Development and evaluation of new Van Genuchten soil-properties ancillary files for JULES and the Unified Model

MCGUIRE, PATRICK; VIDALE, PIER LUIGI; BEST, MARTIN; CASE, DAVID; DHARSSI, IMTIAZ; DURAN ROJAS, CAROLINA; HATCHER, ROSALIN; LISTER, GRENVILLE; MARTINEZ DE LA TORRE, ALBERTO; MONTZKA, CARTSEN; MÜLLER, OMAR V.; PEANO, DANIELE; PREDOI, VALERIO; ROBERTSON, EDDY; WILSON, SIMON; TODT, MARKUS; VERHOEF, ANNA

Workshop; JULES Remote Open Science Meeting; 2020

UK Centre for Ecology & Hydrology

We have updated the soil properties used in JULES (Joint UK Land Environment Simulator), which is the land-surface component of the UM (Unified Model, the UK Met Office?s land-atmosphere coupled model). JULES is generally used for modeling the interaction of the soil and vegetation with the atmosphere, and to simulate the energy, water, and carbon fluxes. Prior work with JULES had normally used either: (i) the Brooks & Corey (BC) model for estimating soil hydraulic properties, or (ii) the van Genuchten (VG) model but using hydraulic parameters translated from the BC model. One advantage of the VG model over the BC model is the smoother dependence of matric potential for soils that are nearly saturated with water. Herein, we report on our work towards fully implementing the VG model in JULES and in the UM, through the implementation and evaluation of several VG pedotransfer functions (PTFs) for estimating the soil hydraulic parameters used in the hydraulic functions.In global offline JULES runs (from 1979-2012 with WFDEI driving data on a 0.5-degree grid) performed on the CEDA JASMIN supercomputer, the three VG PTFs that we have tested are: the combination of Tóth et al. PTFs 17 & 20, the Weynants et al. PTF, and the Zhang & Schaap ROSETTA3 H1 PTF. We opted to modernize the soil basic properties that are conventionally used for JULES and the UM, from the UM version of the HWSD to the SoilGrids database. We also opted to modify the Zhang & Schaap PTF, replacing the PTF values for sand-rich grid cells with the PTF values for loamy sand. This modified version of the Zhang & Schaap ROSETTA3 H1 PTF with SoilGrids soils is currently being run in (coupled) UM runs on the ARCHER supercomputer.We evaluated the global JULES performance using: (i) the ILAMB tool; (ii) custom analysis of bias ratios for model-model comparisons that are more direct; (iii) integrative global comparison of the basin-wide water balance between the models by comparison to river- gauge observations. Our findings show that the modified version of the Zhang & Schaap ROSETTA3 H1 PTF is the best of the three VG options, and it compares favorably with the BC control model (which uses the Cosby et al. PTF and the UM/HWSD soils). We will present further comparative analyses of both the offline JULES runs and the coupled UM runs.