PO.DAAC

The MetOp-A ASCAT Level 2 Ocean Surface Wind Vectors Optimized for Coastal Ocean dataset provides enhanced coastal coverage of ocean surface wind vectors over the global ice-free oceans at a sampling resolution of 12.5 km. The Ocean and Sea Ice Scatterometer Application Facility (OSI SAF) provides this experimental dataset in near-real-time (NRT) to supplement the existing Level 2 NRT ASCAT ocean wind vector datasets.

Figure Caption: "This image compares the wind vectors from the OSI SAF Operational 12.5 km ASCAT data set (left panel) with wind vectors from the OSI SAF Coastal 12.5 km ASCAT data set (right panel) in the western Gulf of Mexico region on the morning (ascending node) of 6 September 2010. Tropical Storm Hermine can be seen just west of the Yucatan Peninsula."

The enhanced coastal coverage is made possible using a spatial box filtering technique (rather than the traditional Hamming filter) which is used to generate a spatial average of the full resolution (Level 1) backscatter (Sigma-0) measurements within a 15 km radius of the wind vector cell center; prior to averaging, all land/ice contaminated Sigma-0 measurements are discarded. This unique sampling approach provides wind retrievals as close as approximately 15 km from the coast, which is in contrast to the static 35 (70) km land mask in the standard (i.e., operational) 12.5 (25) km dataset. The European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) OSI SAF production and quality monitoring information is available at the Royal Netherlands Meteorological Institute (KNMI) through www.knmi.nl/scatterometer/.

Cal/Val Summary (Verhoef and Stoffelen, 2011): The data characteristics of the coastal dataset in the open ocean (beyond 50 km from coastline) have been shown to be very similar to the operational 12.5 km dataset. Direct comparisons with the operational 12.5 km dataset show there is no wind speed bias and very low standard deviations in the vector components (0.39 m/s for U; 0.53 m/s for V). The coastal dataset, however, flags less points in the QC step than the operational dataset does, probably due to its reduced spatial smearing. Comparisons to in situ buoys (winds adjusted to 10 m and equivalent neutral) reveal a slight negative speed bias with the Coastal dataset in the coastal areas (-0.23 m/s) which is a bit less than the negative speed bias observed in open ocean (-0.29 m/s); the latter is very similar to the open-ocean negative speed bias observed with the operational 12.5 km dataset (-0.28 m/s). Standard deviation of the difference with respect to buoys is notably better in open-ocean regions than coastal regions for the U (i.e., Zonal) vector component (1.57 m/s for coastal; 1.48 m/s for open-ocean), where the difference in the V component is statistically identical for both open-ocean (1.61 m/s) and coastal buoys (1.60 m/s). This result was anticipated as coastal winds are known to be more variable than open-ocean winds and thus buoys do not represent the 30-km winds as well here, which leads to the slightly larger differences. The speed standard deviation of difference with buoys is very similar for collocated coastal and operational 12.5 km dataset in open ocean regions (resp. 1.44 and 1.43 m/s for U; 1.54 and 1.56 m/s for V). Spectral analysis shows very similar and consistent spectral slopes for the 12.5 km operational and coastal datasets with no significant flattening at the higher spatial frequencies, the latter of which indicates no significant white noise in both of the wind datasets.
Data: This experimental dataset is provided in the same netCDF, CF-compliant format as the operational ASCAT wind datasets. Due to the experimental nature of this dataset, its usage and any scientific results derived from its usage should be treated with caution and discretion. Unlike the operational wind datasets, the historical data record of this dataset is considerably recent, beginning on 18 August 2010. The spatial distribution of wind vector cells is identical to the operational 12.5 km dataset, with the exception of an additional number of valid wind vector retrievals in coastal regions, which comes as a bi-product of the spatial box filtering technique. All quality flags, metadata, and file naming conventions are consistent with the operational datasets. This data may be accessed through the PO.DAAC web portal. 
 

References:

• Verhoef, A. and A. Stoffelen (2011), Validation of ASCAT coastal winds, Tech. Report, version 1.3, SAF/OSI/CDOP/KNMI/TEC/RP/176, Sponsored by EUMETSAT.
• Verhoef, A., M. Portabella and A. Stoffelen (2011), High-resolution ASCAT scatterometer winds near the coast, submitted, IEEE Trans. Geosci. Remote Sens.

NOTICE: This dataset has been superseded. For the latest information on this data series please visit https://podaac.jpl.nasa.gov/Integrated_Multi-Mission_Ocean_AltimeterData.

The MEaSUREs Integrated Multi-Mission Ocean Altimeter Data for Climate Research v1.0 sea surface height (SSH) anomaly dataset is comprised of TOPEX/Poseidon (T/P), Jason-1, and OSTM/Jason-2 altimeter data integrated to form a single Sea Surface Height (SSH) Climate Data Record (CDR), merged into a single mean reference orbit.

The application of improved GSFC (STD1007) replacement orbits tie the multiple missions to a common well-defined geodetic reference frame (ITRF2008) providing the geodetic consistency requirement necessary for the demanding application of global and regional mean sea level determination.

Figure Caption: This plot shows the global SSH change through time with a trend superimposed on it, showing a global sea level rise of 3.1 mm/yr. The map has the regional sea level rise averaged over the time length of the dataset.

Altimeter data from the multi-mission Geophysical Data Records (GDRs) are interpolated to a common reference orbit facilitating direct time series analysis of the geo-referenced SSH.  The baseline v1.0 file is comprised of 660 10-day repeat cycles spanning from September 1992 to August 2010. As future OSTM cycles become available and are fully validated the direct access structure of the file allows new data to be appended.  All inter-mission biases have been applied to provide a seamless transition throughout the current 18+ year record.
Each SSH data record is a SSH time series at a specific geo-referenced location defined by revolution number and along-track index.  A 3-dimensional directory (rev#, index, cycle) permits direct access of individual locations at specific times (i.e. temporal and spatial sub-sampling). Auxiliary files provide time, mean sea surface reference, terrain type, bathymetry, proximity to coast, and SSH quality assessments (flag word) at each geo-referenced location.

The data are available in NetCDF format by individual cycles or the entire time series in a single file.

The Jason Microwave Radiometer (JMR) Enhanced Product provides an improved rain and sea ice flag valid for the radiometer data products and applies a recently developed coastal processing algorithm to provide wet tropospheric Path Delays (PD) near land, where the data were previously flagged as bad in the Geophysical Data Record (GDR).  The use of satellite altimetry for coastal studies has steadily increased over the past several years, but some of the science processing algorithms used to produce the data products on the GDR are either tuned for the open ocean or only valid in the open ocean and not valid in the coastal zone.  One such correction that is flagged as invalid in the coastal region is the radiometer derived wet tropospheric path delay correction.  Recent work by the Ocean Surface Topography Science Team (OSTST) and others has investigated this issue (Desportes et al. 2007; Mercier et al., 2008; Fernandes et al., 2010) and recently a new algorithm was developed to retrieve wet path delay in the coastal zone from the radiometer data and was first applied to the Advanced Microwave Radiometer (AMR) on Jason-2 (Brown 2010).  This so called Mixed-Pixel (MP) algorithm has been now applied to the Jason-1 radiometer and the data are available on the JMR Enhanced Product.

Algorithm Performance:

The performance of the algorithm is validated through detailed simulations and comparisons with the ECMWF model fields and ground based GPS derived path delays.  For the new JMR coastal product, the path delay errors are found to be unbiased relative to the near-by uncontaminated open ocean measurements and the RMS error is found to be less than 1.2 cm at 25 km from land and less than 1.5 cm at 10 km from land.  This represents a 2 cm reduction in error compared to the product available on the GDR.  The performance of the algorithm applied to the OSTM/Jason-2 Advance Microwave Radiometer (AMR) is even better because of the improved antenna design of the AMR compared to the JMR.  A comparison of the JMR and AMR path delays during the tandem mission in the coastal zone show they are unbiased with an RMS difference of less than 1cm, up to 10km from land.  An assessment of potential seasonal and regional biases revealed no significant geographically or temporally correlated systematic biases with the algorithm.

Data:

The time, latitude and longitude fields included in the enhanced product are identical to those on the Jason-1 GDR, and the number of records in each pass file is the same as on the GDR.  The enhanced wet troposphere path delay is identical to the rad_wet_tropo_corr field on the Jason-1 GDR greater than approximately 75 km from land.  Near the coast, an improved near land wet path delay algorithm is applied to improve the performance up to the coast.  The enhanced radiometer PD land flag that is included with the product is applicable to only the enhanced PD product.  The flag will be 2 (invalid) when the radiometer boresight is centered on the coastline and remain 2 when the radiometer is over land.  The flag will be 1 (valid) if the coastal processing has been applied and will be 0 (valid) if the open ocean processing has been applied.  The radiometer land flag on the GDR is valid only for the GDR PD and not the enhanced PD.  The sea ice flag indicates if the radiometer path delay measurement should be considered invalid due to sea ice contamination.  This flag is applicable to both the GDR PD and the new enhanced PD.  The sea ice flag is 0 if the measurement is valid and 1 if the measurement is invalid due to sea ice.  The rain ice flag indicates if the radiometer path delay measurement should be considered invalid due to rain contamination.  This flag is applicable to both the GDR PD and the new enhanced PD.  The rain flag is 0 if the measurement is valid and 1 if the measurement is invalid due to rain.
All questions/comments/feedback about the product should be directed to Shannon Brown (Shannon.T.Brown@jpl.nasa.gov).

References:

• F. Mercier, G. Dibarboure, C. Dufau, L. Carrere, P. Thibaut, E. Obligis, S. Labroue, M. Ablain, P. Sicard, G. Garcia, T. Moreau, L. Commien, N. Picot, J. Lambin, E. Bronner, A. Lombard, A. Cazenave, J. Bouffard, M. C. Gennero, F. Seyler, P. Kosuth, and N. Bercher, “Improved Jason-2 altimetry products for coastal zones and continental waters (PISTACH Project),” presented at the EUMETSAT Meteorological Satellite Conf., Darmstadt, Germany, 2008.
• C. Desportes, E. Obligis, and L. Eymard, “On the wet tropospheric correction for altimetry in coastal regions,” IEEE Trans. Geosci. Remote Sens., vol. 45, no. 7, pp. 2139–2149, Jul. 2007.
• M. Joana Fernandes, Clara Lázaro, Alexandra L. Nunes, Nelson Pires, Luísa Bastos, and Virgílio B. Mendes, “GNSS-Derived Path Delay: An Approach to Compute the Wet Tropospheric Correction for Coastal Altimetry,”  ,” IEEE Geosci. Remote Sens. Letters, vol. 7, no. 3, pp. 596-600, Jul. 2010.
• Brown, S., "A Novel Near-Land Radiometer Wet Path Delay Retrieval Algorithm:  Application to the Jason-2/OSTM Advanced Microwave Radiometer", IEEE Trans. Geosci. Remote Sens., vol. 48, no. 4, pp. 1986–1992, April, 2010.