SATELLITE SOUNDINGS: LESSONS LEARNED FROM THE ATMOSPHERIC INFRARED SOUNDER (AIRS)

The request of the numerical weather forecasting community for temperature and moisture soundings from space with radiosonde accuracy, nominally referred too as 1K/1km, i.e. 1K rms accuracy with 1 km vertical resolution, has been the driving force for the development of the Atmospheric Infrared Sounder (AIRS) and the following hyperspectral sounders like IASI and CRIS. Based on the experience of more than three years of AIRS data and an intensive effort of global validation the soundings from space, which we refer to as “satellite sondes”, are in many ways superior, and in other ways can never equal conventional radiosondes. Based on comparison of AIRS temperature and moisture retrievals with the operational radiosondes and special validation ARM/CART sites we find that the difference can be expressed as in five altitude dependent error terms: 1. The inherent retrieval accuracy under perfectly cloud-free, uniform scene conditions, 2. the error introduced by the effort to eliminate the effects of clouds, 3. the error introduced by uncertainties in the surface emissivity, 4. the temporal and spatial co-location error, i.e. difference between the state of the atmosphere observed from space in a 15 km beam and that reported by the RAOB as much as three hours earlier or later and as far as 100 km away, and 5. the measurement error in the RAOB. For the less than 1% of the AIRS data fraction which is sufficiently free of clouds a retrieval accuracy of 0.5K rms for temperature and 10% for water near the surface are achieved. For 55% of the cloud-cleared retrievals, identified strictly by internal quality indicators, AIRS achieves about 1K rms accuracy over ocean, about 1.7K over land. In general, we found out that cloud-clearing error dominates over the ocean, emissivity error dominates over land. The validation shows that the AIRS derived retrievals are superior to radiosondes above 400 mbar for water vapour and above 100 mbar for temperature soundings, because the errors in the radiosondes dominate at these altitudes.

AIRS, which was launched into polar 705 km altitude orbit on the EOS Aqua spacecraft on May 4, 2002, AIRS covers the 3.7 to 15.4 micron region of the thermal infrared spectrum with spectral resolution of n/Dn=1200. Since the start of routine data gathering in September 2002 AIRS has returned 3.7 million spectra of the upwelling radiance each day. The operational assimilation of AIRS data by the NWC has produced significant positive impact in the southern and northern hemispheres.