Evaluation of long-term pavement performance (LTTP) climatic data for use in mechanistic-empirical pavement design guide(MEPDG) calibration and other pavement analysis

Improvements in the Long-Term Pavement Performance (LTPP) Program's climate data are needed to support current and future research into climate effects on pavement materials, design, and performance. The calibration and enhancement of the Mechanistic-Empirical Pavement Design Guide (MEPDG) is just one example of these emerging needs. A newly emerging climate data source, the Modern-Era Retrospective Analysis for Research and Applications (MERRA), developed by the National Aeronautics and Space Administration (NASA) for its own in-house modeling needs, provides continuous hourly weather data starting in 1979 on a relatively fine-grained uniform grid. MERRA is based on a reanalysis model that combines computed model fields (e.g., atmospheric temperatures) with ground-, ocean-, atmospheric-, and satellite-based observations that are distributed irregularly in space and time. MERRA data are available at an hourly temporal resolution and 0.5 degrees latitude by 0.67 degrees longitude (approximately 31.1 by 37.30 mi at mid-latitudes) spatial resolution over the entire globe. MERRA data were compared against the best available ground-based observations both statistically and in terms of effects on pavement performance as predicted using the MEPDG. These analyses included a systematic quantitative evaluation of the sensitivity of MEPDG performance predictions to variations in fundamental climate parameters. More extensive analysis of MERRA data included additional statistical analysis comparing operating weather station (OWS) and MERRA data, evaluation of the correctness of MEPDG surface shortwave radiation (SSR) calculations and comparison of MEPDG pavement performance predictions using OWS and MERRA climate data for more sections. The principal conclusion from these evaluations was that the MERRA climate data were as good and in many cases substantially better than equivalent ground-based OWS data. Given these many benefits and very few if any significant limitations, MERRA is strongly recommended as the new future source for climate data in LTPP. Recommendations are provided for incorporating hourly MERRA data into the LTPP database.