Preface – special issue on “coupled atmosphere‐hydrological processes: Novel system developments and cross‐compartment evaluations”

Water and energy fluxes between the land surface and the atmospheric boundary layer entail complex interactions (Betts et al., 1996; Fisher & Koven, 2020). Comprehensive process-based modelling of all major intertwined fluxes, related non-linear feedbacks and scale-dependent properties is a cutting-edge approach to advance the understanding of the interaction of water with both the atmospheric and the land surface system, which requires compartment-crossing strategies. The need for dynamically coupling different model compartments was foreseen already decades ago and grew parallel with increasing technological opportunities, like high-performance computing advances, development of efficient couplers and the availability of comprehensive earth observation datasets. Eagleson (1986) already claimed that the solution to multiple hydrological problems at the local scale requires “global scale dynamic modelling of the coupled ocean-atmosphere-land surface”. Twenty years later, Beven (2007) suggested that dynamic “coupling between atmospheric forcing, catchment response, river runoff and coastal interaction with tidally dominated sea levels” can be crucial for flood protection. Such increasing awareness led hydrologists to overcome previous disciplinary boundaries and decisively become intimate with atmospheric sciences. Today, coupled atmosphere-hydrological models address the challenge by describing the waterand energy cycle from groundwater across the land surface to the top of the atmosphere. The new simulation paradigm investigates the dynamic feedback between land and atmosphere by solving adapted forms of the three-dimensional equations governing both compartments (e.g., Butts et al., 2014; Gochis et al., 2020; Maxwell et al., 2011; Shrestha et al., 2014; Wang et al., 2018). These integrated modelling systems enable new research on coupled hydrologic and meteorological processes. They also promise to be particularly useful for adaptation and resilience to climate change challenges, which often arise as complex compound events (Zscheischler et al., 2018) that are not interpretable through traditional mono-disciplinary approaches. Though critical bottlenecks remain, current research has identified some essential feedbacks due to coupling, such as the sensitivity of precipitation patterns to resolved overland flow (e.g., Arnault et al., 2016) or the connection of water-table depth fluctuations to the land surface and atmospheric parameterizations (e.g., Davison et al., 2018; Sulis et al., 2017). In general, the potential of coupled atmosphere-hydrological modelling approaches for a unified simulation of the regional water cycle has been demonstrated previously (e.g., Fersch et al., 2020; Forester et al., 2018; Lahmers et al., 2019; Senatore et al., 2015; Xiang et al., 2017). Such systems are proposed as the backbone of more complex Earth System models, aimed at making it possible to study the coupled dynamics of the land biosphere and the climate (e.g., Martín Belda et al., 2022; Sellar et al., 2019). This special issue in Hydrological Processes aims to gather some of the latest system developments of atmospheric-hydrological coupled approaches, reporting examples and tools for cross-compartment and multi-variable validation, and discussing the challenges of complex interactions between surface water, groundwater, land surface processes and regional climate. It is made up of 11 papers authored by 46 researchers belonging to 23 institutions. The topics addressed are manifold, as the word-cloud achieved with all papers' abstracts shows (Figure 1). Different criteria could have been adopted to group them into coherent sets, for example, forecast/reanalysis, spatial extent, length of the modelled period, and ensemble/no ensemble. However, according to the aims of this special issue, the grouping criterion selected for their systematic presentation refers to the main coupled processes analysed (Table 1). Therefore, 4 papers out of 11 focus on providing quantitative information about the subsurface-surfaceatmosphere continuum feedbacks; three mainly explore further coupling with the ocean (i.e., ocean-atmospheric-hydrologic modelling); two relate to the effects on the water cycle of groundwater dynamics even in complex systems with not negligible human impact (reservoir regulation); and the other two focus on snow processes. Beyond this partition, most papers describe the effect of different modelling systems on representing the primary hydrological variable, that is, streamflow. In the next section, all the contributions are briefly introduced according to the chosen aggregation criterion before concluding with some remarks looking at future challenges and perspectives.