1. PATMOS-x Reflectance Calibration and Reflectance Time-Series
Andrew Heidinger
NOAA/NESDIS/STAR
Madison, Wisconsin, USA
Christine Molling
UW/CIMSS
February 10, 2010
Joint GRWG and GDWG Meeting & 11 February 2010 at CNES, Toulouse (France)

2. Outline PATMOS-x Introduction Calibration Methodology
Validation using PATMOS-x Time Series
Future Work
I basically have two points make – bullets 3 and 4. Note, I am not speaking about the NGST baseline algorithms. I am speaking in terms of the information provided by the VIIRS IR bands which do ultimately constrain any algorithm. To make these points, I’ll need to delve a bit into radiative transfer, show an euqation or two – please forgive me.

3. What is the AVHRR?
Provides 4 km and 1 km observations roughly 4x - 6x per day.
Spectral information: 0.63, 0.86, 1.6 / 3.75, 10.8 and 12 m
For clouds we can estimate cloud-top phase, opacity, cloud-top height, cloud-top microphysics.
AVHRR does not do well in snow covered regions compared to MODIS and can not compete with sounders on sensitivity to thin cirrus.
Images below demonstrate the spectral information or cloud studies.
0.63 / 0.86 / 10.8 mm False Color
0.63 / 3.75 / 10.8 mm False Color

4. What is PATMOS-x? is a global cloud climatology
AVHRR Pathfinder Atmospheres – Extended
is a unique long-term satellite-based data set (almost 31 years)
includes over 30 parameters including radiance, cloud and surface products in a HDF and NETCDF format.
consists of twice daily fields from all AVHRR/2 and AVHRR/3 data from (AVHRR/1 available since 1978)
Use the spectral information and spatial resolution offered by AVHRR to expand the knowledge available from existing climatologies (ISCCP and HIRS)
Done in collaboration with the AVHRR reprocessing effort in CM-SAF within the SCOPE-CM
TIROS-N AVHRR November 1978

5. Motivation
AVHRR Prelaunch is woefully inadequate (no onboard calibration).
Existing long-term AVHRR reflectance calibration parameter data sets contain some large artifacts that need to be addressed for CDR generation.
Time series below show a region over Greenland from NOAA-12 and NOAA-15.

6. AVHRR Reflectance Calibration Methodology
Our new PATMOS-x AVHRR reflectance calibration is based on 4 different sources of data. Method has been applied to all sensors.
MODIS to AVHRR SNO ( )
AVHRR to AVHRR SNO ( )
Libyan Desert MODIS-derived Reference
DOME-C MODIS-derived Reference
SNO
Libya
DOME-C
Libya - seen in every month by afternoon and mid-morning satellites
DOME-c - seen once a year by every satellite.
Images courtesy of CEOS and STAR websites

7. PATMOS-x Calibration Results for NOAA-7
Blue Points – Calibration slopes generated from MODIS to AVHRR simultaneous nadir overpasses.
Black Points – Calibration slopes generated from AVHRR observations of DOME-C (Antarctica) using a MODIS-derived reflectance model.
Red Points – Calibration slopes generated from AVHRR observations of the Libyan Desert using a MODIS-derived reflectance model.
Green Points – Calibrations slopes from other AVHRR sensors transferred using AVHRR to AVHRR simultaneous nadir overpasses.

8. PATMOS-x Calibration Results for NOAA-18
Blue Points – Calibration slopes generated from MODIS to AVHRR simultaneous nadir overpasses.
Black Points – Calibration slopes generated from AVHRR observations of DOME-C (Antarctica) using a MODIS-derived reflectance model.
Red Points – Calibration slopes generated from AVHRR observations of the Libyan Desert using a MODIS-derived reflectance model.
Green Points – Calibrations slopes from other AVHRR sensors transferred using AVHRR to AVHRR simultaneous nadir overpasses.

9. Verification of Time Series Stability and Inter-satellite Consistency
To demonstrate the consistency of the long-term PATMOS-x reflectance time series, we applied the new calibration to entire record and analyzed results over DOME-C.
While DOME-C was used in the calibration procedure, its was one of four contributing methods.
Time-series shows temporal stability and the expected variation with sun-angle.
DOME-C
NOAA-7 is expected to be darker due to its different and broader SRF.

10. Verification of Time Series Stability and Inter-satellite Consistency
DOME-C December Statistics (Channel-1 Only)

11. Comparison of Reflectance-based CDRs Among Spectrally Similar Sensors
Instantaneous Comparison of LWP for near-nadir simultaneous overpass from MODIS and AVHRR
Results include only those pixels where both MYD06 and PATMOS-x provide data

12. Comparison of Reflectance-based CDRs Among Spectrally Similar Sensors
PATMOS-x applies physically similar algorithms to those in MYD06 (same surface reflectance, rayleigh correction …)
Time series below show mean LWP computed using the pixel-level results coupled with the reflectance distribution to “recover” a truer estimate of the grid-averaged LWP. This technique removes differences due to pixel-level filtering.
Good agreement in EOS/AQUA era (2003 – 2009)
Period with 1.6 micron channel on ( )

13. Comparison of Reflectance-based CDRs Among Spectrally Different Sensors
Comparison of similar products with fundamentally different sensors allows an additional verification of long-term reflectance calibration performance. In the microwave comparison below, the AVHRR reflectance calibration is only part of the picture. AVHRR SRF knowledge and physical differences in the quantities are important as well.
Figure provided by Ralf Bennartz UW/AOS

14. Conclusions
The PATMOS-x reflectance calibration has achieved agreement with MODIS to within 2-3%. (MODIS itself has an error of approximately 2%).
Largest remaining source of uncertainty is the spectral surface reflectance of the DOME-C and Libyan Targets.
Use of Simultaneous Nadir Overpass between AVHRR + MODIS and AVHRR + AVHRR offer key constraints.
Inclusion of the CEOS-sanctioned DOME-C site offers substantial improvements because it allows for afternoon and morning orbiters to view the same calibration target. Deserts are only viewed by afternoon sensors.
Analysis of PATMOS-x time-series is a critical component in verifying the impact of the reflectance calibration.
Future work will include including the 1.6 micron channel using MODIS/AVHRR SNO
Future work will also include incorporation of CEOS and GSICS advancements (more sites, more sensor SNOs) in collaboration with the larger NESDIS team.

15. Thank You