NSIPP AVHRR Pathfinder and Erosion Global 9 km SST Climatology (Casey,
Cornillon)
Summary:
A high resolution SST climatology has been generated from daily 9 km Advanced
Very High Resolution Radiometer (AVHRR)
Pathfinder SST satellite data through a pixel-by-pixel averaging approach.
This guide includes descriptive information on PO.DAAC product 112 under
the product name NSIPP AVHRR Pathfinder Global 9 km SST Climatology
(Casey, Cornillon).
Data Set Introduction:
Using the daily 9 km Pathfinder SST time series from 1985-1997, pixel-by-pixel
averaging was applied to create 9 km climatologies with 5 day (pentad)
and monthly temporal intervals. A cloud erosion filter was applied to the
SST data before averaging.
See the README
file that accompanies the data on the PO.DAAC ftp site for more information.
Objective/Purpose:
To create a high resolution SST climatology suitable for climate studies.
Summary of Parameters:
Sea surface temperature
Discussion:
Time series of satellite SST are of sufficient length and accuracy for
use in the construction of a global climatology.
The high temporal and spatial resolution of a satellite-based climatology
is an inherent advantage over lower
resolution traditional climatologies that depend on in situ or blended
in situ and satellite data.
Related Data Sets:
PO.DAAC product 111: AVHRR Pathfinder Global 9 km SST Climatology (JPL)
2. Investigator(s):
Investigator(s) Name and Title:
This climatology was produced by Dr. Ken Casey.
casey@mohawk.gsfc.nasa.gov
Title of Investigation:
Satellite-based SST Climatology
Contact Information:
Edward M. Armstrong
Jet Propulsion Laboratory
California Institute of Technology
Tel: 818 393-6710
Fax 818 393-2718
email: ed@seanet.jpl.nasa.gov
3. Theory of Measurements:
See documentation on Pathfinder
AVHRR SST and MCSST
for discussion and references of theory of measurements
Each of the NOAA polar-orbiting satellites has carried an AVHRR as one
of three sensors aboard the spacecraft. AVHRR was designed for multispectral
investigations of meteorological, oceanographic, and hydrologic parameters,
measuring emitted and reflected radiance in four or five spectral bands,
spanning the visible portion of the spectrum to the thermal infrared.
Key Variables:
The sensor measures emitted and reflected radiation from Earth in two visible
channels and three infrared channels.
Principles of Operation:
Each AVHRR scan views Earth for 51.282 milliseconds, during which time
each channel of the analog data output is digitized. Scans occur at the
rate of 6 per second, and the sampling rate of the AVHRR sensors is 39,936
samples per second per channel. During a scan, the detectors view an internal
target, cold space, and the external scene. The temperature of the internal
target is monitored, and space is assumed to have a black- body temperature
of 3K. In this way, a simple two-point linear calibration is done internally
(Schwalb, 1978). The nonlinear modification to this calibration is achieved
at the time of post processing, and takes into account sensor nonlinearities,
measurement of internal target temperature, calculation of target radiance,
internal reflections and emissions, etc.
Sensor/Instrument Measurement Geometry:
The AVHRR has a cross-track scanning system which use an elliptical beryllium
mirror rotating at 360 RPM about an axis parallel to the Earth. The 110.8°
cross-track scan equates to a swath width of about 2700 km. This swath
width is greater than the 25.3° separation between successive orbital
tracks, and provides overlapping coverage. Coverage is global, twice daily,
at an instantaneous field of view (IFOV) of ~1.4 milliradians, giving a
ground field of view of ~1.1 km at nadir for a nominal altitude of 833
km.
Manufacturer of Sensor/Instrument:
ITT Aerospace
Calibration:
Specifications:
Channels 1 and 2 are calibrated to produce at-satellite radiances using
a time dependent correction which accounts for sensor
degradation and intercalibrates among the satellites. Channels 4 and 5
are calibrated using a non-linear function based on
the internal calibration targets, baseplate temperatures, instrument dependent
response curves, and NOAA-provided gains and offsets. Channel
3 is calibrated using the gains and offsets in the GAC data record. The
thermal channels are then converted to equivalent
brightness temperatures using a lookup table based on the inverse Planck
function convolved with the
instrument response.
Tolerance:
The instrument is designed to maintain a constant operating
temperature for the IR detectors and provide a signal-to-noise ratio (SNR)
of 3:1 at 0.5% albedo.
Frequency of Calibration:
The thermal infrared channels are calibrated in flight every scan line
by using a view of a stable blackbody and space as a reference. Channels
1 and 2 have no onboard calibration capabilities, however, they are
calibrated before launch.
Other Calibration Information:
In an effort to develop a consistent set of in-flight calibration algorithms
for channels 4 and 5, a radiance-based correction procedure was developed
to account for the non-linear response characteristics of the detectors.
This procedure resulted in a single correction algorithm applicable over
the entire range of AVHRR operating temperatures, representing a significant
improvement over the use of myriad tables to look up temperature corrections.
5. Data Acquisition Methods:
Full resolution AVHRR data are continuously transmitted and recorded in
High Resolution Picture Transmission (HRPT) format. The Global Area Coverage
(GAC) data are subsampled to approximately 4 km IFOV, recorded internally,
and downlinked daily. These data are the starting point for the Pathfinder
SST processing. To produce the NOAA GAC Level-1B data, the Level-0 (unprocessed)
instrument data are quality controlled, assembled into discrete data sets,
and have calibration and Earth location information appended. Data are
then stored as full orbits consisting of both ascending (daytime) and descending
(nighttime) data.
6. Observations:
Data Notes:
Not Applicable
Field Notes:
A buoy matchup data set is used in computing the calibration coefficients
used to calculate sea surface temperature.
7. Data Description:
Spatial Characteristics:
Spatial Coverage:
Global
Spatial Coverage Map:
Spatial Resolution:
9 km
Projection:
Equal-Angle.
Grid Description:
The grid is identical to the daily 9 km Pathfinder SST imagery (4096x2048
map).
Temporal Characteristics:
Temporal Coverage:
Daily Pathfinder data from 1985-1997 were used as the baseline for
this climatology.
Temporal Coverage Map:
Temporal Resolution:
Five-day (pentad) or monthly. There are a total of 73 climatological pentads.
Data Characteristics:
Parameter/Variable:
Sea Surface Temperature
Variable Description/Definition:
SST - temperature of the sea surface.
Unit of Measurement:
Pixel value with slope of 0.15 and y-intercept of -3.0 to convert to °C.
Data Source:
AVHRR
Data Range:
The data range is greater than -3.0°C and less than 35°C.
Sample Data Record:
Not Available
8. Data Organization:
Data Granularity:
A basic granule is the entire climatology file containing all 73 pentads.
A general description of data granularity as it applies to the EDG appears
in the EOSDIS
Glossary.
Data Format:
The data are stored as flat binary as byte data types (chars).
9. Data Manipulations:
Formulae:
Derivation Techniques and Algorithms:
See documentation for the Pathfinder SST products for a complete description
of the SST processing scheme.
Data Processing Sequence:
Processing Steps:
See documentation for the Pathfinder SST products for a complete description
of the SST processing steps.
Processing Changes:
None
Calculations:
Special Corrections/Adjustments:
See documentation for the Pathfinder SST products for a complete description
of the SST processing steps.
Calculated Variables:
Sea surface temperature
Graphs and Plots:
None available
10. Errors:
Sources of Error:
One of the greatest limitations is the obstruction by clouds in the field
of view. Other sources of errors include water vapor and other atmospheric
gases, and aerosols as well as water surface characteristics.
Quality Assessment:
Data Validation by Source:
See Pathfinder SST documentation for a description of the quality assessment
for the daily Pathfinder SST product used in the climatology.
The climatology was "performance tested" using the methodology of Casey and Cornillon (1999).
11. Notes:
Limitations of the Data:
Climatological SST in the polar regions ( > 60N and > 60S) are suspect
due to possible sea ice contamination.
Known Problems with the Data:
None
Usage Guidance:
See the README
file that accompanies the data on the PO.DAAC ftp site.
Any Other Relevant Information about the Study:
None
12. Application of the Data Set:
Global climate studies, studies of ocean circulation and temperature trends.
13. Future Modifications and Plans:
None planned.
14. Software:
Software Description:
On the PO.DAAC ftp site the user will find sample C
and MATLAB programs that can be used as templates
for reading the data.
Software Access:
The user must compile the C software or have MATLAB installed on a local
computer.
15. Data Access:
Contact Information:
User Services Office
Physical Oceanography Distributed Active Archive Center (PO.DAAC)
Jet Propulsion Laboratory (JPL)