**Preface to "Multiscale, Multiphysics Modelling of Coastal Ocean Processes: Paradigms and Approaches"**

Climate change and increasing human activities have resulted in many ocean flow problems, which involve distinct physical processes across varying temporal and spatial scales and present grand challenges to our modeling capabilities. For instance, the 2010 BP oil spill in the Gulf of Mexico started as a fully three-dimensional, high-speed jet flow covering an area of 10 m. Later, it evolved into drifting patches of oil film on the water surface, expanding to hundreds of kilometers of coverage. Although both are fluid flows, the jet and patches comprise multiscale, multiphysics phenomena; they exhibit distinct physical behaviors at vastly different scales, and different sets of governing equations better describe them. More example problems include the interaction of land runoff and coastal water, compound flooding, and tsunami propagation and its impact on coastal structures. Currently, the state-of-the-art simulations for these phenomena (e.g., spill jet and oil patches), are made using wholly disjoint computer models (e.g., from the engineering and ocean science communities). However, these approaches encounter difficulties in appropriately handling such multiscale, multiphysics problems to meet the needs of scientific research and engineering practice. It has become critical to break through barriers and to integrally and simultaneously simulate these flow phenomena and their interactions.

Multiscale simulations of coastal ocean flows trace back to early mariners interested in determining optimal shipping routes owing to the impacts of permanent currents such as the Gulf Stream and changing sea states. Now, such simulations have become common practice within the ocean science community. However, this is not ye<sup>t</sup> the case for multiphysics simulations, which in general cannot be realized merely by local mesh refinement but needs to integrate different models. To attract the attention of communities and to promote further development, this Special Issue of the *Journal of Marine Science and Engineering* entitled "Multiscale, Multiphysics Modelling of Coastal Ocean Processes: Paradigms and Approaches" is a collection of papers on the simulation of multiscale, multiphysics coastal ocean flows. This collection provides perspectives on the status of such simulations, discussions related to current issues, and research ideas to further understand this field of science. This Special Issue covers coastal flooding, model assessment, effects of scales and wind fields, model coupling, parallel computation, and computational methods.

We thank all authors who kindly contributed their papers to this issue and the in-house editors from the *Journal of Marine Science and Engineering* for their kind help and co-operation. We are also indebted to the MDPI staff for their assistance in preparing and publishing this issue.

> **Hansong Tang, C. Reid Nichols, Donald T. Resio, Don Wright** *Editors*
