Comparison of Direct and Telescopic Grid Nesting in a High-Resolution Operational Ocean Model

High-resolution regional ocean models are typically nested within coarser parent models, with resolution ratios limited to avoid spurious artefacts. Consequently, a telescopic grid-nesting strategy is commonly adopted for very high-resolution applications by introducing one or more models of intermediate resolution. However, such an approach is complex and computationally expensive. The purpose of this study is to assess the effectiveness of the recently developed Nesting with Data Assimilation (NDA) method in reducing the need for multi-stage telescopic nesting. The NDA method assimilates observations into the child model indirectly via the 3D outputs from the parent model, which in turn assimilates observations directly. Direct nesting of a 1/60° resolution child into a 1/12° parent model is compared with telescopic nesting via an intermediate 1/20° model in the Southeast Arabian Sea for the year 2016. Validation against observations demonstrates that, when combined with the NDA method, the directly nested model achieves accuracy comparable to, or better than, that of the double-nested configuration. Direct nesting eliminates the intermediate model entirely, reducing the complexity and cost of the modelling system while maintaining at least equivalent, or often better, performance. The findings suggest that an efficient assimilation technique can effectively replace intermediate dynamical downscaling steps in operational ocean forecasting, where reducing complexity is particularly valuable.