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SPIE NEWS ITEM 


NASA’s Advancements in Space-Based Spectrometry Lead to Improvements in 
Weather Prediction and Understanding of Climate Processes 

Joel Susskind, NASA GSFC 
Lena Iredell, SAIC 

AIRS (Atmospheric Infra-Red Sounder), was launched, in conjunction with AMSU-A 
(Advanced Microwave Sounding Unit-A) on the NASA polar orbiting research satellite 
EOS (Earth Observing System) Aqua satellite in May 2002 as a next generation 
atmospheric sounding system. Atmospheric sounders provide information primarily 
about the vertical distribution of atmospheric temperature and water vapor distribution. 
This is achieved by measuring outgoing radiation in discrete channels (spectral 
intervals) which are sensitive primarily to variations of these geophysical parameters. 
The primary objectives of AIRS/AMSU were to utilize such information in order to 
improve the skill of numerical weather prediction as well as to measure climate 
variability and trends. 

AIRS is a multi-detector array grating spectrometer with 2378 channels covering the 
spectral range 650 cm' 1 (15 microns) to 2660 cm' 1 (3.6 microns) with a resolving power 
(v/8 v) of roughly 1200 where 8 v is the spectral channel bandpass. Atmospheric 
temperature profile can be determined from channel observations taken within the 15 
micron (the long-wave C0 2 absorption band) and within the 4.2 micron (the short-wave 
C0 2 absorption band). Radiances in these (and all other) spectral intervals in the 
infrared are also sensitive to the presence of clouds in the instrument’s field of view 
(FOV), which are present about 95% of the time. AIRS was designed so as to allow for 
the ability to produce accurate Quality Controlled atmospheric soundings under most 
cloud conditions. This was achieved by having 1) extremely low channel noise values in 
the shortwave portion of the spectrum and 2) a very flat spatial response function within 
a channel’s FOV. IASI, the high spectral resolution IR interferometer flying on the 
European METOP satellite, does not contain either of these important characteristics. 
The AIRS instrument was also designed to be extremely stabile with regard to its 
spectral radiometric characteristics, which is critical with regard to the ability to measure 
accurate long term trends. 

AIRS, at nadir viewing, has nine 13.5 km by 13.5 km FOV’s that lie within a single 45 
km by 45 km AMSU Field Of Regard (FOR). The AIRS Science Team retrieval 
algorithm generates a single sounding per FOR, using the nine AIRS fields of view to 
derive clear column radiances for each channel i, which represent the radiance channel 
i would have seen if the FOR were completely cloud free. Geophysical parameters are 
determined so as to be consistent with clear column radiances for a select set of 
channels. Following theoretical considerations, coefficients needed to determine clear 
column radiances for all channels are determined using only observations in the 
longwave C0 2 band and longwave window region, while atmospheric and surface 
temperatures are determined using clear column radiances only in the shortwave C0 2 
band and shortwave window region. This optimal approach is practical for AIRS 



because noise on the shortwave channels is very low. The operational AIRS Science 
Team Version 5 retrieval algorithm uses this approach and has resulted in marked 
improvement in the use of AIRS products for data assimilation and also in the 
generation of accurate climate data sets. This new methodology also allowed for the 
generation of error estimates of the retrieved products which are used for Quality 
Control and are critical for their optimal use of in weather and climate applications. A 
number of important results using AIRS Version 5 products are given below. 

1 ) Assimilation of Quality Controlled AIRS Version 5 temperature profiles produced 
superior 5-7 day forecasts than those obtained using the NOAA operational 
procedure of assimilating observed AIRS radiances. Assimilation of Quality 
Controlled AIRS temperature soundings also has resulted in a significant 
improvement in the ability to predict the track of Tropical Storm Nargis,that 
devastated parts of Indonesia in 2006, compared to the operational procedure. 

2) OLR computed from AIRS products confirms the result observed by CERES that 
global mean OLR has been decreasing at a rate of 0.12 W/m 2 /yr over the Aqua 
time period September 2002 through December 2009. The majority of this 
decrease occurs in the tropics as a result of a significant redistribution of cloud 
cover and water vapor in response to a very strong La Nina event starting in late 
2007. 

3) Twice daily global fields of CO derived from AIRS accurately depict the space 
and time propagation of CO coming from fires. 

4) AIRS provided the first accurate global monthly mean fields of C0 2 concentration 
which showed both local seasonal cycles and C0 2 growth over time. 

NASA has a new proposed design, employing two dimensional detector arrays, for a 
new Low Earth Orbiting instrument ARIES, which would have AIRS-like spectral and 
radiometric characteristics but would view the earth with a 1 km spatial resolution. 
Products derived from such an instrument would result in a further significant 
improvement in the ability to improve weather prediction, monitor climate parameters, 
and derive atmospheric water vapor and trace gas distribution. 


References: 

Improved Temperature Sounding and Quality Control Methodology Using AIRS/AMSU 
Data: The AIRS Science Team Version 5 Retrieval Algorithm. Joel Susskind, John 
Blaisdell, Lena Iredell, and Fricky Keita. IEEE TGRS-2009-00127. Accepted for 
publication. 

AIRS impact on the analysis and forecast track of tropical cyclone Nargis in a global 
data assimilation and forecasting system. Oreste Reale, W.K. Lau, J. Susskind, E. Brin, 
E. Liu, L.P. Riishojgaard, M. Fuentes, and R. Rosenberg. Geophysical Research 
Letters, Vol 36, L06812, doi: 10.1029/2008GL037122, 2009.