Implementation of a 3D ocean model to understand upland lake wind-driven circulation

L A Morales-Marín,J R French,H Burningham

doi:10.1007/s10652-017-9548-6

Abstract

A community numerical ocean model is used to extend the understanding of wind-driven circulation in small upland lakes. A 3D model of a case study lake (Llyn Conwy, Wales, UK) is calibrated against measured velocity profiles via adjustment of the bottom roughness coefficient. Validation against a separate set of measured velocity profiles confirms the ability of the model to resolve key features of the flow field. Sensitivity analysis shows that the velocity field responds rapidly to changes in the wind forcing. Analysis of the gross circulation using Empirical Orthogonal Functions reveals a persistent two-gyre circulation pattern in the upper half layer of the water column driven by the interaction of wind and bathymetry. At the bottom, the flow is characterised by locally strong currents and analysis of vertical circulation over short time scales shows strong currents in the deepest parts of the lake basin and the responsiveness of the water column to changes in wind speed and direction. Even in small lakes, the assumption of uniform wind stress across the water surface is not always justified and topographic sheltering or other catchment roughness effects give rise to heterogeneity in the wind field. An idealized experiment for the case study lake shows that differences in circulation emerge if the wind stress is allowed to vary across the lake. Energetic wind forcing in upland areas can drive an energetic lake circulation that has important implications for mixing and sediment dynamics. 3D numerical modelling of wind-driven circulation should be more widely used to provide insights into physical limnology to support a wide range of ecological, biogeochemical and palaeoenvironmental studies.

Highlights

Studies of the occurrence and size distribution of surface freshwater bodies have highlighted the importance of small lakes, especially those smaller than 1 km2 (e.g. [16, 45])
Analysis of the gross circulation using Empirical Orthogonal Functions reveals a persistent two-gyre circulation pattern in the upper half layer of the water column driven by the interaction of wind and bathymetry
The flow is characterised by locally strong currents and analysis of vertical circulation over short time scales shows strong currents in the deepest parts of the lake basin and the responsiveness of the water column to changes in wind speed and direction

Summary

Introduction

Studies of the occurrence and size distribution of surface freshwater bodies have highlighted the importance of small lakes, especially those smaller than 1 km (e.g. [16, 45]). Small lakes have disproportionately high hydrological and nutrient processing rates [62], greater diel temperature ranges [69] and their sheer number means that they are a significant contributor to global geochemical cycles and elemental budgets [24]. Despite their importance, many aspects of their physical limnology are still not adequately understood. Many aspects of their physical limnology are still not adequately understood This is especially true of small upland lakes, which can have complex hydrodynamic and thermal regimes on account of more energetic wind forcing than their lowland counterparts. The nature of their wind-driven circulation remains under-researched and this constitutes a knowledge gap that has implications for palaeoenvironmental as well as hydrological and biogeochemical studies

Methods

Results

Discussion

Conclusion