Land surface model spin‐up behavior in the North American Land Data Assimilation System (NLDAS)

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Abstract
The process of a model adjusting to its forcing (model spin‐up) can severely bias land surface simulations, and result in questionable land surface model (LSM) output during the spin‐up process. To gain a better understanding of how spin‐up processes affect complex spatial and temporal land surface modeling situations in general, and the Retrospective North American Land Data Assimilation System (NLDAS) simulations in particular, a two‐phase study was conducted. The first phase examined results from Control, Wet, and Dry 11 year‐long Mosaic simulations, while the second phase attempted to explain spin‐up behavior in NLDAS Retrospective simulations from the Mosaic, Noah, VIC and Sacramento LSMs based in part on the results from phase 1. Total column and root zone soil moisture spin up slowly, while evaporation and deep soil temperature spin up more quickly. Mosaic soil moisture initialization with NCEP/DOE Global Reanalysis 2 (NCEP/DOE R‐2) data (Control run) leads to a faster spin‐up time than saturated (Wet run) or dry (Dry run) initialization, with the Control run reaching equilibrium 1 to 2 years sooner than the Wet run and 3 to 4 years more quickly than the Dry run. Overall, practical drift of land surface stores and output ceased in the Control run within approximately 1 year, and fine‐scale equilibrium was reached within 5.5 years. Spin‐up times exhibited large spatial variability, and although no single causal factor could be determined, they were correlated most strongly with precipitation and temperature forcing. In general, NLDAS models reach a state of rough equilibrium within the first 1 to 2 years of the 3‐year Retrospective simulation. The Sacramento LSM has the shortest spin‐up phase, followed by the Mosaic, VIC, and Noah LSMs. Initial NCEP/DOE R‐2 conditions were too dry in general for the VIC and Noah LSMs, and too moist for the Mosaic and Sacramento LSMs. These results indicate that in most cases, the 1‐year spin‐up time used in the Retrospective NLDAS simulations eliminated spin‐up problems from the subsequent period that was used for analysis.