Abstract Link

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Abstract

Exploring the intricate relationships between land and marine processes is essential for a comprehensive understanding of climate dynamics. While contemporary coupled climate models have made significant progress in capturing various interactions, the explicit resolution of estuarine and intertidal processes remains a challenge. Building upon the foundation laid by the UKC3 UK national climate model, we present a novel perspective by incorporating a high-resolution (<20 m) flexible mesh model, Delft-3D, to specifically address intertidal and estuary regions. Our study focuses on the dynamic eastern Irish Sea, marked by hyper-tidal conditions and hosting eight estuaries alongside a significant intertidal zone. Employing a comprehensive comparison between the Delft model and the UKC3 model, we emphasize the simulation of extreme water heights during the winter storm season of 2013-2014. The outcomes provide valuable insights into the capabilities of both models in capturing high-water levels, paving the way for future investigations. Looking ahead, our research extends to incorporate the latest UKCP18 climate scenarios into the refined Delft model. This expansion allows us to explore potential variations in climate patterns and their implications for estuarine and coastal regions. The anticipated analysis aims to offer valuable insights into the impact of future climate change on these vital areas. As a final objective, I plan to parameterize estuarine processes within the UKC3 coupled system using an estuarine box model. This simplified approach holds promise in resolving coastal extremes and fluxes for impact studies, marking a crucial step towards enhancing the overall accuracy of climate models in portraying estuarine dynamics.

Background

Estuaries are fundamental features that link land to the sea. These systems are complex ecosystems, that are vital for economic function and provide a home for millions of people worldwide (ref). Therefore, it is important that we have a good understanding of these systems, such as through national environmental prediction models. One such system is the United Kingdom Coupled 4 (UKC4)(ref). These models have advanced our understanding of coastal and marine dynamics. However, these models often encounter limitations in fully capturing the intricacies of estuarine processes, particularly in regions characterised by hyper-tidal conditions, such as the eastern Irish Sea (ref).This study introduces a refined approach by incorporating the Delft-3D model, renowned for its hyper variable resolution (<20 m) flexible mesh capabilities, to generate a nested model for the analysis of a case study in the Morecambe Bay area which hosts several large estuaries. Unlike the UKC4, this new nested model includes wetting and drying as well as improving the resolution from 1.5km of the UKC4 model to better resolve the estuary. This will help us bridge this gap in estuarine dynamics. This model is named UKC4PRIMEA. Working within this framework of better improving national hazard models a more computationally effective scheme us suggested using Estuarine Box Models (EBM). These have been trialled at sites across the Mediterranean for their simplicity and improved capability of estuarine processes in large, complicated models which still only have freshwater discharges into ocean basins with no feedback. Both will then be applied with climate forcings looking forward to the future. This model is named UKC4EBM.

Methods

For the nested model a simulation was generated in Delft 3D Flexible Mesh Suite, with identical forcings except for the model grid. Due to this unique set up, the river boundary forcing condition placement moved from an ocean tile up towards locations where river gauges have been implemented. The ocean forcing is extracted using a cell-by-cell interface, so that no interpolation needs to be carried out between model grid resolutions. We use the UK’s 2013-2014 storm data extrapolated form the UKC4 model for simulations and analysis. For other runs improved riverine simulations have been run as shown by (ref), 15-minute river discharges are important in accurately predicting the freshwater influence, as well as accurately predicting flash flood and storm surge events. The estuarine box model (EBM) simulations involve a similar approach except instead of generating a new model framework that is disconnected form the fully coupled UKC4 I instead plan to couple the EBM directly into the NEMO framework of the modelling suite. The EBM will be built upon and adjusted to work better with the Met Office’s needs, the main file to be adjusted in the NEMO code is the sbcrnf.F90 which contains all the river inputs. In this analogy at present water is discharged at each river cell directly into the ocean taken from a river climatology. By adjusting the river component in this FORTRAN file, creating an estuary with the CMCC EBM code. These can be validated with tide gauges, ADCP’s and satellite data within the domain, with additional data such as a river logger deployed in the Kent Estuary, figure X, over the 2021-2023 period being used for calibration studies. 

Results

At present the results show that the UKC4PRIMEA model is performing better than its counterpart when comparing with measured data through validation, with RMSE of PRIMvsTide at 0.30m, compared to UKC4vsTide at 0.52m. Then again, the same for Liverpool with 0.63m and 0.93m respectively showing a 20-30cm improvement in these areas. In figure X, it can be seen when viewing a maximum surface height analysis over this model simulation period that for the confined area of Morecombe Bay, UKC4 is suggesting higher maximum surface heights yet in the Dee estuary the opposite occurs with the PRIMEA model suggesting a higher max surface height over the simulation period. 

Conclusions

It is seen that improving the resolution of grid cells goes as far as to improve the simulation of results for this period, as we know that estuaries are small scale features and cannot be accurately resolved if the grid cell size is identical to the mouth opening of if the ocean-river interaction which defines an estuary is ignored. 

More work needs to be carried out on the estuarine system to refine methods and further answer the questions proposed, and to find out if the more computationally effective method of the box model provides similar results that it can be justified in operational use for better predicting natural hazards. 

References

• Verri, G., Mahmoudi Kurdistani, S., Coppini, G., & Valentini, A. (2021). Recent Advances of a Box Model to Represent the Estuarine Dynamics: Time-Variable Estuary Length and Eddy Diffusivity. Journal of Advances in Modeling Earth Systems, 13(4). Link to article
• Sun, Q., Whitney, M. M., Bryan, F. O., & Tseng, Y. heng. (2017). A box model for representing estuarine physical processes in Earth system models. Ocean Modelling, 112, 139–153. https://doi.org/10.1016/j.ocemod.2017.03.004
• Lewis, H.W. and Dadson, S.J., 2021. A regional coupled approach to water cycle prediction during winter 2013/14 in the United Kingdom. Hydrological Processes, 35(12), p.e14438.
• Little, B, 2023. GeoVista GitHub Page. Website https://github.com/bjlittle/geovista. Date Accessed 02/10/2023
• Lowe, J. A., Bernie, D., Bett, P. E., Bricheno, L., Brown, S. J., Calvert, D., Clark, R. T., Eagle, K. E., Edwards, T., & Fosser, G. (2018). UKCP18 science overview report. Met Office Hadley Centre: Exeter, UK, 2018 (November 2018), 1–73. www.metoffice.gov.ukwww.metoffice.gov.uk
• Robins, P. E., Lewis, M. J., Freer, J., Cooper, D. M., Skinner, C. J., & Coulthard, T. J. (2018). Improving estuary models by reducing uncertainties associated with river flows. Estuarine, Coastal and Shelf Science, 207, 63–73. https://doi.org/10.1016/j.ecss.2018.02.015
• H. Lewis, J. Manuel Castillo Sanchez, A. Arnold, J. Fallmann, A. Saulter, J. Graham, M. Bush, J. Siddorn, T. Palmer, A. Lock, J. Edwards, L. Bricheno, A. Martínez-De La Torre, J. Clark, The UKC3 regional coupled environmental prediction system, Geosci. Model Dev. 12 (2019) 2357–2400. https://doi.org/10.5194/gmd-12-2357-2019.
• Lewis, H., Mittermaier, M., Mylne, K., Norman, K., Scaife, A., Neal, R., Pierce, C., Harrison, D., Jewell, S., Kendon, M., Saunders, R., Brunet, G., Golding, B., Kitchen, M., Davies, P., & Pilling, C. (2015). From months to minutes – exploring the value of high-resolution rainfall observation and prediction during the UK winter storms of 2013/2014. Meteorological Applications, 22(1), 90–104. https://doi.org/10.1002/met.1493
• EDINA Digimap Ordnance Survey Service (2023), https://digimap.edina.ac.uk
• UK National River Flow Archive (NRFA) (2020), https://nrfa.ceh.ac.uk
• Craig A., Valcke S., Coquart L., 2017: Development and performance of a new version of the OASIS coupler, OASIS3-MCT_3.0, Geoscientific Model Development, 10, pp. 3297-3308, (doi: https://doi.org/10.5194/gmd-10-3297-2017)