Recent Changes in the ISBA‐CTRIP Land Surface System for Use in the CNRM‐CM6 Climate Model and in Global Off‐Line Hydrological Applications

Abstract

In recent years, significant efforts have been made to upgrade physical processes in the ISBA‐CTRIP land surface system for use in fully coupled climate studies using the new CNRM‐CM6 climate model or in stand‐alone mode for global hydrological applications. Here we provide a thorough description of the new and improved processes implemented between the CMIP5 and CMIP6 versions of the model and evaluate the hydrology and thermal behavior of the model at the global scale. The soil scheme explicitly solves the one‐dimensional Fourier and Darcy laws throughout the soil, accounting for the dependency of hydraulic and thermal soil properties on soil organic carbon content. The snowpack is represented using a multilayer detailed internal‐process snow scheme. A two‐way dynamic flood scheme is added in which floodplains interact with the soil hydrology through reinfiltration of floodwater and with the overlying atmosphere through surface free‐water evaporation. Finally, groundwater processes are represented via a two‐dimensional diffusive unconfined aquifer scheme allowing upward capillarity rises into the superficial soil. This new system has been evaluated in off‐line mode using two different atmospheric forcings and against a large set of satellite estimates and in situ observations. While this study is not without weaknesses, its results show a real advance in modeling the physical aspects of the land surface with the new ISBA‐CTRIP version compared to the previous system. This increases our confidence that the model is able to represent the land surface physical processes accurately across the globe and in turn contribute to several important scientific and societal issues.

Evaluation of CMIP6 DECK Experiments With CNRM‐CM6‐1

“…The land surface is represented using the new Interaction Soil‐Biosphere‐Atmosphere‐CNRM TRIP (ISBA‐CTRIP) coupled system (Decharme et al, ). ISBA calculates the time evolution of the energy and water budgets at the land surface, while CTRIP simulates river discharges up to the ocean from the total runoff computed by ISBA.…”

“…Finally, the mean topography is derived from the 1‐km Global Multi‐resolution Terrain Elevation Data 2010 (https://topotools.cr.usgs.gov/gmted_viewer/). More details can be found in Decharme et al ().…”

“…In comparison to its predecessor CNRM‐CM5.1 (Voldoire et al, )⁠, which was the basis for the CNRM‐Cerfacs contribution to CMIP5, CNRM‐CM6‐1 (Figure ) corresponds to a major update of its atmospheric and land surface components. It now embarks ARPEGE‐Climat version 6.3 and SURFEX version 8.0, with fully revised physical parameterizations for the atmosphere (http://www.umr-cnrm.fr/cmip6/references) and the land surface (Decharme et al, ), respectively. All other model components were also updated to their newest respective version but with only limited changes in the choice of parameterizations involved.…”

“…All other model components were also updated to their newest respective version but with only limited changes in the choice of parameterizations involved. In CNRM‐CM6‐1, the ocean, sea ice, and river routing components are based respectively on NEMO version 3.6 (Madec et al, ), GELATO version 6, and a new TRIP version redeveloped at CNRM and called CTRIP (Decharme et al, ). The coupling software was also updated to OASIS3‐MCT (Craig et al, ), which now allows for parallel exchanges of fields.…”

Evaluation of CNRM Earth System Model, CNRM‐ESM2‐1: Role of Earth System Processes in Present‐Day and Future Climate

“…This database includes climatological observations at high resolution over the 2000s, such as LAI, vegetation roughness length, snow‐free land surface albedo (Carrer et al, ), or soil textural properties, from the Harmonized World Soil Database at a 1‐km resolution (HWSD, ). More details on the physical processes represented in the ISBA‐CTRIP system can be found in Decharme et al ().…”