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Characteristics of the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis surface water budget are compared to the surface water budget characteristics in a long‐term simulation with the NCEP global spectral model (GSM) used in the reanalysis. There are many geographic similarities. There are a few differences though, mainly because the reanalysis has a source (artificial) of water from a seasonally varying damping (forcing) term. Because the GSM does not have this (forcing) term, the GSM surface becomes dryer and ultimately has decreased land evaporation and precipitation. Since evaporation variations are strongly related to surface air and skin temperature variations, this decreased GSM surface water is also associated with increased land temperature. However, the GSM surface water variability is greater because the reanalysis time constant (60 days) used to damp (force) the surface water back to the assumed climatology is shorter than characteristic land surface parameterization hydrologic times, which are of the order of 6 months. In agreement with the decreased surface water variability, the reanalysis evaporation variability is also less than the evaporation variability in the GSM. This artificial time constraint may be a reason that the reanalysis temperature predictability by surface water is less than the demonstrated GSM predictability.
Roads et al. (Sun,) studied this question.