necessary for various reasons: understanding of aerosol-climate interactions, understanding of global transport of air pollution, provision of boundary conditions for regional photochemical simulations, etc. This issue should be addressed in the final chapter as well as in the introduction.

Specific remarks:

Some of the recommendations need further clarification.

The designation of recommendations by “R” and “A” for reanalysis and attribution, respectively, further enhances the separation/discontinuity between these techniques. The committee suggests that all recommendations that blend reanalysis and attribution issues be combined.

The biggest challenge with reanalysis is the model and the authors are silent on the fidelity of the model. A discussion of how the model should assimilate temperature and precipitation should be included in the recommendations.

R2 needs to be more specific. What does “optimized for climate purposes” mean? Does this mean detection and attribution? There should be some discussion of the fact that there is a range of climate purposes, and different purposes demand different, incompatible, reanalysis configurations. For example, if one wants a trend-free reanalysis, one uses a sparse subset of the current data, but if one wants the most accurate representation of the atmospheric state at any given time, one uses as much data as possible. This discussion would tie reanalysis and attribution sections together better.

R6 states that it is beneficial to go beyond present ad hoc project efforts to a more coordinated and effective national program in climate analysis and reanalysis. How this approach would be beneficial to improve coordination is unclear from this recommendation. What is the scope of this coordination? What is the rationale behind this recommendation? Would the goal be to coordinate better, have a better use of existing resources? There is a US national interest to continue to do reanalysis – does this recommendation mean that a program should go beyond the current program? Will this approach make better use of existing climate data sets? For example, surface trend problems are evidence that we need to do better – will this be accomplished through a national program? Capability may be a better word that does not necessarily imply new infrastructure.

L5400 “… efforts should include a focus on …”

L5454: Recommendation A1: Is Program for Climate Model Diagnosis and Intercomparison (PCMDI) addressing this need?

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