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Supplementary material to “What Can Water Vapor Reveal About Past and Future Climate Change?”
Published 7 April 2009
Steven C. Sherwood, Department of Geology and Geophysics, Yale University, New Haven, Connecticut
Natalia Andronova, Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor
Eric Fetzer, Jet Propulsion Laboratory, Pasadena, California
Robert Kursinski, University of Arizona, Tucson
Citation:
Sherwood, S. C., N. Andronova, E. Fetzer, and R. Kursinski (2009), What Can Water Vapor Reveal About Past and Future Climate Change?, Eos Trans. AGU, 90(14), 122. [Full Article (pdf)]
A speaker panel and ensuing discussions on the last day of the conference addressed water vapor observing systems. Six general methodologies were considered: in situ instruments on aircraft; instruments observing water isotopes; radiosonde instruments; Global Positioning System observations; microwave remote sounding; and infrared remote sounding. All these techniques have provided records extending from five years to decades. Significant advances are currently available or expected in next few years in all these areas. However, a number of issues about the observational record were raised in the discussions. Continuity, especially from satellite instruments, was of particular concern. Data continuity includes merging records from newer, more advanced technologies with observations from earlier instruments, and is particularly important for such climate change issues as atmospheric response to anthropogenic forcing. The discussions also highlighted the need for merging simultaneous records from multiple sources. A properly formulated merged data set will contain significantly more information than its constituent sets. In particular, rigorously derived error estimates should embody the expert knowledge from the several observing methodologies in the combined data set. New retrieval and analysis methods must be developed to keep pace with improvements in observing technologies.
The need for a set of community-defined scientific hypotheses was also noted. Those hypotheses would be appropriate to current and future water vapor observing systems. For example, questions about local supersaturation and cloud microphysics are best answered with aircraft in situ observations, while long-term humidity trends would best be addressed with a combination of satellite and radiosonde observations. Some phenomena discussed at the conference may not be completely observed by current systems. One such phenomenon is water vapor at frontal boundaries. It is difficult to measure from space (especially over land and in the boundary layer), while in situ methods can provide only limited information about three-dimensional structure. A set of clearly defined hypotheses about humidity pre-conditioning and convection are critical in helping define future observing systems. Similarly, expected small changes in relative humidity over the coming decades may require new measurement technologies and techniques.
Significant improvements in technology can be guided only by a set of well-formulated science questions. Historically, instrument selection has been based largely on technological improvement and cost. We recommend the establishment of a science-driven, systems engineering approach to developing an integrated global observing system. The panel specifically recommends that a multi-stage process be initiated to define the objectives and capabilities of future observing systems. In the first stage, experts in theory, modeling and data analysis would identify the critical science questions, hypotheses and uncertainties followed by formulation of the set of observational objectives needed to answer the critical science questions and hypotheses. This formulation of science questions and observational objectives would be done independently of the observational community. In the second stage, the observational community would determine the feasibility of meeting those objectives, and develop a conceptual design of a global observing system. This design would incorporate elements of existing observing systems, and identify critical new elements. Furthermore, a result of the second stage would be a clearly defined subset of observational objectives that cannot be met because of a combination of inadequate technology and funding. In a third stage, this list of unmet objectives would be used to provide direction for R&D programs at NASA and other United States and international agencies. This set of documents would provide much needed traceability that connects the science questions and objectives to the observational capabilities and R&D strategy.