Global hourly drifter locations and velocities with formal error bars
This page contains instructions to download global hourly location and velocities from the GDP surface drifters as described in Elipot et al. (2016), "A global surface drifter dataset at hourly resolution", J. Geophys. Res. Oceans,121,doi:10.1002/2016JC011716. This study reports the methods used to produce location and velocity estimates along the trajectories of GDP surface drifters, tracked by either Argos or by GPS.
Version 1.01, released 19 January 2017
Release notes for version 1.01:
- The interpolation methods for drifter trajectories tracked by the Argos system and by the Global Positioning Sytem are the same as for version 1.00, described in Elipot et al. (2016).
- This 1.01 update adds newer data to the previous version, one year of quality-controlled data up to 30 June 2016 for Argos-tracked drifters, and more than one year of GPS-tracked data, up to 1 October 2016. In addition, we have added data before 1 September 2005 (the start date of v1.00) by considering all GPS-tracked drifters since the beginning of the GDP, and all Argos-tracked drifter data since the beginning of the GDP, when the average of the uneven sampling intervals per trajectory is less than or equal to 3 hours.
- In the previous version of the dataset (v1.00), some drifters were erroneously categorized as being either Argos-tracked or GPS-tracked. This has now been corrected. In addition, when drifters were tracked by both systems, only the GPS trajectory is now contained in the dataset. Previously, there were repeats when merging the Argos and GPS files.
- The position and velocity estimates from GPS-tracked data are now provided with formal 95% confidence intervals from the weighted least squares solutions of the estimation, assuming an observational error of 22 m for GPS positions. We diagnosed this observational error from the overall distribution of the GPS data received at the Data Assembly Center of the GDP. In order to obtain acceptable confidence intervals, the LOWESS processing of the GPS positions has been implemented to make sure that at least 3 data points are used for each estimation. This differs from v1.00 of the dataset for which only 2 data points were sometimes used.
- The 1.01 dataset now totals over 117.4 million estimates of position and velocity with confidence intervals from 12,287 Argos-tracked drifter trajectories, and over 5.6 million estimates of position and velocity with confidence intervals from 985 GPS-tracked drifter trajectories.
- Simplified versions of the Matlab codes used to generate this dataset are available through a GitHub repository
The data are provided via anonymous ftp as ASCII files (compiled in a compressed tar file) and as NetCDF files. The data from Argos-tracked drifters are in the files containing the acronym WMLE (Weighted Maximum Likelihood Estimation), referring to the interpolation method used for Argos fixes. The data from GPS-tracked drifters are in the files containing the acronym GPS. For ASCII files, please refer to the accompanying README files. For the NetCDF files please refer to the metadata within these files.
Instructions for obtaining all files via ftp:
unix> ftp ftp.aoml.noaa.gov
ftp> cd phod/pub/buoydata/hourly_product/
ftp> prompt off
ftp> mget *
Or go directly to :
ftp://ftp.aoml.noaa.gov/pub/phod/buoydata/hourly_product and download the files you need.
List of files:
driftertrajGPS_1.01.nc - All GPS-tracked drifters in NetCDF format
driftertrajWMLE_1.01.nc - All Argos-tracked drifters in NetCDF format
driftertrajGPS_1.01.tar.gz - All GPS-tracked drifters in ASCII format
driftertrajWMLE_1.01.tar.gz - All Argos-tracked drifters in ASCII format
README_driftertrajGPS_1.01.txt - Readme file for GPS-tracked ASCII files
README_driftertrajWMLE_1.01.txt - Readme file for Argos-tracked ASCII files
Queries, questions, comments, and the reporting of issues with these products should be directed to Drs. Rick Lumpkin, Shane Elipot, or Renellys Perez. If you would like e-mail notifications of updates to the hourly data set, send your request to Rick.Lumpkin "at" noaa.gov.
If you use this dataset in your studies and publications, please cite:
Elipot, S., R. Lumpkin, R. C. Perez, J. M. Lilly, J. J. Early, and A. M. Sykulski (2016), "A global surface drifter dataset at hourly resolution", J. Geophys. Res. Oceans, 121, doi:10.1002/2016JC011716.
Preliminary analyses of the drifter velocity variance showed that this dataset is a valuable new tool for the study of relatively small-scale and high-frequency, oceanic processes. In particular, velocity rotary spectra suggest that high frequency tidal and internal wave motions are detectable globally. Below is a figure adapted from Elipot et al. (2016) which illustrate that the new dataset exhibits more velocity variance compared to the 6-hourly velocity product.
Figure caption: (a) zonally averaged drifter rotary velocity spectra in 1 degree latitude bins from 2048-hour (~85 days) trajectory segments at 1-hour resolution, overlapping by 50 percent, from the new global hourly product. (b) same as in (a) but zoomed in for frequencies in the range +/- 3.2 cpd. The black dashed curve indicates the inertial frequency (-f/(2*pi) cpd) and the white dashed curve the Coriolis frequency (f/(2*pi) cpd). Note that the color scales indicate different ranges of values and are both saturated. (c) same as in (b) but calculated from the post-2005 6-hourly Kriged global product for the same trajectories.