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Project titleThe investigation of small-scale processes of convective character in the near-surface layer of the coastal zone of the sea

Research area 07 - EARTH SCIENCES, 07-505 - Turbulence and small-scale processes

Keywordsfree convection, negative buoyancy, near-shore waters, near-slope water dynamics, autumn-winter cooling from the surface, mixing processes, small-scale water dynamics, non-stationarity, laboratory experiment, along-slope current, numerical model, South-East Baltic, pollution transport

Annotation
The project is devoted to the study of one of the fields of free convection research, namely, the study of small-scale dynamics and mixing processes that occur during autumn-winter cooling or early spring warming in the surface layer of coastal waters of the sea. During the autumn-winter cooling of water in the near-surface layer of water, a thin boundary layer is formed with a temperature lower than that of the underlying waters. This layer is the initial stage of the formation of a negative buoyancy flow into the underlying sea waters. As it turned out, the penetration of colder, and therefore with a higher density of waters, can occur in various forms. The main goal of the project will be to study various small-scale forms of penetration of waters with negative buoyancy into the underlying waters. Previously performed by the team laboratory and numerical studies of the dynamics of the near-surface layer of coastal waters of the sea of convective nature made it possible to study the details of the behavior of individual spots of salt water surrounded by fresh water. However, the stage of origin and subsequent evolution of water spots with negative buoyancy in the surface layer of water during its cooling from the surface was not considered and reproduced in laboratory experiments. In calculations on the model, individual elements of this process were reproduced. In particular (Chubarenko, Gritsenko, 2006), numerical modeling allows to obtain the phenomenon of the formation of a near-surface layer by the negative buoyancy of its waters in the near-shore zone of the sea and, at the same time, the initiation of a along-slope density current, when some of the thermals that reached the bottom slope were combined into a layer. However, this direction of model calculations was not developed further. The project is supposed to use a combination of laboratory and numerical experiments, as well as statistical methods for processing natural data. In particular, it is planned to study the formation of a random ensemble of thermals under conditions of a change in the thickness of the near-surface boundary layer and the values of the density difference, as well as the process of active interaction of individual thermals with each other when they are immersed in the underlying waters. The fundamental novelty of the proposed research will be the use of a system of tracers both for tinting the waters in the flume and for their identification in the calculations. Laboratory and numerical experiments will be supplemented by the analysis of observations of the temperature of the surface water layer of the South-East Baltic (data from the thermal streamer: 10 sensors from the surface to 28 m, 20 km from the coast, 3 years of observations in 1 min). This allows to identify periods favorable for the development of convective processes in the coastal zone of the South-East Baltic. A description of the main stages of the generation of a random ensemble of water spots with negative buoyancy in the near-surface water layer and details of their behavior will be obtained, during performing experiments. The joint use of the results of visual observation of the behavior of laboratory flows and the detail of possible numerical estimates in model calculations with close characteristic scales of flows can supplement the understanding of the processes of subsidence of finite volumes of salt water from the surface of fresh water and their interaction with each other. The combination of experiments in the flume and on a numerical model with the analysis of natural conditions in the coastal waters of the South-East Baltic allows to add the understanding of small-scale processes occurring in the near-surface water layer. All the above circumstances make obvious the relevance of the implementation of the program planned in the project for the study of small-scale processes of a convective nature occurring in a thin surface layer of the waters of the World Ocean.

Expected results
As a result of the project implementation, knowledge of small-scale dynamics and water mixing processes that occur under seasonal convection conditions in the Baltic Sea will be expanded. Laboratory experiments allows to describe the stages of the formation of a stochastic ensemble of spots and the scheme of interaction between individual small volumes of waters of negative buoyancy in the near-surface layer of the sea, which arise during its cooling in the coastal zone. Verification of the series obtained during the experiments, photographs will help to quantify the approximate speed and volume of immersing spots, as well as to identify the features of the origin of spots, depending on the conditions specified in the near-surface water layer. Model calculations on a nonlinear two-dimensional model of the dynamics of a stratified fluid will provide new results for the team, generalizing experiments with different initial and boundary conditions into a single picture of small-scale dynamics and mixing processes of waters that are inhomogeneous in density in the near-surface layer. Fundamentally new for the team will be the results of a study of seasonal convection in the coastal zone of the sea through the analysis of time series from a thermospit (10 horizons, three-year measurements in 1 min), located 20 km from the coast of the Baltic Sea on platform D6. Field observations of the Laboratory of Marine Physics (AB IO RAS) in the coastal zone will make it possible to verify the significance of the results of laboratory and numerical experiments in the general process of variability in the state of coastal waters. Significant for the development of new scientific topics will be an understanding of the physics of the process of vertical dynamics of coastal and coastal waters, as well as a complete description of the dynamics of waters and the degree of their possible cleaning through the formation of cross-slope water exchange of deep and coastal waters. Thus, the results of the work can be included in the strategy of rational nature management for the coastal zone, and will also make it possible to give practical suggestions when planning water protection measures for the South-East Baltic.

Annotation of the results obtained in 2023
The aim of the project is to study peculiarities of free convection process: small-scale dynamics of near-surface waters and associated mixing processes that occur under conditions of autumn-winter cooling of the surface layer of coastal sea waters. An investigation of the forming of convective elements (patches, thermals) in a near-surface boundary layer with a hydrostatically unstable density distribution were carried out. The process of interaction of individual convective elements, deepening in the underlying waters, was analyzed. The laboratory experiments and numerical modeling with similar characteristics allows to describe the main stages of the formation, an interaction and features of processes of deepening of a random set of convective elements. A field data on water temperature variability in the coastal waters of the south-eastern Baltic was analyzed. Laboratory experiments, using the hydroflume, shown that the flow of negative buoyancy into the underlying waters deepens as a random set of convective elements (thermals or patches). Each convective element is a small volume of water with negative buoyancy. Convective element mixes with the surrounding water under the influence of vortices of a baroclinic nature and molecular diffusion. The vortex nature of the flow is determined by the rate of dominance of the entrainment of surrounding water over the molecular diffusivity and leads to interlayering of the density field near convective element. Analysis of the laboratory flows of deepening convective elements in a hydroflume and numerical experiments allows to identify and characterize a new stage (the advective-vortex nature) of transformation of their waters. The obtained result made it possible to propose a new interpretation of the presence of fluctuations in the density (temperature) field noted in other scientific publications. The appearance of fluctuations in the density field can be explained by the presence of many high-gradient layers during the interpenetration of waters of convective elements with the surrounding water. The relation between the process of forming of convective elements and the thickness of the near-surface layer of water shows the main features of the propagation of many convective elements into the underlying waters. Observations of laboratory flows have shown that the formation of prototypes of future convective elements occurs in the form of local deflections of isopycnals. After the start of deepening, vorticity of a baroclinic nature at the outer lateral boundaries leads to the formation of mushroom shape of convective elements. This issue of research will continue in the next year of the project. Empirical estimates of the vertical deepening rate of an ensemble of convective elements showed that the average speed of deepening of their density front is in the range from 0.1 mm/s to 0.5 mm/s. The local deflections of isopycnals at the lower boundary of the density front ranged from ~4.7 number per centimeter at the initial stage of formation (8-10 seconds from the beginning) to ~5.6 number per centimeter at the stage (about 23-25 seconds from the beginning) of the development of intense convective movements. This issue of research will be continued. Calibration of numerical modeling was done using a new calculation condition for upper boundary of the model space. This boundary condition allows to implement horizontal movements on the free surface. The water cooling process is parametrized by adding a random small additive density for the upper layers of the computational grid. Calculation of the density structure of the calculated flows using an additional set of flow parameters (density field gradients vertically and horizontally, along and across streamlines, as well as histograms of the values of these gradients) made it possible to analyze the effect of interlayering of the density field for model flows. Analysis of experimental data on the deepening of individual convective elements (with a volume of 2 - 3 ml) using serial photography (1.78 - 10 Hz), made it possible to obtain estimates of their vertical deepening rate - from 0.2 to 0.5 cm/s (for different initial values of volume and negative buoyancy difference). The rapid increase in the deepening rate begins immediately after the formation of a convective element. At the stage of development of a vortex structure of the flow causes to rapid deceleration in speed (~time-2). At the viscous stage, the vertical deepening rate and its variability were small. The features of the interpenetration of convective elements into each other during their deepening were analyzed. The experiments made it possible to identify weak and active types of their interaction. Photographs of individual phases of the movement of convective elements state the laminar nature of the deepening flows. The non-symmetry interaction was observed in most experiments. Dynamic (quasi-wave) interaction of convective elements without interpenetration of waters was observed for symmetry case of the initial position and parameters. The study of this section will continue. Analysis of the measured values of sea water temperature (data from the thermocouple on the oil production platform D-6, SE part of the Baltic Sea) and air temperature for 2016–2020 showed that the nature of the temporal variability of the process of cooling of coastal waters can be divided into typical and anomalous. Under typical regime, cooling to late-autumn temperatures occurs over a period of 1.5–2 months (7 weeks in 2018), and under anomalous regime (in 2016), cooling occurred in 7–8 days. Trends in the cooling rate of water from the surface under typical conditions were –0.11 ºС/day and –0.22 ºС/day under anomalous conditions. In an anomalous regime, a cold and strong wind from the coast leads to the intensive development of convection. An analysis of the Rayleigh number values for 2016 confirmed that the critical value had been exceeded and had arisen condition for the development of thermogravitational convection. Thus, all the expected scientific results for the first year of the project were obtained. The results of individual stages of research were presented at scientific conferences and seminars. The results were published in peer-reviewed journals.

Publications

1. Kupriyanova A.E., Gritsenko V.A. Experimental Estimates of the Rate of Deepening of Individual Thermals Processes in GeoMedia—Volume VII. Springer Geology. Springer, Singapore., Volume VII. Springer Geology. P. 139–152 (year - 2023) https://doi.org/10.1007/978-981-99-6575-5_13

2. Kupriyanova A.E., Gritsenko V.A. Бароклинный фактор в смешении воды погружающегося термика с окружающей его водой Фундаментальная и прикладная гидрофизика, Т. 16, № 4 (year - 2023)

3. Korobchenkova K.D., Kupriyanova A.E., Kileso A.V. Гидрометеорологические условия выхолаживания поверхностных вод юго-восточной части Балтийского моря Гидрометеорология и физика атмосферы: современные достижения и тенденции развития : Материалы Всероссийской научно-практической конференции, Санкт-Петербург, 21–23 марта 2023 года, С. 255-258 (year - 2023)

4. Kupriyanova A.E., Gritsenko V.A. Лабораторные исследования приповерхностной конвекции Современные методы и средства океанологических исследований (МСОИ-2023) : Материалы XVIII международной научно-технической конференции, Москва, 23–25 мая 2023 года / Институт океанологии им. П.П. Ширшова РАН, Том 2., С. 99-102. (year - 2023)

5. Kupriyanova A.E., Gritsenko V.A. О смешении вод погружающегося термика и подстилающих вод Гидрометеорология и физика атмосферы: современные достижения и тенденции развития : Материалы Всероссийской научно-практической конференции, Санкт-Петербург, 21–23 марта 2023 года, С. 263-265 (year - 2023)

6. Gritsenko V.A., Kileso A.V., Kupriyanova A.E., Korobchenkova K.D. The process of deepening of individual volumes with negative buoyancy in freshwater environment Mendeley Data, Mendeley Data, V1 (year - 2023) https://doi.org/10.17632/djm4r3vxb7.1

7. Kupriyanova A.E., Gritsenko V.A., Kileso A.V. An interaction of several volumes with negative buoyancy during their deepening in freshwater environment Mendeley Data, Mendeley Data, V1 (year - 2023) https://doi.org/10.17632/48x9wdtxmw.1

8. Kupriyanova A.E., Gritsenko V.A., Kileso A.V. Laboratory experiments of forming and deepening of set of small individual volumes with negative buoyancy Mendeley Data, Mendeley Data, V1 (year - 2023) https://doi.org/10.17632/x8pjgwyx94.1