AVHRR Visible Band Calibration / Intercalibration (for Climate Studies)

1. AVHRR Visible Band Calibration / Intercalibration (for Climate Studies) Andrew Heidinger and Michael Pavolonis* Changyong Cao, Aleksandar Jelenak, Jerry Sullivan, Fred Wu NOAA/NESDIS Office of Research and Applications *Cooperative Institute for Meteorological Satellite Studies (CIMSS) Madison, Wisconsin 2nd CORP Science Symposium – Madison Wisconsin, July 13, 2005

2. Background • This effort is part of much larger effort within ORA. ORA has funded the AVHRR reprocessing as a pilot project for Scientific Data Stewardship (SDS) – thank you! • We hope to continue as a funded project under the NESDIS SDS initiative. • ORA now has the entire GAC archive (1979-present; 32 TB) from CLASS. • For the first time, we have demonstrated the ability to reprocess this data within ORA. So far we have generated time-series of GVI-x and PATMOS-x. Polar Winds and SST projects are also underway. • A large part of this effort is to improve the quality of the AVHRR data and this involves both reflectance and thermal calibration and geolocation. PATMOS-x GVI-x Polar Winds SST

3. Why use the The AVHRR for Climate Studies The Advanced Very High Resolution Radiometer (AVHRR) was launched in the 1979 for non-quantitative cloud imagery and SST. It flies on the NOAA Polar Orbiting Satellites (POES) 2. AVHRR provides enough spatial resolution (1 or 4 km) to resolve many atmospheric and surface features 1. AVHRR Provides enough spectral information for several applications 3. Combined with its long data record (1981-2012) make the 5-channel AVHRR data-set the best we have for decadal studies for many key climate parameters.

4. Why Improve the AVHRR Reflectance Calibration? • Without onboard calibration, the prelaunch reflectance calibrations can be many % in error. Accurate reflectances are critical for cloud, aerosol and vegetation climate records. • Analysis of existing post-launch calibrations (esp those use at NESDIS) has shown room for improvement for climate use • The calibration of many of the early and the morning orbiting AVHRRs has received little attention • New sensors with onboard calibration and new processing techniques (SNO) warrant a new look at this issue. • There is still disagreement in the AVHRR reflectance calibrations from different techniques (see right) D.R. Doelling, 2001: Proceedings AMS 11th Conference on Satellite Meteorology and Oceanography, Madison, Wisconsin, October 15 – 18, pp. 614-617

5. Goals of this Work • Improve upon the existing AVHRR reflectance calibrations for ORA climate work • Derive a new self-consistent set of calibrations for whole series of AVHRR data (NOAA-6,7,8,9,10,11,12,14,15,16,17,18,…) that allows for meaningful climate work. • Try and build a consensus calibration through multiple collaborations and transfer these new calibrations to the community.

6. My Reflectance Calibration Background • Starting working on AVHRR in 1999. • Collaborated with Nagaraja Rao and Jerry Sullivan on extending N. Rao’s Libyan Desert Technique to NOAA-12 (a morning satellite). • Heidinger, A. K., J. T. Sullivan and N. Rao, 2003: Calibration of visible and near-infrared channels of the NOAA-12 AVHRR using time-series of observations over deserts.I.J.R.S., 24, 3635-3649. • After death of N. Rao, Mike Weinreb asked that I develop initial post-launch calibration for NOAA-16 (a dual gain instrument). Worked with C. Cao on the first SNO’s and wrote the following paper. • Heidinger, A. K., C. Cao, and J. T. Sullivan, Using Moderate Resolution Imaging Spectrometer (MODIS) to calibrate advanced very high resolution radiometer reflectance channels, J. Geophys. Res., 107(D23), 4702, doi:10.1029/2001JD002035. • Fred Wu (formerly of CIMSS) was later hired for AVHRR calibration and I am now only interested in the calibrations for climate work.

7. Proposed Methodology – Simultaneous Nadir Observations (SNO) • Our goal is to use Simultaneous Nadir Observations (SNO) to improve both the relative and absolute calibration. This method provides new information and constraints not used in past studies, • Polar orbiting satellites intersect each other at high latitudes. This occurs for satellites even in very different orbits. • We have primarily analyzed MODIS and AVHRR SNO data though could do VIRS, ATSR and geo imagers. • We also study AVHRR to AVHRR SNO’s to fix the relative calibration from one instrument to another. Taken from Changyong Cao: http://www.orbit.nesdis.noaa.gov/smcd/spb/calibration/intercal/

8. Example Imagery from a SNO Sensor #1 data projected on to Sensor #2 strip. These points comprise the SNO ± 5° Nadir Strip from Sensor #1 ± 5° Nadir strip from Sensor #2

9. Example of one July’s SNO’s for ch1 and ch2 of TERRA and NOAA-16 (note y-axis should be ch1 not ch2 on left-hand plot) SNO’s from MODIS and AVHRR allow us to transfer MODIS’s calibration to the AVHRR directly.

10. Example of SNO’s for one month of PATMOS-x data (July 1992) For July 1992, NOAA-11 and NOAA-12 gave 78 grid-cells that met SNO criteria. Note dark counts are removed so line should pass through origin. This data provides a constraint on the ratio of NOAA-11 to NOAA-12 calibration slopes. Does not provide any information on absolute calibration by itself.

11. ORA (C. Cao and others) has automated SNO’s from AVHRR, AMSU and HIRS and offers a real-time monitoring capability.

12. A New Libyan Desert Reference for pre-MODIS Calibration In addition to SNO data (AVHRR/MODIS and AVHRR/AVHRR), we also employ a New Libyan Desert Reflectance Reference value to provide an estimate of the absolute calibration for the afternoon satellites. • This new Libyan Reflectance Reference was constructed using the following steps: • Selection of Stable Target (Used same as N. Rao) • Acquisition of MODIS data over Target • BRDF modeling • Spectral Adjustment due to Water Vapor (MODIS + MODTRAN) • Spectral Adjustment from Hyperion data This method gives us the absolute calibration for the pre-MODIS AVHRR data. MODIS TRUE COLOR

13. Ch1 Calibration Slopes from All Satellites and All Methods • Reflectance = Calibration Slope x ( Count – Dark Count) • Note the relative agreement between the calibration slopes derived from different methods.

14. Comparison of Ch 1 Equations for NOAA-7,9,11,14,16 14 9 7 11 16 RCS = Rao, Chen and Sullivan VS = Vermote and El Saleous TC = Tahnk and Coakley WU = Fred Wu (Operational NOAA)

15. Testing the New Reflectance Calibrations • Because the ORA effort also involves generation of climate products, we can test the impact of new calibrations on climate records. • Aerosol optical depths should test the consistency of the low end Testing Long Term Consistency Testing Absolute Accuracy AOT Tables from A. Ignatov

16. Testing the Consistency for bright scenes • Greenland’s Reflectance should be stable and tests the high end stability • Need to compare to absolute standards published by Tahnk and Coakley.

17. Conclusions • ORA is undertaking activity to improve the AVHRR 1b data. An efforts are underway to communicate these improvements to the wider community through metadata / ancillary files. • We are seeking a consensus calibration and are seeking collaborators currently working with El Saleous and Vermote from NASA. • Using the SNO technique, we can transfer MODIS’s calibration directly to the AVHRR • SNO’s also provide a direct method to ensure reflectance continuity across AVHRR transitions without assumptions about vicarious targets. • SNO’s coupled with a new MODIS-based vicarious desert calibration target appear to have produced an AVHRR reflectance calibration that is consistent for all AVHRR’s (and is consistent with MODIS).