Assessing land-surface-atmosphere coupling in the ERA-40 reanalysis with boreal forest data

This analysis uses long time-series of daily mean data from the three BERMS flux sites in central Saskatchewan to explore biases in ERA-40 at a close-by grid-point, and to study the relationships between surface variables and fluxes and cloud cover in the observed and model data sets. On the seasonal timescale the biases in ERA-40 of temperature and humidity are small, but the model has a high bias in evaporation, and except in mid-summer a low bias of reflective cloud, which gives a high bias in the surface downward net shortwave flux. In summer, on days with high observed cloud cover, the model has too little cloud, and a corresponding high sensible heat flux and a warm, dry bias; while on summer days when observed cloud is lower, the model biases are generally reversed. The internal relationships between near-surface relative humidity, linked to the mean lifting condensation level, cloud cover and the surface radiation fluxes are however very similar in model and data. Although cause and effect cannot be determined from these biases alone, given the complexity of the interactions, they suggest possible errors in both the land-surface model and the atmospheric cloud parameterizations.