Evaluation of vegetation cover and land-surface albedo in MPI-ESM CMIP5 simulations

In recent generation Earth system models (ESMs), land-surface grid cells are represented as tiles covered by different plant functional types such as trees or grasses. Here, we present an evaluation of the vegetation-cover module of the ESM developed at the Max Planck Institute for Meteorology in Hamburg, Germany (MPI-ESM) for present-day conditions. The vegetation continuous fields (VCF) product that is based on satellite observations in 2001 is used to evaluate the fractional distributions of woody vegetation cover and bare ground. The model performance is quantified using two metrics: a square of the Pearson correlation coefficient, r(2), and the root-mean-square error (RMSE). On a global scale, r(2) and RMSE of modeled tree cover are equal to 0.61 and 0.19, respectively, which we consider as satisfactory values. The model simulates tree cover and bare ground with r(2) higher for the Northern Hemisphere (0.66) than for the Southern Hemisphere (0.48-0.50). We complement this analysis with an evaluation of the simulated land-surface albedo using the difference in net surface radiation. On a global scale, the correlation between modeled and observed albedos is high during all seasons, whereas the main disagreement occurs in spring in the high northern latitudes. This discrepancy can be attributed to a high sensitivity of the land-surface albedo to the simulated snow cover and snow-masking effect of trees. By contrast, the tropics are characterized by very high correlation and relatively low RMSE (5.4-6.5 W/m(2)) during all seasons. The presented approach could be applied for an evaluation of vegetation cover and land-surface albedo simulated by different ESMs.