The high-latitude regions of the Northern Hemisphere are a nexus for the interaction between land surface physical properties and their exchange of carbon and energy with the atmosphere. At these latitudes, two carbon pools of planetary significance – those of the permanently frozen soils (permafrost), and of the great expanse of boreal forest – are vulnerable to destabilization in the face of currently observed climatic warming, the speed and intensity of which are expected to increase with time. Improved projections of future Arctic and boreal ecosystem transformation require improved land surface models that integrate processes specific to these cold biomes. To this end, this study lays out relevant new parameterizations in the ORCHIDEE-MICT land surface model. These describe the interactions between soil carbon, soil temperature and hydrology, and their resulting feedbacks on water and CO₂ fluxes, in addition to a recently developed fire module. Outputs from ORCHIDEE-MICT, when forced by two climate input datasets, are extensively evaluated against (i) temperature gradients between the atmosphere and deep soils, (ii) the hydrological components comprising the water balance of the largest high-latitude basins, and (iii) CO₂ flux and carbon stock observations. The model performance is good with respect to empirical data, despite a simulated excessive plant water stress and a positive land surface temperature bias. In addition, acute model sensitivity to the choice of input forcing data suggests that the calibration of model parameters is strongly forcing-dependent. Overall, we suggest that this new model design is at the forefront of current efforts to reliably estimate future perturbations to the high-latitude terrestrial environment.

This paper presents a new version of HCLIM, a regional climate modelling system based on the ALADIN–HIRLAM numerical weather prediction (NWP) system. HCLIM uses atmospheric physics packages from three NWP model configurations, HARMONIE–AROME, ALARO and ALADIN, which are designed for use at different horizontal resolutions. The main focus of HCLIM is convection-permitting climate modelling, i.e. developing the climate version of HARMONIE–AROME. In HCLIM, the ALADIN and ALARO configurations are used for coarser resolutions at which convection needs to be parameterized. Here we describe the structure and development of the current recommended HCLIM version, cycle 38. We also present some aspects of the model performance. HCLIM38 is a new system for regional climate modelling, and it is being used in a number of national and international projects over different domains and climates ranging from equatorial to polar regions. Our initial evaluation indicates that HCLIM38 is applicable in different conditions and provides satisfactory results without additional region-specific tuning. HCLIM is developed by a consortium of national meteorological institutes in close collaboration with the ALADIN–HIRLAM NWP model development. While the current HCLIM cycle has considerable differences in model setup compared to the NWP version (primarily in the description of the surface), it is planned for the next cycle release that the two versions will use a very similar setup. This will ensure a feasible and timely climate model development as well as updates in the future and provide an evaluation of long-term model biases to both NWP and climate model developers.

This work is motivated by the identification of the land‐atmosphere interactions as one of the key sources of uncertainty in climate change simulations. It documents new developments in related processes, namely, boundary layer/convection/clouds parameterizations and land surface parameterization in the Earth System Model of the Institut Pierre Simon Laplace (IPSL). Simulations forced by prescribed oceanic conditions are produced with different combinations of atmospheric and land surface parameterizations. They are used to explore the sensitivity to the atmospheric physics and/or soil physics of major biases in the near surface variables over continents, the energy and moisture coupling established at the soil/atmosphere interface in not too wet (energy limited) and not too dry (moisture limited) soil moisture regions also known as transition or “hot‐spot” regions, the river runoff at the outlet of major rivers. The package implemented in the IPSL‐Climate Model for the Phase 6 of the Coupled Models Intercomparison Project (CMIP6) allows us to reduce several biases in the surface albedo, the snow cover, and the continental surface air temperature in summer as well as in the temperature profile in the surface layer of the polar regions. The interactions between soil moisture and atmosphere in hotspot regions are in better agreement with the observations. Rainfall is also significantly improved in volume and seasonality in several major river basins leading to an overall improvement in river discharge. However, the lack of consideration of floodplains and human influences in the model, for example, dams and irrigation, impacts the realism of simulated discharge.

[1] Accurate estimates of basin‐wide water and energy cycles are essential for improving the integrated water resources management (IWRM), especially for relatively dry conditions. This study aims to evaluate and apply a fine‐resolution global data set (Global Land Data Assimilation System with Noah Land Surface Model, GLDAS/Noah; 3‐h, 0.25‐degree) in a semiarid mesoscale basin (∼15000 km²). Four supporting objectives are proposed: (1) validating a Water and Energy Budget‐based Distributed Hydrological Model (WEB‐DHM) for GLDAS/Noah evaluation and application; (2) evaluating GLDAS forcing data (precipitation; near‐surface air temperature, T_air; downward shortwave radiation, R_sw,d; downward longwave radiation, R_lw,d); (3) investigating GLDAS/Noah outputs (land surface temperature, LST; evapotranspiration; fluxes); (4) evaluating the applicability of GLDAS forcing in modeling basin‐wide water cycles. Japanese 25‐year reanalysis and in situ observations (precipitation; T_air; R_sw,d; discharge) are used for GLDAS/Noah evaluation. Main results include: (1) WEB‐DHM can reproduce daily discharge, 8‐day LST and monthly surface soil moisture (point scale) fairly well; (2) the GLDAS is of high quality for daily and monthly precipitation, T_air, monthly R_lw,d, while it overestimates monthly R_sw,d; (3) the GLDAS/Noah agrees well with the verified WEB‐DHM and JRA‐25 in terms of LST, upward shortwave and longwave radiation. While the net radiation, evapotranspiration, latent and sensible heat fluxes modeled by GLDAS/Noah are larger than WEB‐DHM and JRA‐25 simulations in wet seasons; (4) the basin‐integrated discharges and evapotranspiration can be reproduced reasonably well by WEB‐DHM fed with GLDAS forcing except linear corrections of R_sw,d. These findings would benefit the IWRM in ungauged or poorly gauged river basins around the world.

A conceptual model based on the surface energy budget is developed to compute the sensitivity of the climatological mean diurnal amplitude and mean daily surface temperature to the soil thermal inertia. It uses the diurnal amplitude of the net surface radiation, the sensitivity of the turbulent fluxes to the surface temperature and the soil thermal inertia. Its performance is evaluated globally with numerical simulations using the atmospheric and the land surface modules of a state‐of‐the‐art climate model. The only regions where the thermal inertia has a limited impact are the moist regions. In dry areas, together with the stability of the boundary layer it plays a major role. It also has a significant impact at high and midlatitudes especially in winter when the turbulent fluxes are weak. In semiarid regions, the soil moisture exhibits a high day‐to‐day variability strongly correlated to the evaporation and the day‐to‐day variability of the surface temperature is generally explained by the soil moisture via its control on the evaporation. However, the soil moisture via its control on the thermal inertia reduces the impact of the day‐time evaporative cooling by reducing the nocturnal cooling. This newly identified moisture‐related negative feedback can reduce the variability of the surface temperature in semiarid regions by up to a factor of 2. The model also provides a simple framework to understand the role of the thermal properties in the frequent cold bias identified in stratified stable atmospheric situations in the northern midlatitudes.