A Bayesian data assimilation framework for lake 3D hydrodynamic models with a physics-preserving particle filtering method using SPUX-MITgcm v1

We present a Bayesian inference for a threedimensional hydrodynamic model of Lake Geneva with stochastic weather forcing and high-frequency observational datasets. This is achieved by coupling a Bayesian inference package, SPUX, with a hydrodynamics package, MITgcm, into a single framework, SPUX-MITgcm. To mitigate uncertainty in the atmospheric forcing, we use a smoothed particle Markov chain Monte Carlo method, where the intermediate model state posteriors are resampled in accordance with their respective observational likelihoods. To improve the uncertainty quantification in the particle filter, we develop a bi-directional long short-term memory (BiLSTM) neural network to estimate lake skin temperature from a history of hydrodynamic bulk temperature predictions and atmospheric data. This study analyzes the benefit and costs of such a stateof- the-art computationally expensive calibration and assimilation method for lakes.Swiss Data Science Center (SDSC) DATALAKES C17-17Eawag Discretionary Fundin

Flushing the Lake Littoral Region: The Interaction of Differential Cooling and Mild Winds

The interaction of a uniform cooling rate at the lake surface with sloping bathymetry efficiently drives cross-shore water exchanges between the shallow littoral and deep interior regions. The faster cooling rate of the shallows results in the formation of density-driven currents, known as thermal siphons, that flow downslope until they intrude horizontally at the base of the surface mixed layer. Existing parameterizations of the resulting buoyancy-driven cross-shore transport assume calm wind conditions, which are rarely observed in lakes and thereby restrict their applicability. Here, we examine how moderate winds (≲5 m s −1) affect this convective cross-shore transport. We derive simple analytical solutions that we further test against realistic three-dimensional numerical hydrodynamic simulations of an enclosed stratified basin subject to uniform and steady surface cooling rate and cross-shore winds. We show cross-shore winds modify the convective circulation, stopping or even reversing it in the upwind littoral region and enhancing the cross-shore exchange in the downwind region. The analytical parameterization satisfactorily predicted the magnitude of the simulated offshore unit-width discharges in the upwind and downwind littoral regions. Our scaling expands the previous formulation to a regime where both wind and buoyancy forces drive cross-shore discharges of similar magnitude. This range is defined by the non-dimensional Monin-Obukhov length scale, χMO: 0.1 ≲ χMO ≲ 0.5. The information needed to evaluate the scaling formula can be readily obtained from a traditional set of in situ observations.Swiss National Science Foundation (SNSF) European Commission 175919ETH-Bereich Forschungsanstalte

Development of overturning circulation in sloping waterbodies due to surface cooling

This work was supported by the Swiss National Science Foundation (project Buoyancy driven nearshore transport in lakes, HYPOlimnetic THErmal SIphonS, HYPOTHESIS, reference 175919) and by the Physics of Aquatic Systems Laboratory (APHYS), EPFL.Cooling the surface of freshwater bodies, whose temperatures are above the temperature of maximum density, can generate differential cooling between shallow and deep regions. When surface cooling occurs over a long enough period, the thermally induced cross-shore pressure gradient may drive an overturning circulation, a phenomenon called ‘thermal siphon’. However, the conditions under which this process begins are not yet fully characterised. Here, we examine the development of thermal siphons driven by a uniform loss of heat at the air–water interface in sloping, stratified basins. For a two-dimensional framework, we derive theoretical time and velocity scales associated with the transition from Rayleigh–Bénard type convection to a horizontal overturning circulation across the shallower sloping basin. This transition is characterised by a three-way horizontal momentum balance, in which the cross-shore pressure gradient balances the inertial terms before reaching a quasi-steady regime. We performed numerical and field experiments to test and show the robustness of the analytical scaling, describe the convective regimes and quantify the cross-shore transport induced by thermal siphons. Our results are relevant for understanding the nearshore fluid dynamics induced by nighttime or seasonal surface cooling in lakes and reservoirs.Swiss National Science Foundation (SNSF) European Commission 175919Physics of Aquatic Systems Laboratory (APHYS), EPF

Fate of Artificially Injected Oxygen in the Hypolimnion of a Two-Basin Lake: Amisk Lake, revisited

The first author visited Texas A&M University funded by U.S. National Science Foundation grant CBET 1033514. It was during that visit that, under the supervision of Scott Socolofsky, the double-plume model was implemented into the 3D hydrodynamic model. Meteorological data for Atmore AGDM station was provided by the Alberta Climate Information Service, found at https://acis.alberta.ca. The data displayed in the figures can be accessed at https://zenodo.org/record/4565311.Bubble-plume diffusers are increasingly used to add dissolved oxygen (DO) to the hypolimnion of lakes and reservoirs. Bubble plumes are successful at replenishing hypolimnetic DO, but they also introduce mixing energy that induces subtle changes in the thermal structure of the reservoir, driving changes in plume behavior. To account for this complex plume-reservoir interaction, a double bubble-plume model is coupled with a three-dimensional hydrodynamic model. The coupled model is used to reassess a field-scale analysis of the bubble-plume diffuser in two-basin Amisk Lake, aiming at evaluating the relative role of bubble-induced circulation and internal-seiching in driving inter-basin transport under stratified conditions. A large-scale plume-induced circulation was previously thought to be the main driver of inter-basin oxygen transport. This interpretation was based on the attribution of the time-averaged circulation in the channel due to plume operation. However, the intrinsic complexity of the hydraulic system and the sparseness of the field data introduced large uncertainties in the previous analysis. Here, we demonstrate that the time-averaged circulation is primarily the result of wind-driven internal seiches. Oxygen exchange is shown to be controlled by the interaction between internal seichedriven horizontal transport along the channel, and, the rate at which added oxygen reaches the layers above the sill, which is mainly controlled by plume-induced circulation. Internal-seiche driven transport through basin constrictions will vary depending on the magnitude of the wind forcing, depth of the thermocline and the channel geometry. These results highlight the importance of understanding water movement prior to introducing restoration actions in lakes.National Science Foundation (NSF) CBET 103351

Hidrodinámica y mezcla en la confluencia de ríos: Influencia de los contrastes de densidad y de la marea

River confluences are critical points in river networks where strong physical and chemical gradients develop, resulting in a wide range of distinctive environmental conditions (habitats) for biological growth. Large variations in water temperatures, organic matter, nutrients, for example, and in general, in water chemistry have been reported to occur at these sites. As a consequence of their high spatial and temporal heterogeneity of habitats and resources, river confluences behave as biological hotspots, where the number of species appears to increase very significantly in comparison with other river reaches. The effects of river confluences persist downstream, therefore, affecting biological communities and ecological processes at scales of river reaches and channel networks. The spatial extent of the reaches downstream of river junctions where heterogeneous habitat conditions persist, largely depends on the rate at which mixing between the mainstream and tributary waters occurs. The literature on mixing in river confluences is extensive, but still, our understanding of flow and mixing dynamics in these sites is far from complete. In particular, the effect of density contrast between the confluent streams on mixing has traditionally been neglected, which has been justified by differences in the inertia of the confluent flows being much higher than density differences. However, as the scale of the confluent channels increases, the probability of draining different geological terrains also increases which results in an increasing potential for significant differences in density. In this work, we present results of a series of field experiments carried out in a confluence in Northern Spain where the presence of density contrast is important for both the spatial arrangement of the rivers once at the confluence and river mixing.Tesis Univ. Granada. Programa Oficial de Doctorado en: Ciencias de la TierraWork on the confluence between the Ebro and Segre rivers was funded through a collaborative agreement between the University of Barcelona and the University of Granada to work jointly in the project “Gestión hidráulica y técnicas de detección remota aplicada al control de poblaciones mejillón cebra: el caso del embalse de Ribarroja y el tramo inferior del río Ebro”, funded by the Spanish Ministry of the Environment. Work on the Sacramento River was funded through the project “North Delta Salmon Out-migration Study”, funded by the United States Department of Interior ― Bureau of Reclamation (USBR), solicitation No. 09SS200013. The PhD student was supported by a PhD grant (Programa Estatal de Promoción del Talento y su Empleabilidad, subprograma de Formación de Profesorado Universitario) from the Spanish Government

Seasonality of density currents induced by differential cooling

This study was financed by the Swiss National Science Foundation ("Buoyancy driven nearshore transport in lakes" project; HYPOlimnetic THErmal SIphonS, HYPOTHESIS, grant no. 175919).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 a thermal siphon (TS). TSs flush the littoral region and increase water exchange between nearshore and pelagic zones; thus, they may potentially impact the lake ecosystem. Past observations of TSs in lakes are limited to specific cooling events. Here, we focus on the seasonality of TS-induced lateral transport and investigate how seasonally varying forcing conditions control the occurrence and intensity of TSs. This research interprets 1-year-long TS observations from Rotsee (Switzerland), a small wind-sheltered temperate lake with an elongated shallow region. We demonstrate that TSs occur for more than 50 % of the days from late summer to winter and efficiently flush the littoral region within similar to 10 h. We further quantify the occurrence, intensity, and timing of TSs over seasonal timescales. The conditions for TS formation become optimal in autumn when the duration of the cooling phase is longer than the time necessary to initiate a TS. The decrease in surface cooling by 1 order of magnitude from summer to winter reduces the lateral transport by a factor of 2. 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 diurnal flushing by TSs relate to daily heating and cooling phases. The longer cooling phase in autumn increases the flushing duration and delays the time of maximal flushing relative to the summer diurnal cycle. Given their scalability, the results reported here can be used to assess the relevance of TSs in other lakes and reservoirs.Swiss National Science Foundation (SNSF) European Commission 17591

Penetrative Convection Modifies the Dynamics of Downslope Gravity Currents

Gravity currents contribute to the transport of heat and mass in atmospheric and aquatic environments. In aquatic systems subject to daily surface cooling, gravity currents propagate through turbulent convective surroundings. Yet, the effects of thermal convection on aquatic gravity currents remain to be quantified. This paper demonstrates how the interaction between penetrative convection and downslope gravity currents impacts the fluid dynamics and transport across littoral aquatic systems. We performed field experiments in a wind-sheltered lake experiencing differential cooling to resolve the dynamics of thermally driven gravity currents in convective environments. Our in situ observations reveal that convective plumes penetrate gravity currents, generating large vertical fluctuations that foster the erosion of the stratified layer. This enhanced vertical mixing destroys the stratified downslope flow and limits the basin-scale transport. Our results demonstrate that the interaction between penetrative convection and downslope gravity currents controls the littoral-pelagic connectivity in aquatic ecosystems.Swiss National Science Foundation (SNSF) 17591