2
17
Quality of sigma0 estimate 2nd frequency JAS2
18
Quality of off nadir angle from
waveforms
JAS2
19
Quality of off nadir angle from platform
JAS2
20
Ice flag from database
all
2130
spare
31
Flag rejection
all
For GFO, TOPEX/Poseidon, Jason-1, Jason-2 and Envisat Bit 8 of the rejection_flags variable
(corruption of radiometer) is equivalent to the rain flag and will be set if rain is detected.
This flag is taken into account when calculating the quality level. If Bit 8 of the rejection_flags is set
then quality level will be set to “1” for an otherwise good measurement.
1.3.5 Ancillary variables
Ancillary fields are quantities additional to those associated with wave measurements that may have
an effect on the measurements or their application. The ancillary variables are:
wind_speed_model_u
wind_speed_model_v
bathymetry
distance_to_coast
sea_surface_temperature
surface_air_temperature
surface_air_humidity
surface_air_pressure
Some of these ancillary fields are provided within the L2 data products, but in order to maintain
consistency across the L2P all the values are updated from external sources.
10
For the meteorological parameters (all except bathymetry and distance to coast) the values are
obtained from numerical models operated at ECMWF.
For near real-time L2P data, output from the operational global 0.5 degree analysis is used.
For delayed mode data, output from the ERA40 0.75 degree reanalysis is used. This output does not
include the surface_air_humidity values, so humidity values are absent from the initial release
(2010) of the L2P GDR data products.
The ice flag bit of the rejections_flag variable is also taken from the ECMWF model output, but is
planned to be updated using the high resolution OSI-SAF dataset later in the project.
The bathymetry and distance_to_coast are calculated from the GEBCO_08 Grid — The General
Bathymetric Chart of the Oceans (GEBCO) consists of an international group of experts who work on
the development of a range of bathymetric data sets and data products, including gridded
bathymetric data sets, the GEBCO Digital Atlas, the GEBCO world map and the GEBCO Gazetteer of
Undersea Feature Names. GEBCO_08 is provided on a global grid at 30 arc-second intervals. The grid
is largely based on ship track soundings with interpolation between soundings guided by satellitederived gravity data. However, in areas where they improve on the existing GEBCO_08 grid, data
sets generated by other methods have been included. See http://www.gebco.net/.
1.4
Product Validation
Full details of how product quality is assessed are given in Annex B of the GlobWave PUG, available
at www.globwave.org.
The validation for each 1 second altimeter record cannot be guaranteed to separate out all bad and
good records; they only indicate records which are likely to be good from those which are most likely
bad. Those likely to be good are further divided into those which are generally acceptable –
acceptable for most purposes – and those which are almost certainly good.
For example, if an altimeter has a rain flag then this is ignored when deciding whether a record is
generally acceptable, since only very heavy rain adversely affects the altimeter radar, so frequently
records with rain give good estimates of wave height and wind speed, and applying this check would
remove many good records; often rain is associated with strong winds and high waves so discarding
rain-flagged records for climate studies is likely to skew the results. However for some purposes,
such as when calibrating wave height against buoys or studying sea state bias, it might be
appropriate to use only those records which are almost certainly good.
The Globwave quality variable is set to:
2
1
0
probably bad
generally acceptable
almost certainly good
(abbreviated to ‘bad’ below).
(abbreviated to ‘acceptable’ below).
(abbreviated to ‘good’ below).
The checks are on the quality of both the significant wave height and the sigma-0 value (from which
wind speed is derived). Often both wave height and wind speed are required for analysis, and if one
is dubious the other is unlikely to be good. (An exception is that wave height is less affected by mispointing than is sigma-0.)
11
Note that this product has been generated through the Globwave project, and has not been
independently validated by the eSurge project, other than to confirm that the data have been
ingested correctly into our database.
The Globwave project has carried out extensive validation on the product. These are summarised in
the quarterly validation reports, which are available on the GlobWave website at:
http://www.globwave.org/Validation.
12
2. THE NRT WAVE HEIGHT PRODUCT (NRT3)
2.1
Introduction
In general, the NRT product is essentially a special case of the product described in section 1.
However there are some differences, which are outlined in this section.
At the time of writing, the Globwave project is due to finish at the end of December 2012, however
it is expected that the NRT product will continue to be produced by Ifremer.
2.2
Data origins
Given the limited number of active altimetry satellites, the only sources for NRT altimetry inputs are
the two Jason spacecraft.
Table 2-1: Space Agency L2 NRT Satellite Data for GlobWave
Source
L2 Product
Type
Dates
Official Source
CNES/NASA
Jason-1
OSDR
Altimeter
15/01/2002
onwards
AVISO
www.aviso.oceanobs.com,
PODAAC
http://podaac.jpl.nasa.gov
CNES/NASA
Jason-2
OGDR
Altimeter
/12/2008
onwards
NOAA
www.noaa.gov,
EUMETSAT
www.eumetsat.int
2.3
Variable Calculation
This section discusses changes to the historical data variable calculation (section 1.3) applied to the
NRT data by the GlobWave project.
2.3.1 Altimetry Correction
Recommended corrections have been applied to the altimeter 1Hz estimates of significant wave
height (swh) to derive the calibrated values (swh_calibrated).
For near real-time data (NRT) the calibration of Jason-2 should be the same as for the historical data
(GDR). For Jason-1 the calibrations are different and are taken from Durrant et al. (2009).
2.3.2 Ancillary Variables
For near real-time L2P data, output from the operational global 0.5 degree analysis is used.
13
3. PRODUCT DETAILS
3.1
Technical Description
An overview of the available delayed mode products is given in the table below. These are identical
apart from the satellite that they originate from.
Note that not all products will be available for any given event.
Table 3-1: Overview of L2P product file types within the WA1a product (* is a generic
marker for date, time, cycle and orbit information)
L2P product
Description
GW_L2P_ALT_ERS1_GDR_*
L2P product from ERS-1 altimeter
GW_L2P_ALT_ERS2_GDR_*
L2P product from ERS-2 altimeter
GW_L2P_ALT_ENVI_GDR_*
L2P product from Envisat altimeter
GW_L2P_ALT_GEOS_GDR_*
L2P product from Geosat altimeter
GW_L2P_ALT_GFO__GDR_*
L2P product from Geosat Follow-On altimeter
GW_L2P_ALT_TOPX_GDR_*
L2P product from TOPEX/Poseidon altimeter
GW_L2P_ALT_JAS1_GDR_*
L2P product from Jason-1 altimeter
GW_L2P_ALT_JAS2_GDR_*
L2P product from Jason-2 altimeter
GW_L2P_ALT_CRYO_GDR_*
L2P product from CryoSat-2 altimeter
The L2P format is netCDF-3 and the convention Climate and Forecast CF-1.4.
For more information on the netCDF format visit:
http://www.unidata.ucar.edu/software/netcdf/index.html
For more information on Climate and Forecast conventions visit:
http://cf-pcmdi.llnl.gov/
The L2P data files are named in the following way:
GW_L2P_instr_satid_type_startdate_starttime_enddate_endtime_cycle_orbit.nc
where the lower case word labels are substituted as follows:
Table 3-2: L2P file name components
Label
File name
components
Comments
instr
ALT
Gives satellite instrument type: altimeter
satid
one of:
GEOS
ERS1
ERS2
ENVI
TOPX
GFO_
JAS1
Gives satellite:
Geosat (altimeter only)
ERS1
ERS2
Envisat
Topex/Poseidon (altimeter only)
Goesat Follow-on (altimeter only)
Jason-1 (altimeter only)
14
Label
File name
components
JAS2
CRYO
type
Comments
Jason-2 (altimeter only)
CryoSat-2 (altimeter only)
Gives type of L2 source data used:
Delayed mode data products (often called
Geophysical Data Records, GDR)
Near real-time data products
one of:
GDR
NRT
start date
8 digit date in the
form YYYYMMDD
Data start date
starttime
6 digit time in the
form hhmmss
Data start time
enddate
8 digit date in the
form YYYYMMDD
Data end date
endtime
6 digit time in the
form hhmmss
Data end time
cycle
3 digit number
Cycle of satellite mission
orbit
3 or 4 digit number
Orbit or pass within the cycle
3.2
File contents
A netCDF file contains Dimensions, Variables, Variable Attributes, Global Attributes and Data. The
content of the L2P data are described with respect to these below.
3.2.1 Dimensions
For altimetry data (GW_L2P_ALT) data are one-dimensional along the satellite track and there is just
the time dimension:
Table 3-3: Altimeter Variable Dimensions
Name
time
Length
variable
Notes
This dimension is variable in length
according to the number of data points
in time in the L2 data.
3.2.2 Variables
The altimetry variables are described in Table 3-4. Variables in blue type are those which were
derived or manipulated within the GlobWave processing (see section 1),while those in black are
transcribed directly from the L2 data products.
Table 3-4: Altimetry Variables
No.
Name
Description
NetCDF
Type
Units
1
time
Seconds since 1985-01-01
double
s
2
lat
Latitude
double
deg
3
lon
Longitude, range 0 to 360
double
deg
15
No.
Name
Description
NetCDF
Type
Units
4
swh
Significant wave height (primary, e.g. short
Ku band)
10-3 m
5
swh_calibrated
Calibrated significant wave height.
short
10-3 m
6
swh_quality
Significant wave height quality (3
levels).
byte
N/A
7
swh_standard_error
Significant wave height best estimate short
of standard error.
10-3 m
8
swh_2nd
Significant wave height 2nd frequency short
(e.g. S or C band)
10-3 m
9
swh_2nd_calibrated
Calibrated significant wave height 2nd short
frequency.
10-3 m
10
swh_2nd_quality
Significant wave height quality 2nd
frequency (3 levels) .
byte
N/A
11
sigma0
Backscatter coefficient (primary, e.g.
Ku band)
short
10-2 dB
12
sigma0_calibrated
Calibrated backscatter coefficient.
short
10-2 dB
13
sigma0_quality
Sigma0 quality (3 levels).
byte
N/A
nd
14
sigma0_2nd
Backscatter coefficient 2 frequency short
(e.g. S or C band)
10-2 dB
15
sigma0_2nd_calibrated
Calibrated significant wave height 2nd short
frequency.
10-2 dB
16
sigma0_2nd_quality
sigma0 quality 2nd frequency (3
levels) .
byte
N/A
17
wind_speed_alt
Altimeter wind speed
short
10-2 m/s
18
wind_speed_alt_calibrated
Calibrated Altimeter wind speed.
short
10-2 m/s
19
wind_speed_rad
Radiometer wind speed
short
10-2 m/s
20
wind_speed_model_u
U component of model wind vector
short
10-2 m/s
21
wind_speed_model_v
V component of model wind vector
short
10-2 m/s
22
rejection_flags
32 bundled flags, all relevant flags
included with spare bits for future
missions.
int
N/A
23
swh_rms
RMS/std of significant wave height
(primary, e.g. Ku band)
short
10-3 m
24
swh_rms_2nd
RMS/std of significant wave height
2nd frequency (e.g. S or C band)
short
10-3 m
25
swh_num_valid
Number of valid points used to
compute Hs (primary, e.g. Ku band)
short
N/A
26
swh_num_valid_2nd
short
Number of valid points used to
compute Hs 2nd frequency (e.g. S or C
band)
N/A
27
sigma0_rms
RMS/std of backscatter coefficient
10-2 dB
short
16
No.
Name
Description
NetCDF
Type
Units
(primary, e.g. Ku band)
10-2 dB
28
sigma0_rms_2nd
RMS/std of backscatter coefficient
2nd frequency (e.g. S or C band)
29
sigma0_num_valid
Number of valid points used to
short
compute sig0 (primary, e.g. Ku band)
N/A
30
sigma0_num_valid_2nd
short
Number of valid points used to
compute sig0 2nd frequency (e.g. S or
C band)
N/A
31
peakiness
Pulse peakiness (primary, e.g. Ku
band, Envisat only)
short
10-3
32
peakiness_2nd
Pulse peakiness 2nd frequency (e.g. S
band, Envisat only)
short
10-3
33
off_nadir_angle_wf
Off nadir angle from waveforms
short
Deg(2)
34
off_nadir_angle_pf
Off nadir angle from platform
short
Deg(2)
35
range_rms
RMS/std of range (primary, e.g. Ku
band)
short
10-4 m
36
range_rms_2nd
RMS/std of range 2nd frequency (e.g.
S or C band)
short
10-4 m
37
bathymetry
Bathymetry from GEBCO 08 /
DTM2000.1
short
m
38
distance_to_coast
Distance to coast.
short
km
39
sea_surface_temperature
SST from model.
short
K
40
surface_temperature
Surface air temperature from model. short
K
41
surface_air_humidity
Surface air humidity from model.
short
%
42
surface_air_pressure
Surface air pressure from model.
short
Pa
short
Table 3-4 is a list of all variables, but for each individual satellite the included variables are a subset
of this. The variables included for each satellite are summarized in Table 3-5.
Table 3-5: Altimetry variables included for each satellite
Satid
Included variable numbers
ERS1
1-7, 11-13, 17-18, 20-23, 25, 27, 33, 35, 37-42
ERS2
1-7, 11-13, 17-18, 20-23, 25, 27, 33, 35, 37-42
ENVI
1-18, 20-42
GEOS
1-7, 11-13, 17-18, 20-22, 33, 35, 37-42
GFO_
1-7, 11-13, 17-18, 20-23, 25, 27, 33, 35, 37-42
TOPX
1-18, 20-25, 27, 29, 33-35, 37-42
JAS1
1-30, 33-42
JAS2
1-30, 33-42
CRYO
1-7, 11-13, 17-18, 20-23, 25, 27, 34-35, 37-42
17
3.2.3 Variable attributes
The variable attributes used in the L2P are described in Table 3-6. The attributes associated with
specific variables are given in the example L2P header listings in ANNEX A: L2P dimension, variable
and attribute listings.
Table 3-6: Variable Attributes
Variable Attribute
Description
_FillValue
Default value for missing or undefined data
long_name
A descriptive name that indicates a
variable’s content. This name is not
standardized
standard_name
A standard name that references a
description of a variables content in the
standard name table
units
Unit of a variable’s content
calender
Reference time calender
source
Data source (e.g. model)
institution
Institution which provides the data
calibration_formula
Formula used in applying calibration
calibration_reference
Reference for calibration formula
quality_flag
Name of variable or quality flag
representing quality of current variable
valid_min
Smallest theoretical valid value of a variable
(this is not the maximum of actual data)
valid_max
Largest theoretical valid value of a variable
(this is not the maximum of actual data)
flag_values
Provide a list of the flag values. Use in
conjunction with flag_meanings
flag_masks
Use in conjunction with flag_meanings to
allocate variable bit settings
flag_meanings
Use in conjunction with flag_values to
provide descriptive words or phrase for
each flag value
scale_factor
Scale factor to be applied to integer to
achieve correct units
add_offset
Offset to be applied to integer to achieve
correct value
coordinates
Identified auxiliary coordinate variables
comment
Miscellaneous information about the data
or the methods used to produce it
Comment
18
3.2.4 Global attributes
The global attributes used in the L2P are described in Table 3-7.
Table 3-7: Global Attributes
Global Attribute
Description
Conventions
CF-1.4
title
GlobWave L2P
source
radar altimeter
project
ESA GlobWave
institution
GlobWave
history
Date and time of product creation
contact
Contact information
references
GlobWave Product User Guide
processing_center
Ifremer
software_version
GlobWave product user guide
source_provider
mission_name
source_name
source_version
source_software
altimeter_sensor_name
radiometer_sensor_name
acq_station_name
Acquisition station
cycle_number
Mission cycle number
pass_number
Pass or orbit within cycle
equator_crossing_time
equator_crossing_longitude
start_time
Date and time of first measurement in file
stop_time
Date and time of last measurement in file
3.2.5 Data
The data themselves are of the specified binary type and presented in blocks in the order of the
variables listing, however this order is not normally relevant when reading netCDF files. The number
of data points for a variable corresponds to the product of the dimension values of that variable.
19
3.3
Known Issues
The following known issues are reported by the GlobWave project:
3.3.1 Historic data (GDR)
Altimeter product version 1.3:
Unfilled variables
• swh_2nd_calibrated, sigma0_calibrated, sigma0_2nd_calibrated, wind_speed_alt_calibrated
These variables are included in the l2p for future use if robust calibrations become available.
• swh_2nd_quality, sigma0_2nd_quality
These variables are included in the l2p for future use if robust quality criteria become
available.
• surface_air_humidity
This parameter was not available in the ECMWF reanalysis but may be included in a future
release of the l2p.
Other variables to be updated
• rejections_flag
The ice flag component of this variable may be updated with higher resolution data in a future
release of the L2P.
Data quality issues
• Envisat: certain files (e.g. Cycle 047 pass 194) have some bad latitude values in the L2 data
that have not yet been corrected for in the L2P.
• ERS2: Some negative swh values to be investigated.
• GFO: some L2 files contain no data and are omitted from the L2P.
It is recommended not to use sigma0 or altimeter wind speed data after 2nd August 2006.
• TOPEX: it is recommended not to use sigma0 or altimeter wind speed data for the first 10
cycles of TOPEX, up until December 31st 1992, and cycles 433 to 437, from June 15th to August
2nd 2004 inclusive. Quality levels of sigma0 for these periods have been set accordingly and
may differ from the quality levels of significant wave height.
Other issues
• Geosat: swh_calibrated values are the same as swh as the data are calibrated in the L2. The
calibration reference in given incorrectly and should be Carter et al. (1992)
• All satellites: the swh_standard_error variable was calculated using calibrated data instead of
uncalibrated data giving a difference of <1%.
• CryoSat-2: Quality levels, error estimates and calibrations are not yet available and will be
calculated once there is a suitable time series of buoy co-locations.
3.3.2 NRT
Unfilled variables
•
surface_air_humidity
20
4. ACQUIRING THE PRODUCT
4.1
The eSurge Database
The eSurge database provides a wide variety of data sets for a range of storm surge events (SEVs).
For each SEV, we provide suitable data sets either by extracting the relevant subset of the full data
set from another source, or in some cases by providing new processed data.
4.2
Accessing the Product
The product is available from the eSurge website: www.storm-surge.info/data-access.
Help for downloading is available at www.storm-surge.info/how-to.
4.3
Additional sources of data
The data provided by eSurge is a subset of the data produced by the globwave project, available
through their website at www.globwave.eu.
4.4
Conditions of Use
This data is provided by the eSurge project. By accessing this data, you are agreeing to the GlobWave
terms and conditions, which are as follows:
The data provided by eSurge has been compiled from several sources. Any additional terms
and conditions required by the data provider should be considered as part of the eSurge
conditions. In case of any conflict, the conditions of the original data provider shall prevail.
Unless required otherwise by the data provider, eSurge data is public domain and may be
used, copied and distributed free of charge.
eSurge data may be exploited commercially to develop revenue-generating services where
measurable value is added to the data by a service provider. Distribution or copying of
unmodified eSurge data for commercial purposes or financial gain is strictly prohibited. Users
are required to acknowledge the eSurge project in any resulting papers, products,
presentations or other outreach material.
Any data made available through eSurge is not guaranteed to be: up to date, true, not
misleading, free from viruses (or anything else which may have a harmful effect on any
technology), or to always be available for use. No liability shall be accepted for any losses
incurred as a result of the use of this data
This data was originally generated by the GlobWave project. By accessing this data, you are agreeing
to the GlobWave terms and conditions, which are as follows:
“GlobWave data is public domain and may be used, copied and distributed free of charge.
GlobWave data may be exploited commercially to develop revenue-generating services
where measurable value is added to the GlobWave data by a service provider. Distribution or
copying of unmodified GlobWave data for commercial purposes or financial gain is strictly
prohibited. Users are required to acknowledge GlobWave in any resulting papers, products,
presentations or other outreach material.
Any data made available through GlobWave is not guaranteed to be: up to date, true, not
misleading, free from viruses (or anything else which may have a harmful effect on any
technology), or to always be available for use.
No liability shall be accepted for any losses incurred as a result of the use of this data.”
21
5. Frequently Asked Questions
This section will remain un-compiled for version 1.0 of this handbook.
This is where the eSurge consortium needs your input as users to guide what we do next. What
questions do you have regarding the data? What clarifications do you need or regularly need when
assessing other datasets? Which storm surge events would you like to see data gathered for during
Phase 2 of the eSurge project?
To this end, we have made available a quick online survey to gather your views on our handbooks,
and shape them further into what you need as a user. The simple Survey Monkey ® questionnaire
can be found http://www.surveymonkey.com/s/XX9VH5S, and will take a maximum of five minutes.
22
6. Further Information and Contacts
For queries regarding the eSurge Project, please do not hesitate to contact:
General eSurge Contact:
Email:
Website
Mr. P. Harwood
[email protected]
http://www.storm-surge.info
For queries regarding the products, please contact the GlobWave project team via their website at
www.globwave.eu.
For queries regarding the European Space Agency (ESA) Data User Elements (DUE) Programme, see
http://due.esrin.esa.int/
23
7. REFERENCES
Carter D J T, Challenor P G & Srokosz M A 1992
An assessment of Geosat wave height and wind speed measurements
J. Geophys. Res. 97 (C7), 11383-11392.
Durrant T H, Greenslade D J M & Simmonds I 2009
Validation of Jason-1 and Enivsat remotely sensed wave heights
J. Atmos, Oce. Tech. 26, 123-134.
Queffeulou P & Croizé-Fillon D June 2009
Global altimeter SWH data set
IFREMER ([email protected])
Queffeulou, P., Ardhuin, F., and Lefevre, J.M. 2011.
Wave Height Measurements from altimeters: Validation status and applications.
Presentation at GlobWave User Consultation Meeting, Cork, October 2011
Additional reference material may be found on the globwave website at www.globwave.int.
24
ANNEX A: THE ESURGE PROJECT
8. B.1
About eSurge
Despite the potential utility of satellite data, the storm surge community has not made as much use
of it as they could. Largely this is due to the lack of easy data access. Different datasets are stored in
different locations, in different data formats and with different access requirements. eSurge aims to
change this, bringing relevant datasets together in an east to use, web-accessible database of data
products, downloadable in a standardised format.
The eSurge project is being run in two phases. During the initial Development Phase (Phase 1) we
have built the database, known as SEARS (Surge Event Analysis and Repository Service), and
populated it with initial data for a selection of historical surge events. This will give a useful library
that can be used for assessing and improving the performance of numerical models. Whilst most of
the datasets are already available, and just need to be imported into the database, others are being
created during the project.
Following the launch of the SEARS database, eSurge will move into a Service Demonstration phase
(Phase 2). During this phase we will continue to add more historical data, but will also look at making
data available for surge events as they occur. The aim is to show that it is feasible to provide satellite
data in near real time, so that it could potentially be used in forecasting and warning systems.
It is important to note that eSurge is not itself a forecasting and warning system, it is a system to
make data available to forecasters. There are dedicated agencies (such as the UK Environment
Agency) whose role it is to warn of impending flooding.
Making the data available is just part of the process of getting people to use it; we must also show
the value of the data. To this end our partners at DMI and NOC will perform a series of experiments,
focussing on the North Sea and North Indian Ocean. These will take existing models, such as DMI’s
HBM model and NOC’s operational CS3X surge model, and will look at how incorporating satellite
data could improve the models’ hindcast accuracy. These experiments will also investigate the best
way to incorporate satellite data into models. This is a complex subject, and we do not expect to be
able to resolve it in this project, but we aim to pave the way for future work
25
9. B.2
The eSurge Consortium
The eSurge consortium consists of Logica (UK), the National Oceanography Centre (UK), the Danish
Meteorological Institute (DK), University College Cork’s Coastal and Marine Research Centre (IRL)
and the Royal Dutch Meteorological Institute (NL).
Logica is now part of CGI. Founded in 1976, CGI is a global IT and business process services provider
delivering business consulting, systems integration and outsourcing services. With 72,000
professionals operating in 400 offices in 40 countries, CGI fosters local accountability for client
success while bringing global delivery capabilities to clients’ front doors. CGI applies a disciplined and
creative approach to achieve an industry-leading track record of on-time, on-budget projects and to
help clients leverage current investments while adopting new technology and business strategies. As
a result of this approach, our average client satisfaction score for the past 10 years has measured
consistently higher than 9 out of 10. We have a dedicated international Space and Satcoms business
with over 300 specialists and a long track record in delivering mission critical software systems
across the Space sector, and in particular for Navigation and GNSS systems. We have worked on
many ESA Earth Observation projects, including GlobWave, CCI, GECA, PALSAR and many others.
The National Oceanography Centre (NOC) is a wholly owned centre of the Natural Environment
Research Council (NERC). The NOC was formed by bringing together the NERC-managed activity at
Liverpool’s Proudman Oceanographic Laboratory and the National Oceanography Centre,
Southampton, creating the UK’s leading institution for sea level science, coastal and deep ocean
research and technology development. The NOC hosts both the National Tidal and Sea Level Facility,
and the Permanent Service for Mean Sea Level (since 1933), and contributes to the Storm Tide
Forecasting Service (STFS), developing operational tide-surge models that provide UK coastal flood
warning (in partnership with the Met Office and the Environment Agency). It has been at the
forefront in developing interfaces to data sources and information. NOC have been involved in ESA
funded projects such as COASTALT, GlobColour and GlobWave.
The Danish Meteorological Institute (DMI) is a public institute, providing meteorological,
oceanographic and related services for the people of the Kingdom of Denmark (Denmark, the Faroe
Islands and Greenland). DMI’s area of activity comprises forecasting and warning services as well as
continuous monitoring of weather, sea state, climate, and related environmental conditions in the
atmosphere, over land and in the sea. As such, it has national responsibility for carrying out storm
surge model forecasts and issuing warnings for Danish areas to the Danish coastal authorities and
the public in general. DMI is part of the Baltic Sea Operational Oceanographic System (BOOS) and
North West Shelf Operational Oceanographic System (NOOS). DMI play the role as the real-time insitu sea level centre for the BOOS and NOOS communities. In the MyOcean project DMI leads the
Baltic Model Forecasting Centre providing real time ocean forecasting for the Baltic Sea. DMI is part
26
of the High Resolution Local Area Modelling (HIRLAM) developing consortium within numerical
weather predictions. DMI is operationally running a number of numerical forecast models for
European and Arctic regions, alongside regional and large scale ocean models (HBM and HYCOM).
DMI is part of a collaboration developing a coupled atmosphere, ocean and sea ice climate model
(EC-Earth), whilst a high resolution coupled ocean and ice forecast model (HYCOM/CICE) is currently
being developed at the institute.
The Coastal and Marine Research Centre (CMRC) in University College Cork was established in 1994
to undertake research into coastal and marine resource management. It is part of the Environmental
Research Institute (ERI) and the Irish Maritime and Energy Resource Cluster (IMERC). Research and
consultancy in the CMRC is undertaken by staff with a range of specialist backgrounds, all of whom
work collaboratively in a project orientated environment. The Centre’s expertise and skill sets are
highly regarded both nationally and internationally. Fundamental and applied research in the CMRC
is organised according to five specialist areas of interest: marine geomatics; applied remote sensing
and GIS; marine and coastal governance; coastal processes and seabed mapping and marine ecology.
The CMRC works with data from a wide range of satellite EO instruments including MERIS, MODIS,
SAR and higher resolution optical datasets (e.g. Landsat, IRS, SPOT, and IKONOS) for land, coastal
and marine applications. It lies at the forefront of geomatics research with Europe and
internationally, with an ability to work with a variety of data in projects such as FP7 NETMAR, FP6
InterRisk and FP5 DISMAR. It has a track record of engaging end users and stakeholders in projects,
organising the CoastColour users’ workshop in 2008 and, was part of the organising committee for
the UK remote Sensing and Photogrammetric annual conference held at UCC in September 2010.
The Koninklijk Nederlands Meteorologisch Instituut, KNMI, (Royal Netherlands Meteorological
Institute) is a government agency operating under the responsibility of the Dutch Ministry of
Transport. It provides weather observations, weather forecasts and vital weather information, whilst
carrying out applied and fundamental research in support of its operational tasks and as a global
change research centre. Skilled and experienced groups, specialising in diverse topics such as
instrument development and electronic read-out, automation, computing, operations control and
quality control are employed within the institute, providing quality controlled, and cost effective
data acquisition and data processing services. As an operational meteorological data centre and
research institute in one, KNMI combines its international networks and collaborative projects in a
practical way. It is an active member of the World Meteorological Organisation (Geneva, CH), the
European Centre for Medium-range Weather Forecasts (Reading, UK) and the European
Organisation for the Exploitation of Meteorological Satellites (Darmstadt, G), and Eumetsat's Ocean
and Sea Ice Satellite Application Facility (SAF).
27
For more information on this product please contact
the GlobWave project at
www.globwave.org
For more information on eSurge please contact
Phillip Harwood, eSurge PM, at
[email protected]
28