Koppen bioclimatic evaluation of CMIP historical climate simulations

Koppen bioclimatic classification relates generic vegetation types to characteristics of the interactive annual-cycles of continental temperature (T) and precipitation (P). In addition to predicting possible bioclimatic consequences of past or prospective climate change, a Koppen scheme can be used to pinpoint biases in model simulations of historical T and P. In this study a Koppen evaluation of Coupled Model Intercomparison Project (CMIP) simulations of historical climate is conducted for the period 1980-1999. Evaluation of an example CMIP5 model illustrates how errors in simulating Koppen vegetation types (relative to those derived from observational reference data) can be deconstructed and related to model-specific temperature and precipitation biases. Measures of CMIP model skill in simulating the reference Koppen vegetation types are also developed, allowing the bioclimatic performance of a CMIP5 simulation of T and P to be compared quantitatively with its CMIP3 antecedent. Although certain bioclimatic discrepancies persist across model generations, the CMIP5 models collectively display an improved rendering of historical T and P relative to their CMIP3 counterparts. In addition, the Koppen-based performance metrics are found to be quite insensitive to alternative choices of observational reference data or to differences in model horizontal resolution.