A METHODOLOGY FOR OPTIMIZING FREE GLOBAL CLIMATE MODEL PARAMETERS USING NASA OBSERVATIONAL DATASETSGENERAL CIRCULATION MODELS (GCM) ARE OFTEN JUDGED BY METRICS THAT ARE TIED TO HOW WELL CLIMATOLOGICAL MEANS SPATIAL VARIATIONS AND SEASONAL CYCLES OF VARIOUS OBSERVABLE PARAMETERS ARE SIMULATED. HOWEVER SUCH EVALUATIONS ARE MADE AGAINST IMPERFECT OBSERVATIONAL DATASETS. A LARGE COMPONENT OF UNCERTAINTY STEMS FROM OBSERVING SYSTEM LIMITATIONS (E.G. RADIATION/WAVELENGTH SPECTRA UTILIZED) RETRIEVAL ALGORITHM LIMITATIONS IN DIFFERING ENVIRONMENTS AND SAMPLING LIMITATIONS (E.G. SUN- VS. NON-SUN SYNCHRONOUS SATELLITES). A NUMBER OF STUDIES HAVE FOUND THAT UNCERTAINTIES ARISING FROM THESE LIMITATIONS MAY APPROACH 50% IN MANY REGIONS FOR A NUMBER OF WIDELY-USED HYDROLOGICAL PRODUCTS. PERVASIVE BIASES OF THIS MAGNITUDE IF FORMALLY INCORPORATED INTO MODEL PERFORMANCE COST FUNCTIONS MAY SUBSTANTIALLY ALTER THE PERCEIVED SKILL OF A GCM AND FURTHER INFLUENCE THE MODEL DEVELOPER S CHOICE OF GCM PARAMETERIZATION TUNING AND FREE PARAMETER COMBINATIONS. USING THE GISS GCM AS A TEST-BED PRELIMINARY RESULTS SHOW THAT WHEN LARGE UNCERTAINTY ESTIMATES ARE ACCOUNTED FOR IN A PERFORMANCE METRIC THE GCM FREE PARAMETER STATE SPACE COMPRISING GOOD OR ADMISSIBLE GCM CONFIGURATIONS SUBSTANTIALLY CHANGES. WIDE VARIATIONS IN ACCEPTABLE GCM FREE PARAMETER COMBINATIONS ARE FOUND AND ARE FURTHER ASSOCIATED WITH WIDELY VARYING REGIONAL BIASES IN RADIATION (INCLUDING OVER THE SOUTHERN OCEANS) DESPITE SIMILAR GOODNESS METRICS (E.G. TAYLOR SCORES) AND TOA RADIATIVE BALANCE. VARYING BIASES IN RADIATION FIELDS SUGGESTING VARIATIONS IN THE EXTENT TO WHICH THE ATMOSPHERE IS CONFLICTED MAY SUGGEST FURTHER IMPACTS ON SIMULATED CLIMATE SENSITIVITY. WE PROPOSE TO FURTHER EXPLORE THESE FINDINGS BY (A) USING A NUMBER OF NASA DATASETS (OBS4MIPS ARCHIVE) AS REFERENCES FOR TRUTH (B) COMPUTING THE SYSTEMATIC DIFFERENCES BETWEEN TRUTH DATASETS AND ALTERNATIVE WIDELY-USED SATELLITE PRODUCTS; AND (C) CASTING THE DIFFERENCES AS QUANTITATIVE PROXIES FOR SATELLITE PRODUCT UNCERTAINTIES. WE FORMALLY INCLUDE THE UNCERTAINTIES IN A MODEL PERFORMANCE INDEX AND ADDRESS THE FOLLOWING QUESTIONS: 1. BY INCORPORATING SUCH LARGE ERRORS INTO MODEL PERFORMANCE METRICS WHAT IS THE IMPACT ON GCM TUNING PROCEDURES? 2. IF THE PLAUSIBLE PORTION OF THE PARAMETER TUNING STATE SPACE CHANGES UPON INCLUSION OF OBSERVATIONAL ERROR IN GOODNESS METRICS IS THERE ADDITIONAL LEEWAY FOR IMPROVING REGIONAL RADIATION BIASES EXHIBITED BY A GCM? 3. DO OUR DECISIONS ON PARAMETER COMBINATIONS TODAY HAVE A NOTICEABLE IMPACT ON THE SIMULATED CLIMATE SEVERAL CENTURIES FROM NOW? WE MAXIMIZE THE USE OF SEVERAL COMPLEMENTARY NASA SATELLITE DATASETS IN THIS PROJECT. WE ANTICIPATE NOVEL RESULTS REGARDING THE IMPORTANCE OF INCORPORATING ERROR ESTIMATES (OF ANY TYPE) INTO GCM PERFORMANCE METRICS. OUR RESULTS WILL YIELD THE SPREAD IN EQUILIBRIUM CLIMATE SENSITIVITY ESTIMATES ARISING FROM SETTLING ON VERY DIFFERENT BUT SEEMINGLY OPTIMAL COMBINATIONS OF TUNING PARAMETERS. THE WORK FURTHER REPRESENTS A CONCERTED EFFORT TOWARD ACHIEVING TRANSPARENCY AND DOCUMENTATION IN THE GCM ADJUSTMENT/TUNING AND FREE-PARAMETER-SETTING PROCESS.