Satellite altimeter data provide estimates of significant wave height (Hs) and near-surface (10 m) wind speed (U10) at 1 Hz ,i.e. at 6 - 7 km intervals along the altimeter tracks; together with other parameters and flags which can be used to validate the Hs and U10 values.
We have validated the data (off-line products, not 'fast delivery') and applied calibrations derived from comparisons with USNODC buoy measurements; and have then taken the median values of Hs and U10 from each altimeter transect of a 2° latitude by 2° longitude bin - given at least 5 1 Hz values in the bin.
We have estimated the zero-upcross wave period (Tz) based on algorithms given by Mackay E B L, Retzler C H, Challenor P G and Gommenginger C P (2008) A parametric model for ocean wave period from Ku band altimeter data, J. Geophys. Res., 113, C03029, doi:10.1029/2007JC004438. These algorithms were developed using US NDBC open ocean buoys and have not been tested at other locations.
The median values are stored by altimeter, month and year on a database.
The 1 Hz data from one transect across a bin are highly correlated, so little information is lost by taking the median. This also reduces the impact of any 'outlier' in the 1 Hz data which passed the, inevitably imperfect, validation process.
This web site enables you to extract the median Hs, U10 and Tz from any 2° bin (0°-2°, 2°-4° etc, specified by the latitude and longitude of the middle of the bin).
Histograms of Hs, U10 and Tz are plotted to the screen, and can be downloaded. Monthly statistics and the median values are output to files which can either be viewed on screen or (by 'right clicking') down loaded. The first line of data output is 'N 10' where N is the number of records and 10 the number of parameters. The second line is a header describing the ten parameters:
"sat_id lat. long. year month day hour Hs(cal) U10(cal) Tz"
The satellite identity (sat_id) code is:
01: Geosat
02: Geosat Follow-On
10: ERS-1
11: ERs-2
12: Envisat (Ku band)
15: Topex
17: Jason-1 (Ku band)
The output is in chronological order.
If you want data for one bin from all altimeters, all months and all years, the extraction process will take less than 1 minute.
We have done our best using automated validation procedures to remove bad data, but cannot guarantee any individual median values. (Validation is particularly difficult when the altimeter encounters very heavy rain such as found in typhoons and when within a few km of land, which can distort the shape of the radar return pulse.) However, values are generally well within the altimeter design specification of 0.5 m and 1 m/s - except that wind speeds above about 20 m/s are of dubious accuracy.
The database will be expanded as further data are processed. At present it contains:
Geosat Nov.1986 - Sept.1989
GFO Jan.2000 - Jan.2004
ERS-1 Jan.- Dec.1992, Jan.- Oct.1993, June 1994 - March 1995
ERS-2 May 1995 - Sept.20076 [Limited after June 2003 to coverage within sight of receiving stations.]
Envisat Oct.2002 - Sept.2007
Topex Jan.1993 - Sept.2005
Jason-1 Jan.2002 - Sept.2007.
If you would like further information or have any problem accessing the database, please e-mail me at d.carter@satoc.eu
David Carter
February 2009.