David T. Gregorich

AIRS hyper-spectral measurements for climate research: Carbon dioxide and nitrous oxide effects

[1]xa0Mid-tropospheric temperature soundings over tropical oceans by the Atmospheric Infrared Sounder, AIRS, using 4.3 micron CO2 R-branch and P-branch channels independently measure about 260 K with one Kelvin semi-annual variability. The difference between the soundings, which cancels seasonal variability, has increased over the past two years by 47 ± 9 mK/year. This trend is explained by the increase of 2.2 ± 0.4 ppmv/year and 0.6 ± 0.2 ppbv/year in the abundance of CO2 and N2O, respectively, which results in a 35 mK/year trend. The ability to achieve closure at this level with only two years of AIRS data is very encouraging for measurements of other trends of atmospheric temperatures relevant to climate research. AIRS covers the 3.7 to 15.4 micron region with spectral resolution of λ/Δλ = 1200. AIRS was launched into a polar 705 km altitude orbit on the EOS Aqua spacecraft on May 4, 2002, and has an expected on-orbit lifetime of seven years.

Formulation and validation of simulated data for the Atmospheric Infrared Sounder (AIRS)

Models for synthesizing radiance measurements by the Atmospheric Infrared Sounder (AIRS) are described. Synthetic radiances have been generated for developing and testing data processing algorithms. The radiances are calculated from geophysical states derived from weather forecasts and climatology using the AIRS rapid transmission algorithm. The data contain horizontal variability at the spatial resolution of AIRS from the surface and cloud fields. This is needed to test retrieval algorithms under partially cloudy conditions. The surface variability is added using vegetation and International Geosphere Biosphere Programme surface type maps, while cloud variability is added randomly. The radiances are spectrally averaged to create High Resolution Infrared Sounder (HIRS) data, and this is compared with actual HIRS2 data on the NOAA 14 satellite. The simulated data under-represent high-altitude equatorial cirrus clouds and have too much local variability. They agree in the mean to within 1-4 K, and global standard deviation agrees to better than 2 K. Simulated data have been a valuable tool for developing retrieval algorithms and studying error characteristics and will continue to be so after launch.

Verification of AIRS boresight accuracy using coastline detection

The longitude and latitude of the centroids of the Atmospheric Infrared Sounder (AIRS) infrared spectrometer footprints are calculated by the Level 1a calibration software based on transformations of scan angles, instrument alignment angles relative to the Earth Observing System Aqua spacecraft, and the spacecraft ephemeris. The detection of coastline crossings is used to determine the accuracy of these coordinates. Tests using simulated AIRS data derived from real Moderate Resolution Imaging Spectroradiometer (MODIS) Terra satellite 10-/spl mu/m window data indicate that an accuracy of 1.7 km is easily achievable with modest amounts of data, such as should be available from AIRS by launch +90 days. This accuracy is a small fraction of the 13.5-km AIRS footprint and is consistent with the accuracy required by the Level 2 software. Preliminary results from actual AIRS data indicate that the algorithm works as predicted. For combined use of the AIRS 13.5-km footprints with MODIS 1-km footprints, accuracy of the order of 0.5 km is desirable. This accuracy may be achievable with several months of data, but depends on the accuracy of the reference map and whether a sufficient number of large clear homogeneous surface scenes can be found.

Seasonal correlations of SST, water vapor, and convective activity in tropical oceans: A new hyperspectral data set for climate model testing

The analysis of the response of the Earth Climate System to the seasonal changes of solar forcing in the tropical oceans using four years of the Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Sounding Unit (AMSU) data between 2002 and 2006 gives new insight into amplitude and phase relationships between surface and tropospheric temperatures, humidity, and convective activity. The intensity of the convective activity is measured by counting deep convective clouds. The peaks of convective activity, temperature in the mid-troposphere, and water vapor in the 0-30 N and 0-30 S tropical ocean zonal means occur about two months after solstice, all leading the peak of the sea surface temperature by several weeks. Phase is key to the evaluation of feedback. The evaluation of climate models in terms of zonal and annual means and annual mean deviations from zonal means can now be supplemented by evaluating the phase of key atmospheric and surface parameters relative to solstice. The ability of climate models to reproduce the statistical flavor of the observed amplitudes and relative phases for broad zonal means should lead to increased confidence in the realism of their water vapor and cloud feedback algorithms. AIRS and AMSU were launched into a 705 km altitude polar sun-synchronous orbit on the EOS Aqua spacecraft on May 4, 2002, and have been in routine data gathering mode since September 2002.