Using AIRS to Assess MODIS Radiances Dave Tobin

Using AIRS to Assess MODIS Radiances Dave Tobin CIMSS / SSEC / UW-Madison NOAA Cooperative Research Program Second Annual Science Symposium 13 July 2005

Introduction • Overall Goal: – Create a long term record of well calibrated and characterized satellite radiances – Emphasis on high spectral resolution (AIRS, TES, IASI, Cr. IS, …) but also including broadband sensors (MODIS, VIIRS, GOES, …) • Post-launch AIRS Radiance Cal/Val activities at CIMSS/SSEC – – Noise characterization Early radiance comparisons with GEOs Spatial co-registration Spectral Radiance validation with aircraft underflights with Scanning-HIS and NAST-I Ø AIRS/MODIS comparisons – Obs-Calc Analyses

• AIRS / MODIS Comparisons – Review of Scanning-HIS validation of AIRS – AIRS / MODIS Comparison Approach • Match spectral resolutions • Match spatial resolution and sampling and select uniform fields of view – Differences characterized as a function of scene temperature, scan angle, and solar zenith angle for global data collected on 6 Sept 2002 and 18 Feb 2004 – Tobin, D. C. , H. E. Revercomb, C. C. Moeller, and T. S. Pagano, Use of AIRS high spectral resolution infrared spectra to assess the calibration of MODIS on EOS Aqua, J. Geophys. Res. , submitted, April 2005. • Important for: – Diagnosing the calibration of both sensors – Understanding differences between AIRS products and MODIS products – Development of applications utilizing data from both sensors (e. g. AIRS cloud-clearing using MODIS, synergistic use of AIRS and MODIS for cloud property retrievals)

AIRS underflight by the Scanning-HIS, 21 November 2002 Gulf of Mexico Daytime AIRS S-HIS AIRS / S-HIS comparison, without accounting for viewing geometry or spectral resolution/sampling differences: Tobin, D. C. , H. E. Revercomb, R. O. Knuteson, F. A. Best, W. L. Smith, P. van Deslt, D. D. La. Porte, S. D. Ellington, M. W. Werner, R. G. Dedecker, R. K. Garcia, N. N. Ciganovich, H. B. Howell, S. B. Dutcher, J. K. Taylor, K. Vinson, T. S. Pagano, S. A. Mango, Radiometric and Spectral Validation of AIRS Observations with the Aircraft based Scanning High resolution Interferometer Sounder, J. Geophys. Res. , submitted, April 2005.

A sample AIRS brightness temperature spectrum overlaid with the Aqua MODIS Spectral Response Functions 36 35 34 30 25 33 29 24 32 28 23 wavenumber 31 27 22, 21 20

To match the MODIS spectral resolution, the AIRS spectra are convolved with the MODIS SRFs RMONO SRFMODIS – (RMONO SRFAIRS) SRFMODIS Convolution Correction: factor that accounts for small gaps in AIRS spectra when convolving AIRS radiance spectra with the MODIS SRFs.

Corrections for standard atmospheres 4. 0 4. 1 4. 4 4. 5 6. 8 7. 3 8. 5 9. 7 11. 0 12. 0 13. 4 13. 7 13. 9 14. 2 m for 6 September 2002 AIRS data In the following comparisons, the correction is represented as the mean of the six standard atmosphere values shown above for each band. This treatment is more accurate for bands for which the correction is small and for which the correction does not vary largely with the profile/spectrum.

The 1 km MODIS data is collocated with AIRS by representing the AIRS FOVs as slightly oversized circular footprints, and computing the mean MODIS value within those footprints for each band. Spatially uniform scenes are selected by requiring the standard deviation of the MODIS data within each AIRS footprint to be 0. 2 K or less.

Example comparisons for band 22 (4. 0 m) on 6 Sept 2002. mean= -0. 05 K Little Dependence on Scene Temperature Little Dependence on X-track View Angle Little Dependence on Solar Zenith Angle AIRS BT (K) AIRS minus MODIS (K)

Example comparisons for band 34 (13. 7 m) on 6 Sept 2002. AIRS MODIS

Histograms of brightness temperature differences. 6 September 2002 18 February 2004 m 14. 2 13. 9 13. 7 (Light gray curves are distributions without the convolution corrections) 13. 4 12. 0 11. 0 9. 7 7. 3 6. 8 4. 5 4. 4 4. 1 4. 0

Brightness temperature differences as a function of scene temperature. 6 September 2002 18 February 2004 m 1 K 14. 2 13. 9 13. 7 13. 4 12. 0 11. 0 9. 7 7. 3 6. 8 4. 5 4. 4 4. 1 4. 0

Band 35 (13. 9 m) brightness temperature differences for one orbit of data on 6 Sept 2002 using (1) the nominal MODIS SRF and (2) the MODIS SRF shifted by +0. 8 cm-1. MODIS SRF out-of-band response also currently being investigated. unshifted unshifted

Brightness temperature differences as a function of scan angle. 6 September 2002 18 February 2004 m 1 K 14. 2 13. 9 13. 7 13. 4 12. 0 11. 0 9. 7 7. 3 6. 8 4. 5 4. 4 4. 1 4. 0

Band 24 (4. 4 m), descending Scan Angle Asymmetry using non-polar clear sky swaths 6 September 2002 AIRS MODIS

Band: 24 33 (Low Yield) 34 35 36

The Longwave CO 2 band biases are making a large (positive) impact on MODIS CO 2 slicing algorithm performance Channel Pair Representing the Best CTP Retrieval Pink, cyan, and green are 36/35, 35/34, 34/33, respectively c/o Rich Frey Zonal Plus Constant Radiometric Bias Zonal Plus Variable Radiometric Bias

Comparisons with CERES Window Channel Radiances: Monochromatic AIRS CERES FM 4 SRF Convolution Errors (K): -0. 44 Tropical -0. 52 MLS -1. 07 MLW -0. 80 SAS -1. 77 SAW -0. 70 US Std.

AIRS / CERES-FM 4 Window Channel Comparisons 18 Feb 2004, Nighttime, Ocean only, 60 S to 60 N PRE LIM INA RY

Summary • Comparison of EOS Aqua AIRS and MODIS infrared radiances for spatially uniform scenes collected on 6 September 2002 and 18 February 2004 have been presented. • A simple approach to account for spectral gaps in the AIRS spectra when convolving with the MODIS SRFs has been introduced. • Estimates of the absolute uncertainty of the comparisons are 0. 1 K or less for the majority of the MODIS bands. • Mean differences between AIRS and MODIS are ~1 K or less for all bands and many bands show agreement of 0. 1 K or better. But at the same time, only band 22 (3. 9 m) shows good absolute agreement and no significant dependence on scene temperature, scan angle, or solar zenith angle. • Differences for MODIS bands 27 (6. 8 m), 28 (7. 3 m), 34 (13. 7 m), 35 (13. 9 m), and 36 (14. 2 m) display clear and significant dependencies on scene temperature. • Results for the two days are very similar with changes in mean differences of 0. 1 K or less for most bands. • Preliminary AIRS/CERES comparisons look good; more accurate comparisons require a more sophisticated approach to account for the AIRS spectral gaps.