Assessing the Accuracy of Water Vapor Measurements in the UT/LS:Problems, Paradigms, and Prescriptions

Because accurate water vapor measurements are critically important for assessing the atmosphere's response to global climate change, the assessment of water vapor instrument accuracy as presented in the widely disseminated 2000 SPARC report requires careful attention. Rather than judging any instrument on its own merit, the report uses in-flight instrument intercomparison as the only tool with which to evaluate instrument performance. Figure 1 of the report that provides a summary of intercomparison results in the troposphere and stratosphere, plots the % difference of water vapor measurements relative to the satellite-borne Halogen Occultation Experiment (HALOE). While the report does not explicitly draw conclusions from the figure about instrument accuracy, the figure invites inferences regarding the accuracy of those instruments. As with other publications that similarly have utilized intercomparisons alone to judge instrument accuracy, the effect of the SPARC evaluation is to ignore other criteria that can be, should be, or have already been used to validate some of those same instruments. The purpose of this talk is to: 1. Critically evaluate the implications of the SPARC intercomparison figure. 2. Provide a list of criteria for judging instrument performance and accuracy that include laboratory, airborne, and on-orbit strategies for establishing absolute accuracy. 3. Use those criteria to evaluate the accuracies of representative instruments included in the intercomparison figure. 4. Suggest potential guidelines that enable independent and impartial instrument evaluation to take place. 5. Suggest other measurements that require the same care and attention now being focused on water vapor. 6. Enlist community support to establish the framework necessary to evaluate instrument accuracy on an ongoing basis.