Box 2.1

Summary and Findings

Present Status and Future Needs of Space-Based Atmospheric Soundings

The past 20 years have witnessed considerable progress in passive infrared remote sensing of temperature profiles using radiance data obtained from filter radiometers. Currently, the combination of the High Resolution Infrared Sounder and the Microwave Sounding Unit (MSU) provides atmospheric temperature profiles with an average root mean square (rms) error of approximately 2 K and a vertical resolution of 3 to 5 km in the troposphere. Temperature retrieval algorithms applied to data from this current suite of operational sounders are mature and well understood; however, the accuracy and resolution of temperature retrieved from current sounder data fall short of numerical weather prediction (NWP) requirements. In addition, even when identical retrieval algorithms and instruments are employed, discrepancies in the data products from one spacecraft to the next reduce the utility of the data for climate monitoring. Although new technologies such as Global Positioning System satellites are expected to improve portions of the retrieval, it is hoped that the next generation of full-column sounders will overcome the shortfalls mentioned. The situation is even worse for water vapor, where characteristics of water vapor retrievals and the accuracy of retrieved water vapor measurements are less advanced.

Issues that need to be considered in the remote sensing of temperature and moisture on the National Polar-orbiting Operational Environmental Satellite System (NPOESS) for climate research include the following:

  • Current sounding systems fall short of the requirements of numerical weather prediction. This shortfall has prompted the development of next-generation sounders, beginning with the Atmospheric Infrared Sounder (AIRS) on the Earth Observing System (EOS) PM satellite followed by the Cross-track Infrared Sounder (CrIS) and Infrared Atmosphere Sounding Interferometer radiometer as part of NPOESS, that provide higher-spectral-resolution measurements. The traditional goal of sounding measurements is to deliver profiles of temperature and moisture for use in numerical weather prediction. These sounding data products also have direct and obvious climatological value. However, assimilation of radiance data has progressed at operational prediction centers, where explicit retrievals are no longer carried out in this traditional sense. Sounding data products are now derived as outputs from NWP models rather than as direct outputs from retrieval schemes.

  • There are additional reasons to move toward increased spectral resolution in sounders: (1) improvements in signal-to-noise ratios can be realized by averaging channels with the same characteristic absorption; (2) clearer discrimination of thin clouds is possible using more highly resolved line absorption information; and (3) spectral measurements enhance the capability of providing other nonsounding information. Although additional advantages exist, sounding information content does not increase proportionally with increasing spectral resolution. Within this context it is legitimate to ask (1) what are the optimal placement and number of channels of a high-resolution instrument like AIRS or CrIS that actually contribute to retrieved soundings, (2) which channels are redundant, and (3) what is to be gained by combining a number of redundant channels.

  • From the climate perspective, the question of whether the calibrated radiance data obtained from sounders are more basic than the sounding products derived from these radiances is unresolved. This is an especially relevant debate today, given the changing ways these radiance data are used in assimilation systems and the likelihood of changes in analysis systems in the future.

  • For purposes of monitoring climate change, it is critical to establish the extent to which any retrieved quantity relies on the first estimate of that quantity, which is usually derived from a climatological database. Properties that are too dependent on such databases cannot provide proper measures of evolving climate change. Current analysis systems, especially as they apply to water vapor, are inadequate and unfortunately rely too much on existing but poorly known climatologies or time records. It is not obvious that this situation will improve substantially with the next generation of infrared sounders.

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