AMIP-style global soil simulations with JULES and the Unified Model: The role of soil hydraulics model, pedotransfer function, and basic soil property map

In prior work (McGuire et al., AGU Meeting Abstracts 2020) we selected the Zhang & Schaap H1 LS pedotransfer function (PTF) as the best of a set of three van Genuchten PTFs tested in land-only JULES simulations with SoilGrids basic soil properties with atmospheric forcing data from the WFDEI database from 1979-2012. In this study we present AMIP-style (Atmospheric Model Intercomparison Project) global simulations of both land and atmosphere at N216 resolution (432 × 324 grid cells) with the van Genuchten soil hydraulics model enabled and with soil parameters computed from the Zhang & Schaap H1 LS PTF and SoilGrids soil basic properties. The model was run on the ARCHER supercomputer for the years 1989-2008, and we compare the outputs of this experimental soils simulation to a control simulation with the Brooks & Corey soil hydraulics model and with the soil parameters computed by the Cosby et al. PTF and the standard Unified Model version of the HWSD soil basic properties. The outputs of the new experimental soils configuration compares favorably with the control soils configuration, when evaluated with the AutoAssess software. However, the global long-time-average model-sensitivity maps of the 1.5-meter JJA air temperature indicate 1-3° C cooling when using the experimental soils configuration over significant areas of North America, South America, and Eurasia in the 1989-2008 study period, relative to the control soils configuration. When compared to the global, gridded, time-averaged CRUTEM3 observational data for the 1.5-meter JJA air temperature from 1979-1998, the RMS of the bias for the experimental soils configuration (at 2.13° C) is slightly smaller than the RMS of the bias of the control soils configuration (at 2.22° C). The demonstrated model sensitivity has large impacts on the interpretation of global climate models and shows the importance of adequate soil parameter and hydraulic model selection.

Reference:McGuire et al., AGU Meeting Abstracts 2020, "Improving the Global Modeling of Soils in JULES and the Unified Model: Updating from UM/HWSD to SoilGrids Soil Properties and from the Brooks & Corey to the Van Genuchten Soil-hydraulics Model", https://ui.adsabs.harvard.edu/abs/2020AGUFMH199.0014M/abstract