Downwelling longwave flux over Summit, Greenland, 2010–2012: Analysis of surface observations and evaluation of ERA-Interim using wavelets

This study analyzes the downwelling longwave radiation (DLW) over the Greenland Ice Sheet (GIS) using surface-based observations from Summit Station (72N 38W 3200 m) and ECMWF Interim Reanalysis (ERA-Interim) DLW fields. Since surface-based observations are sparse in the Arctic, the feasibility of including reanalyses for spatial context is assessed. The DLW at Summit is assessed using time-frequency decomposition (wavelet analysis), which yields the dominant timescales of variability. A new method for evaluating reanalyses is introduced that also uses wavelet analysis, enabling evaluation of different timescales separately. ERA-Interim DLW performs well at Summit under clear skies when the DLW is due to atmospheric gaseous emission. But it is biased low overall, because ERA-Interim produces too many thin clouds and too few thick clouds,. Correlation between the observations and ERA-Interim drops from r2 > 0.8 to nearly zero at timescales less than ~4 days when differences in the spatial resolution of the data sets are important. The bias is low across all timescales and is likely tied to cloud generation processes in the model rather than the spatial representation of the atmosphere across the GIS. The exception is autumn when ERA-Interim overestimates the influence of clouds at timescales of 1 and 4 weeks. The spatial distribution of cloud influence on the DLW across the GIS from ERA-Interim indicates that Summit is located in a transition zone in cloud properties between the south and northeast. The gradient across this transition zone is steepest near Summit in autumn, suggesting that the spatial characteristics of the atmosphere near Summit may contribute to the ERA-Interim bias during this time.