Abstract. State-of-the-art Earth system models typically employ grid spacings of O(100 km), which is too coarse to explicitly resolve main drivers of the flow of energy and matter across the Earth system. In this paper, we present the new ICON-Sapphire model configuration, which targets a representation of the components of the Earth system and their interactions with a grid spacing of 10 km and finer. Through the use of selected simulation examples, we demonstrate that ICON-Sapphire can (i) be run coupled globally on seasonal timescales with a grid spacing of 5 km, on monthly timescales with a grid spacing of 2.5 km, and on daily timescales with a grid spacing of 1.25 km; (ii) resolve large eddies in the atmosphere using hectometer grid spacings on limited-area domains in atmosphere-only simulations; (iii) resolve submesoscale ocean eddies by using a global uniform grid of 1.25 km or a telescoping grid with the finest grid spacing at 530 m, the latter coupled to a uniform atmosphere; and (iv) simulate biogeochemistry in an ocean-only simulation integrated for 4 years at 10 km. Comparison of basic features of the climate system to observations reveals no obvious pitfalls, even though some observed aspects remain difficult to capture. The throughput of the coupled 5 km global simulation is 126 simulated days per day employing 21 % of the latest machine of the German Climate Computing Center. Extrapolating from these results, multi-decadal global simulations including interactive carbon are now possible, and short global simulations resolving large eddies in the atmosphere and submesoscale eddies in the ocean are within reach.
Diurnal sea surface temperature (SST) anomalies and their interplay with the atmosphere and in particular with the diurnal cycle of convection have been an object of study for many decades. In this study, we investigate this connection for the first time using simulations that can explicitly resolve both the diurnal temperature variations in the ocean and convection in the atmosphere on a global scale.Diurnal variations in SST have already been described in Sverdrup et al. (1942). Since then, there have been numerous studies describing the physics and the conditions of appearance of diurnal SST variations, the seminal work by Price et al. (1986) being the first detailed description of this phenomenon. Under low-wind conditions and with sufficient insolation, a stable near-surface layer forms during the day in the upper layers of the ocean (until the depth of O(10 m)) that leads to a surface warming of up to 5 K (see Wick and Castro (2020)). In the absence of solar radiation during the night, the stratification dissolves as vertical turbulent mixing takes overhand, until a homogeneous mixed layer is restored. The physics of this phenomenon is described in detail in a monograph by Soloviev and Lukas (2013). This stratified, warm layer is known as diurnal warm layer (DWL) and it is ubiquitous in all latitudes, causing SST fluctuations of 0.2 K or more in the entire Northern hemisphere and beyond during boreal summer (see Gentemann et al. (2003)). A comprehensive discussion of its definition and properties can be found in a review by Kawai and Wada (2007). In particular, the authors of the review point out that the presence of DWLs in observations as well as in single column simulations leads to stronger latent and sensible heat fluxes. As surface fluxes connect the surface to the atmospheric boundary layer and since changes in boundary layer properties affect the development of convection, the question of the impacts of DWLs on atmospheric convection arises.Investigating this question in models requires both fine enough vertical resolution in the ocean, to resolve DWLs, and fine enough horizontal grid spacing in the atmosphere, to resolve atmospheric convection. With the development of deca-to kilometer scale simulations in a coupled configuration (Hohenegger et al. ( 2023)) such investigations are becoming possible. Prominent among the newest studies are the papers by Voldoire et al. (2022) and Brilouet et al. (2021). In Voldoire et al. (2022), a single column coupled model has been considered, while
<p>The phenomenon of sea surface temperature (SST) anomalies created by oceanic diurnal warm layers has been extensively studied for the last decades, but the assessment of its importance for atmospheric convection has come within reach only very recently, thanks to the development of kilometre-scale simulations. We use the output of a global coupled simulation with a 5km horizontal grid spacing and near-surface ocean layers of order O(0.5m) to explicitly resolve both atmospheric convection and diurnal warm layers. As expected, the simulations produce daily SST fluctuations of up to several degrees. The increase of SST during the day causes an abrupt afternoon increase of atmospheric moisture due to enhanced latent heat flux. This increase is followed by an increase in cloud cover and cloud liquid water content. However, although the daily SST amplitude is exaggerated in comparison to reanalysis, the impact on cloud cover and cloud liquid water content only lasts for 5-6 hours. Moreover, the global daily average of these&#160;quantities is not influenced by their increase. All in all, we conclude that the global&#160;short-timescale impact of diurnal warm layers is negligible.</p>
This manuscript presents a study of oceanic diurnal warm layers in kilometer-scale global coupled simulations and their impact on atmospheric convection in the tropics. With the implementation of thin vertical levels in the ocean, diurnal warm layers are directly resolved, and sea surface temperature (SST) fluctuations of up to several Kelvin appear in regions with low wind and high solar radiation. The increase of SST during the day causes an abrupt afternoon increase of atmospheric moisture due to enhanced latent heat flux, followed by an increase in cloud cover and cloud liquid water. However, although the daily SST amplitude is exaggerated in comparison to reanalysis, this effect only lasts for 5-6 hours and leads to an absolute difference of 1% for cloud cover and 0.01 kg m-2 for cloud liquid water. All in all, the impact of diurnal warm layers on convective cloud cover is found to be negligible in the tropical mean.
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