Using AIRS to Assess MODIS Radiances

1
Using AIRS to Assess MODIS Radiances

• Dave Tobin
• CIMSS / SSEC / UW-Madison
• NOAA Cooperative Research Program
• Second Annual Science Symposium
• 13 July 2005

2
Introduction

• Overall Goal
• Create a long term record of well calibrated and
  characterized satellite radiances
• Emphasis on high spectral resolution (AIRS, TES,
  IASI, CrIS, ) 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
• AIRS/MODIS comparisons
• Obs-Calc Analyses

3

• 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)

4
AIRS underflight by the Scanning-HIS, 21 November
2002 Gulf of Mexico Daytime AIRS / S-HIS
comparison, without accounting for viewing
geometry or spectral resolution/sampling
differences
AIRS S-HIS
Tobin, D. C., H. E. Revercomb, R. O. Knuteson, F.
A. Best, W. L. Smith, P. van Deslt, D. D.
LaPorte, 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.
5
A sample AIRS brightness temperature spectrum
overlaid with the Aqua MODIS Spectral Response
Functions
36 35 34 33
32 31
30 29
28 27
25 24
23 22,21
20
wavenumber
6
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.
7
?m 14.2 13.9 13.7 13.4 12.0 11.0 9.7 8.5 7.3 6.8
4.5 4.4 4.1 4.0 4.0
Corrections for standard atmospheres 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.
8

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 0.2K or
less.
9
Example comparisons for band 22 (4.0 ?m) on 6
Sept 2002.
Little Dependence onScene Temperature
mean -0.05 K
AIRS BT (K)
Little Dependence onX-track View Angle
Little Dependence onSolar Zenith Angle
AIRS minus MODIS (K)
10
Example comparisons for band 34 (13.7 ?m) on 6
Sept 2002.
AIRS MODIS
11
Histograms of brightness temperature
differences. (Light gray curves are
distributions without the convolution corrections)
6 September 2002
18 February 2004
?m 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 4.0
12
Brightness temperature differences as a function
of scene temperature.
6 September 2002
18 February 2004
?m 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 4.0
13
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
shifted
unshifted shifted
unshifted shifted
14
Brightness temperature differences as a function
of scan angle.
6 September 2002
18 February 2004
?m 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 4.0
15
Scan Angle Asymmetry using non-polar clear sky
swaths6 September 2002
16
Band 24 33 34 35
36
(Low Yield)
17
The Longwave CO2 band biases are making a large
(positive) impact on MODIS CO2 slicing algorithm
performance
c/o Rich Frey
18
Comparisons with CERES Window Channel Radiances
Monochromatic AIRS CERES FM4 SRF
Convolution Errors (K) -0.44 Tropical -0.52 ML
S -1.07 MLW -0.80 SAS -1.77 SAW -0.70 US Std.
19
AIRS / CERES-FM4 Window Channel Comparisons 18
Feb 2004, Nighttime, Ocean only, 60S to 60 N
PRELIMINARY
20
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.