Density currents induced by differential cooling in lake

2020 ◽  
Author(s):  
Damien Bouffard ◽  
Cintia Ramón ◽  
Tomy Doda ◽  
Hugo Ulloa
Keyword(s):  
High Resolution ◽  
Numerical Simulations ◽  
Water Column ◽  
Lake Ecosystem ◽  
Density Current ◽  
Field Observations ◽  
Density Currents ◽  
Lake Surface ◽  
Littoral Region ◽  
Negatively Buoyant

<p>Differential cooling occurs in the littoral region of lakes, during calm and cold nights in summer and continuously in fall/ winter. For uniform heat loss over the lake surface, shallower regions cool faster than deeper regions leading to horizontal density gradients. Nearshore waters become negatively buoyant and start to plunge creating a cold downslope density current that can reach the pelagic zone. This cross-shore flow, also referred to as “thermal siphon”, has the potential to transport biogeochemical constituents offshore and deeper into the water column. However, its significance for the lake ecosystem remains unknown. Here we combine high resolution numerical simulations and field observations to evaluate the global significance of lateral boundaries and differential cooling on the lake dynamic. We focus on a small elongated lake: Lake Rotsee (Switzerland) and investigated the dynamic of the density current as well as the consequences of this flow such as the flushing of the littoral water.</p>

scholarly journals Direct numerical simulations of particle-laden density currents with adaptive, discontinuous finite elements

2014 ◽  
Vol 7 (3) ◽  
pp. 3219-3264 ◽  
Author(s):  
S. D. Parkinson ◽  
J. Hill ◽  
M. D. Piggott ◽  
P. A. Allison
Keyword(s):  
High Resolution ◽  
Numerical Simulations ◽  
Turbidity Current ◽  
Direct Numerical Simulations ◽  
Three Dimensions ◽  
Turbidity Currents ◽  
Normal Velocity ◽  
Density Currents ◽  
Conservation Of Energy ◽  
Computationally Expensive

Abstract. High resolution direct numerical simulations (DNS) are an important tool for the detailed analysis of turbidity current dynamics. Models that resolve the vertical structure and turbulence of the flow are typically based upon the Navier–Stokes equations. Two-dimensional simulations are known to produce unrealistic cohesive vortices that are not representative of the real three-dimensional physics. The effect of this phenomena is particularly apparent in the later stages of flow propagation. The ideal solution to this problem is to run the simulation in three dimensions but this is computationally expensive. This paper presents a novel finite-element (FE) DNS turbidity current model that has been built within Fluidity, an open source, general purpose, computational fluid dynamics code. The model is validated through re-creation of a lock release density current at a Grashof number of 5 × 106 in two, and three-dimensions. Validation of the model considers the flow energy budget, sedimentation rate, head speed, wall normal velocity profiles and the final deposit. Conservation of energy in particular is found to be a good metric for measuring mesh performance in capturing the range of dynamics. FE models scale well over many thousands of processors and do not impose restrictions on domain shape, but they are computationally expensive. Use of discontinuous discretisations and adaptive unstructured meshing technologies, which reduce the required element count by approximately two orders of magnitude, results in high resolution DNS models of turbidity currents at a fraction of the cost of traditional FE models. The benefits of this technique will enable simulation of turbidity currents in complex and large domains where DNS modelling was previously unachievable.

scholarly journals The Intrusion Depth of Density Currents Flowing into Stratified Water Bodies

2009 ◽  
Vol 39 (8) ◽  
pp. 1935-1947 ◽  
Author(s):  
Mathew Wells ◽  
Parthiban Nadarajah
Keyword(s):  
Water Column ◽  
Large Scale ◽  
Laboratory Experiments ◽  
Buoyancy Flux ◽  
Density Current ◽  
Density Currents ◽  
Coriolis Parameter ◽  
Entrainment Ratio ◽  
Geostrophic Balance ◽  
Intrusion Depth

Abstract Theory and laboratory experiments are presented describing the depth at which a density current intrudes into a linearly stratified water column, as a function of the entrainment ratio E, the buoyancy flux in the dense current B, and the magnitude of the stratification N. The main result is that Z ∼ E−1/3B1/3/N. It is shown that the depth of the intrusion scales as Z ∼ (3 ± 1)B1/3/N for laboratory experiments, and as for oceanic density currents. The velocity of a large-scale density current is controlled by a geostrophic balance defined as Ugeo = 0.25g′s/f, where s is the slope and f is the Coriolis parameter. The geostrophic buoyancy flux is then defined by Bgeo = g′Ugeoh, with g′ the reduced gravity and h the thickness of the current. The scaling herein implies that the depth of an oceanic intrusion is relatively insensitive to changes in source water properties but is very sensitive to changes in the stratification of the water column, consistent with the previous scaling of Price and Baringer. For example, if the buoyancy flux of a dense current were to double while the stratification remained constant, then there would only be a 25% increase in the intrusion depth, whereas doubling the stratification would result in a 50% decrease of the intrusion depth.

scholarly journals Seasonality of density currents induced by differential cooling

2021 ◽  
Author(s):  
Tomy Doda ◽  
Cintia L. Ramón ◽  
Hugo N. Ulloa ◽  
Alfred Wüest ◽  
Damien Bouffard
Keyword(s):  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Late Summer ◽  
Global Scale ◽  
Lake Ecosystem ◽  
Density Currents ◽  
Surface Cooling ◽  
Lateral Transport ◽  
Littoral Region ◽  
Cooling Phase

Abstract. When lakes experience surface cooling, the shallow littoral region cools faster than the deep pelagic waters. The lateral density gradient resulting from this differential cooling can trigger a cold downslope density current that intrudes at the base of the mixed layer during stratified conditions. This process is known as thermal siphon (TS). TS flushes the littoral region and increases water exchange between nearshore and pelagic zones, with possible implications on the lake ecosystem. Past observations of TS in lakes are limited to specific cooling events. Here, we focus on the seasonality of the TS-induced lateral transport and investigate how the seasonally varying forcing conditions control the occurrence and intensity of TS. We base our analysis on one year of observations of TS in Rotsee (Switzerland), a small wind-sheltered temperate lake composed of an elongated shallow region. We demonstrate that TS occurs for more than 50 % of the days from late summer to winter and efficiently flushes the littoral region in ~10 hours. We further quantify the seasonal evolution of the occurrence, intensity and timing of TS. The conditions for the formation of TS are optimal in autumn, when the duration of the cooling phase is longer than the initiation timescale of TS. The decrease in surface cooling by one order of magnitude from summer to winter reduces the lateral transport by a factor of two. We interpret this transport seasonality with scaling relationships relating the daily averaged cross-shore velocity, unit-width discharge and flushing timescale to the surface buoyancy flux, mixed layer depth and lake bathymetry. The timing and duration of the diurnal flushing by TS are associated with the duration of the daily heating and cooling phases. The longer cooling phase in autumn increases the flushing duration and delays the time of maximal flushing, compared to the summer period. Our findings based on scaling arguments can be extended to other aquatic systems to assess, at a global scale, the relevance of TS in lakes and reservoirs.

scholarly journals Development and evaluation of a high-resolution reanalysis of the East Australian Current region using the Regional Ocean Modelling System (ROMS 3.4) and Incremental Strong-Constraint 4-Dimensional Variational (IS4D-Var) data assimilation

2016 ◽  
Vol 9 (10) ◽  
pp. 3779-3801 ◽  
Author(s):  
Colette Kerry ◽  
Brian Powell ◽  
Moninya Roughan ◽  
Peter Oke
Keyword(s):  
High Resolution ◽  
Data Assimilation ◽  
Water Column ◽  
Marked Improvement ◽  
Sea Surface ◽  
East Australian Current ◽  
Strong Constraint ◽  
Ocean Modelling ◽  
Model Dynamics ◽  
Modelling System

Abstract. As with other Western Boundary Currents globally, the East Australian Current (EAC) is highly variable making it a challenge to model and predict. For the EAC region, we combine a high-resolution state-of-the-art numerical ocean model with a variety of traditional and newly available observations using an advanced variational data assimilation scheme. The numerical model is configured using the Regional Ocean Modelling System (ROMS 3.4) and takes boundary forcing from the BlueLink ReANalysis (BRAN3). For the data assimilation, we use an Incremental Strong-Constraint 4-Dimensional Variational (IS4D-Var) scheme, which uses the model dynamics to perturb the initial conditions, atmospheric forcing, and boundary conditions, such that the modelled ocean state better fits and is in balance with the observations. This paper describes the data assimilative model configuration that achieves a significant reduction of the difference between the modelled solution and the observations to give a dynamically consistent “best estimate” of the ocean state over a 2-year period. The reanalysis is shown to represent both assimilated and non-assimilated observations well. It achieves mean spatially averaged root mean squared (rms) residuals with the observations of 7.6 cm for sea surface height (SSH) and 0.4 °C for sea surface temperature (SST) over the assimilation period. The time-mean rms residual for subsurface temperature measured by Argo floats is a maximum of 0.9 °C between water depths of 100 and 300 m and smaller throughout the rest of the water column. Velocities at several offshore and continental shelf moorings are well represented in the reanalysis with complex correlations between 0.8 and 1 for all observations in the upper 500 m. Surface radial velocities from a high-frequency radar array are assimilated and the reanalysis provides surface velocity estimates with complex correlations with observed velocities of 0.8–1 across the radar footprint. A comparison with independent (non-assimilated) shipboard conductivity temperature depth (CTD) cast observations shows a marked improvement in the representation of the subsurface ocean in the reanalysis, with the rms residual in potential density reduced to about half of the residual with the free-running model in the upper eddy-influenced part of the water column. This shows that information is successfully propagated from observed variables to unobserved regions as the assimilation system uses the model dynamics to adjust the model state estimate. This is the first study to generate a reanalysis of the region at such a high resolution, making use of an unprecedented observational data set and using an assimilation method that uses the time-evolving model physics to adjust the model in a dynamically consistent way. As such, the reanalysis potentially represents a marked improvement in our ability to capture important circulation dynamics in the EAC. The reanalysis is being used to study EAC dynamics, observation impact in state-estimation, and as forcing for a variety of downscaling studies.

scholarly journals Rip current evidence by hydrodynamic simulations, bathymetric surveys and UAV observation

2017 ◽  
Author(s):  
Guido Benassai ◽  
Pietro Aucelli ◽  
Giorgio Budillon ◽  
Massimo De Stefano ◽  
Diana Di Luccio ◽  
...  
Keyword(s):  
Grain Size ◽  
High Resolution ◽  
Numerical Simulations ◽  
Current System ◽  
Current Evidence ◽  
Rip Current ◽  
Rip Currents ◽  
Transport Pathways ◽  
Fine Sediments ◽  
Scientific Challenge

Abstract. The prediction of the formation, spacing and location of rip currents is a scientific challenge that can be achieved by means of different complementary methods. In this paper the analysis of numerical and experimental data, including UAV observation, allowed to detect the presence of rip currents and rip channels at the mouth of Sele river, in the Gulf of Salerno, southern Italy. The dataset used to analyze these phenomena consisted of two different bathymetric surveys, a detailed sediment 5 analysis and a set of high-resolution wave numerical simulations, completed with satellite and UAV observation. The grain size trend analysis and the numerical simulations allowed to identify the rip current system, forced by topographically constrained channels incised on the seabed, which were detected by high resolution bathymetric surveys. The study evidenced that on the coastal area of the Sele mouth grain-size trends are controlled by the contribution of fine sediments, which exhibit suspended transport pathways due to rip currents and longshore currents. The results obtained were confirmed by satellite and UAV 10 observations in different years.

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