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 2 nd CORP Science Symposium – Madison Wisconsin, July 13, 2005

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. GVI-xPolar WindsSST PATMOS-x

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) 1. AVHRR Provides enough spectral information for several applications 2. AVHRR provides enough spatial resolution (1 or 4 km) to resolve many atmospheric and surface features 3. Combined with its long data record ( ) make the 5-channel AVHRR data- set the best we have for decadal studies for many key climate parameters.

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

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.

My Reflectance Calibration Background Starting working on AVHRR in 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, 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: /2001JD Fred Wu (formerly of CIMSS) was later hired for AVHRR calibration and I am now only interested in the calibrations for climate work.

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: bration/intercal/

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

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.

Example of SNO’s for one month of PATMOS-x data (July 1992) 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. 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.

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

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

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.

Comparison of Ch 1 Equations for NOAA-7,9,11,14, RCS = Rao, Chen and SullivanVS = Vermote and El Saleous TC = Tahnk and CoakleyWU = Fred Wu (Operational NOAA)

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

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

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).