Real-time, global, sea surface temperature (RTG_SST_HR) experimental analysis

NOTE for frequent visitors: Go to Changes to the RTG_SST_HR analysis for information on changes in the operational 1/12 deg. Real-Time Global SST analysis.

A daily, high-resolution, real-time, global, sea surface temperature (RTG_SST) analysis has been developed at the National Centers for Environmental Prediction/Marine Modeling and Analysis Branch (NCEP / MMAB). The analysis was implemented in the NCEP parallel production suite 16 August 2005. It became fully operational on September 27, 2005.

The daily sea surface temperature product is produced on a twelfth-degree (latitude, longitude) grid, with a two-dimensional variational interpolation analysis of the most recent 24-hours buoy and ship data, satellite-retrieved SST data, and SST's derived from satellite-observed sea-ice coverage. The algorithm employs the following data-handling and analysis techniques:

Satellite retrieved SST values are averaged within 1/12 ^o grid boxes with day and night 'superobs' created separately for each satellite;

Bias calculation and removal, for satellite retrieved SST, is the technique employed in the 7-day Reynolds-Smith climatological analysis;

Currently, the satellite SST retrievals are generated by a physically-based algorithm from the Joint Center for Satellite Data Assimilation. Retrievals are from NOAA-17 and NOAA-18 AVHRR data;

SST reports from individual ships and buoys are separately averaged within grid boxes;

The first-guess is the prior (un-smoothed) analysis with one-day's climate adjustment added;

Late-arriving data which did not make it into the previous SST analysis are accepted if they are less than 36 hours old;

Surface temperature is calculated for water where the ice cover exceeds 50%, using salinity climatology in Millero's formula for the freezing point of salt water:

t(S) = -0.0575 S + 0.0017 S^3/2 - 0.0002 S²,

with S in psu.

An inhomogeneous correlation-scale-parameter l, for the correlation function: exp(-d²/l²) , is calculated from a climatological temperature gradient, as

l = min ( 450 , max( 2.25 / |grad T| , 100 )),

with d and l in kilometers. "grad T" is in ^oC / km

Evaluations of the analysis products have shown it to produce realistically tight gradients in the Gulf Stream regions of the Atlantic and the Kuroshio region of the Pacific, and to be in close agreement with SST reports from moored buoys in both oceans. Also, it has been shown to properly depict the wintertime colder shelf water -- a feature critical in getting an accurate model prediction for coastal winter storms.

The evaluation of the RTG_SST_HR analysis is available by clicking the link to the RTG_SST_HR Verification page.

GRIB files for the global SST product are available at the following FTP sites:

ftp://ftpprd.ncep.noaa.gov/pub/data/nccf/com/gfs/prod (most recent day)

Inspect subdirectories of the form "sst.YYYYMMDD",
where "YYYYMMDD" represents an 8-character date string, e.g., "20020717".
The RTG_SST_HR analysis is contained in the file "rtgssthr_grb_0.083".
This file is scheduled to disappear from this FTP site in September 2008.
Its replacement is in GRIB2 format, in the file "rtgssthr_grb_0.083.grib2".

ftp://polar.ncep.noaa.gov/pub/sst/ophi (2-7 days ago)
ftp://polar.ncep.noaa.gov/pub/history/sst/ophi (2-366 days ago)

When using these last two links, look for files of the form "rtg_sst_hr_grb_0.083.YYYYMMDD"
where "YYYYMMDD" represents an 8-character date string, e.g., "20020717".

For information on reading and unpacking GRIB files, go to the MMAB Tools page

The RTG_SST_HR analysis is done over all ocean areas and the Great Lakes. There is no analysis over land. The land values are filled by a Cressman interpolation to produce a complete grid for possible interpolation to other grids. The ocean and land areas are defined by a land sea mask. This data set is an ASCII file, ls.dat, which may be found at the FTP site:

ftp://polar.ncep.noaa.gov/pub/history/sst/ophi/lsmask

The spatial grid is defined identically to the grid for the SST arrays, with indices starting just east of the Greenwich Meridian and near the South Pole. The values in ls.dat are set to 0 over the ocean and 3 over land. It can be read by the following Fortran code:

DIMENSION LS(4320,2160)
OPEN (11,FILE='ls.dat')

C
C Read in land sea tags (0 for ocean; 3 for land)
C

READ (11,'(80I1)') LS
STOP
END

For additional information about data-management and analysis techniques, contact William.Gemmill@noaa.gov.

For information about the run cycle and digital data format, contact Bert.Katz@noaa.gov.