Evaluation of Noah-MP land-model uncertainties over sparsely vegetated sites on the Tibet Plateau

Uncertainties in the Noah with multiparameterization (Noah-MP) land surface model are assessed through physics ensemble simulations in four sparsely vegetated sites located in the central and western Tibetan Plateau. The simulated hydrological components are evaluated using observations at those sites during the third Tibetan Plateau Experiment from August 1st, 2014 to August 1st, 2015. By using natural selection, the crucial subprocesses impacting the hydrological component simulations are identified. The effects of precipitation uncertainties and soil organic matter on the energy fluxes and water cycles are analyzed through a set of sensitivity experiments based on an optimal scheme set. The uncertainty analyses indicate that the greatest uncertainties are in the subprocesses of runoff (RNF) and groundwater, surface-layer parameterization and frozen soil permeability, along with the subprocesses of snow surface albedo and the lower boundary of soil temperature for the bare ground site but the subprocesses of the canopy resistance and soil moisture limiting factors for evaporation for the three alpine grassland sites. The sensitivity analyses reveal that more precipitation can increase the annual total net radiation (Rn), latent heat flux (LE) and RNF but decrease sensible heat flux (SH). Compared to the insufficient precipitation, the relatively small increase in precipitation results in the LE increase during the growing season at the Amdo and Baingoin sites but an RNF increase at the Nagqu site (sandy soil). However, when more precipitation was added, a greater proportion of the added water was distributed to the RNF at the Nagqu site and to the soil liquid water at the Amdo and Baingoin sites. The organic soil increases the annual total LE but reduces the annual total Rn, SH, and RNF. The effect of the soil organic matter on the LE and RNF at the Nagqu site (sandy soil), is greater than that at the other three sites (sandy loam soil).