Stable Thermal Stratification Research Articles

Electric Buoyancy-driven convection in stable and unstable thermal stratifications

Thermo-electro-convective modes induced by a dielectrophoretic force in a differentially heated horizontal rectangular cavity have been investigated using direct numerical simulations in stable and unstable thermal stratifications. The variation of the electric tension applied to the plates of the cavity leads to multiple modes under microgravity as well as under both stable and unstable stratifications in terrestrial conditions. An effective electric Rayleigh number incorporating the effects of both the electric potential and the thermal stratification has been introduced in order to analyze the heat transfer induced by thermoelectric convection, leading to a unique curve of the variation of the Nusselt number with the effective electric Rayleigh number. The results can be used for modeling the heat transfer in microfluidic devices where the Archimedean buoyancy is very weak or to simulate natural convection at any planet using experiments performed on the Earth.

Seasonal Evolution of Stable Thermal Stratification in Central Area of Lake Ladoga

The complete climatic courses of the parameters of stable thermal stratification for the central part of Lake Ladoga, the largest European lake, are presented on the basis of empirical relationships, taking into account the physical processes governing water temperature variations. For the first time, the seasonal cycle of the surface water temperature, the temperature and the depth of the thermocline, and the hypolimnion temperature are calculated using the vertical profiles of the temperature obtained from the central area of Lake Ladoga. Temperature data are used for the period of in situ observations from 1897 to the present. The proposed functional forms of the temporal temperature cycle and the course of thermocline’s boundaries deepening are useful for examination and simulation of the heat vertical transport from air to water. Approximation curves for the parameters of heating and cooling periods were developed with high significant determination coefficients. Time dependencies of the climatic rates of change in water temperature and the depth of the thermocline boundaries were determined from the onset of stable stratification to its dissipation. The highest rate of water temperature change in the heating stage takes place in late June–early July, which at the water surface, is 0.32 °C/day, while in the thermocline layer, it is 0.18 °C/day. The peak velocity during the cooling stage at the surface occurs in late August–early September and is 0.14 °C/day, whereas in the thermocline, it is 0.08 °C/day and takes place between September and early October. During the period of heating, the deepening parameters of the thermocline layer do not fluctuate very much, only within the range of 0.1–0.3 m/day. During the cooling period, under the influence of free convection, rates increase drastically. The maximum rates of deepening during the period of full autumn mixing reach 1.8 m/day. When the autumn overturn occurs, the epilimnion thickness equals the bottom depth, and the bottom temperature reaches its maximum during the annual cycle. Climatic norms of the stratification parameters against which it is necessary to assess climate change are calculated.

Effects of the 2018 European heatwave and drought on coastal biogeochemistry in the German Bight

In 2018, Europe experienced an unprecedented heatwave and drought, especially in central and northern Europe, which caused decreased terrestrial production and affected ecosystem health. In this study, the effects of this event on the marine environment are investigate, with a focus on the biogeochemical response in the German Bight of the North Sea. Using time series data from FerryBoxes, research cruises, monitoring programs and remote sensing we compare conditions in 2018 to climatological values. We find that (1) the heatwave caused rapid warming of surface waters, (2) the drought reduced river discharge and nutrient loads to the coast, and (3) these combined effects altered coastal biogeochemistry and productivity. During 2018, both water discharge and nutrient loads from rivers discharging into the German Bight were below the seasonally variable 10th percentile from March onward. Throughout the study domain, water temperature was near or below that threshold in March 2018, but higher than in other years during May 2018, representing not only a heat wave, but also the fastest spring warming on record. This extreme warming period saw concurrent high peaks in chlorophyll a, dissolved oxygen and pH, consistent with the development of a strong spring bloom. It appears that productivity was above 75th percentile of the 21-year record in most of the nearshore region, while offshore it was widely below the 25th percentile in 2018. The drought-related low discharge limited nutrient supply from the rivers, but likely increased water residence time nearshore, where a surge in primary production with efficient nutrient utilization during the spring depleted nutrients available for transport offshore. There, the heatwave-related rapid warming of surface water resulted in the establishment of a stable thermal water column stratification, hindering vertical nutrient supply to the surface layer during the summer.

Direct numerical simulation of thermal stratification of supercritical water in a horizontal channel

A direct numerical simulation of water at a supercritical pressure is carried out to study the fundamental characteristics of turbulence and heat transfer in the flow between two parallel plates. The top and bottom plates are differentially heated to achieve stable thermal stratification. Both forced and mixed convection are studied to investigate buoyancy and variable property effects in a stably-stratified horizontal flow. The simulation results show that the variable property effect causes turbulence deterioration and flow laminarization near the cold wall while increasing the turbulence stress intensity near the hot wall. The direct buoyancy effect weakens the turbulence development throughout the channel. The indirect buoyancy effect strongly enhances flow laminarization near the cold wall and it increases turbulence near the hot wall. However, the net buoyancy effect near the hot wall is insignificant due to a cancelling of the competing direct and indirect buoyancy effects. The turbulence behaviour near the hot wall is dominated by the variable property effect. The heat flux transferred through the wall under the condition of fixed wall temperature is strongly reduced by buoyancy, while it is less sensitive to property variation.

Modeling the infection risk and emergency evacuation from bioaerosol leakage around an urban vaccine factory

Mounting interest in modeling outdoor diffusion and transmission of bioaerosols due to the prevalence of COVID-19 in the urban environment has led to better knowledge of the issues concerning exposure risk and evacuation planning. In this study, the dispersion and deposition dynamics of bioaerosols around a vaccine factory were numerically investigated under various thermal conditions and leakage rates. To assess infection risk at the pedestrian level, the improved Wells-Riley equation was used. To predict the evacuation path, Dijkstra’s algorithm, a derived greedy algorithm based on the improved Wells-Riley equation, was applied. The results show that, driven by buoyancy force, the deposition of bioaerosols can reach 80 m on the windward sidewall of high-rise buildings. Compared with stable thermal stratification, the infection risk of unstable thermal stratification in the upstream portion of the study area can increase by 5.53% and 9.92% under a low and high leakage rate, respectively. A greater leakage rate leads to higher infection risk but a similar distribution of high-risk regions. The present work provides a promising approach for infection risk assessment and evacuation planning for the emergency response to urban bioaerosol leakage.

О МЕХАНИЗМЕ РЕЗКОГО ПОНИЖЕНИЯ ТЕМПЕРАТУРЫ ПОВЕРХНОСТИ В СЕВЕРО-ЗАПАДНОЙ ЧАСТИ ЧЕРНОГО МОРЯ И У ПОБЕРЕЖЬЯ КРЫМА

An analysis of high-resolution satellite data on sea surface temperature (SST) and surface wind for 2008-2021 confirmed that the areas of a dramatic SST drop exceeding several degrees per day regularly occur in the northwestern Black Sea and off the western coast of Crimea. These areas are located not only in coastal upwelling zones. Extensive areas of this type are found in the open water areas in the northwestern part of the sea and in the vicinity of the western large-scale cyclonic gyre. A reason for the formation of such areas during the period of the stable vertical thermal stratification in the upper layer is the strengthening of wind in the atmospheric surface layer and the intensification of cyclonic vorticity. This leads to the generation of upward motions, additional turbulent mixing in the upper sea layer, and, as a consequence, the SST drop within a few days after the passage of the cyclone over the sea. This mechanism is especially effective in the case of slowly moving cyclones in the presence of a blocking or a high-pressure ridge northeast of the Black Sea region.

Numerical experiments on thermal convection of highly compressible fluids with variable viscosity and thermal conductivity in 2-D cylindrical geometry: implications for mantle convection of super-Earths

SUMMARY We conduct a series of numerical experiments of thermal convection of highly compressible fluids in 2-D cylindrical annulus, in order to study the mantle convection on super-Earths. The variations in thermodynamic properties (thermal expansivity and reference density) with depth are taken to be relevant for the super-Earths with 10 times the Earth’s mass, while those in transport properties (viscosity and thermal conductivity) are modelled by an exponential dependence on temperature and/or depth. From our experiments we identified a distinct regime of convecting flow patterns induced by the interplay between the adiabatic temperature change and the spatial variations in viscosity and thermal conductivity. That is, for the cases with strong temperature-dependent viscosity and large increase in thermal conductivity with depth, a ‘deep stratosphere’ of stable thermal stratification is formed at the base of the mantle, in addition to thick stagnant lids at their top surfaces. In the ‘deep stratosphere’, the fluid motion is insignificant particularly in the vertical direction in spite of smallest viscosity owing to its strong dependence on temperature. From the comparison with the experiments with the Cartesian geometry, we also found that the occurrence of ‘deep stratosphere’ tends to be suppressed for the cases with cylindrical geometry, owing to the reduction of the surface area with depth which helps increase the temperature gradient in the lowermost mantle. Our finding may further imply that both the effects of adiabatic compression and those of spherical (or cylindrical) geometry of mantle are of crucial importance in understanding the mantle dynamics of massive super-Earths in the presence of spatial variations in physical properties.

Light as a controlling factor of winter phytoplankton in a monomictic reservoir

Factors affecting the seasonal succession of plankton communities in freshwater temperate lakes have been thoroughly studied for decades. However, there are still relatively few data describing the winter season patterns in detail, as the focus has been mostly on spring to autumn conditions. Ice cover is often the crucial factor limiting light availability for winter phytoplankton, but in warm monomictic lakes is usually lacking and the gradually increasing solar radiation should, theoretically, drive phytoplankton growth. In this study conducted in 2002–2010, we documented regular sharp increases in phytoplankton chlorophyll a, starting just after the winter solstice and lasting throughout the total circulation and/or unstable inverse stratification period in the monomictic Slapy reservoir (Czechia). Chlorophyll a concentrations analysed in one-week intervals reached their yearly minimum of 0.2–0.8 µg L−1 in the solstice period, and the spring peak occurred before the onset of stable thermal stratification. The regular pattern was slightly disrupted in some years, associated with short periods of ice cover. Winter phytoplankton were species poor and dominated by diatoms, cryptophytes, green algae, and cyanobacteria. Using semiparametric regression approach, we aimed to test if selected environmental parameters had a significant effect on the observed winter trend. The resulting model revealed that solar radiation and water temperature positively influenced log chlorophyll a concentrations, whereas water age had a significant negative effect. On the other hand, zooplankton density and ice cover effects were not significant. The shapes of the marginal effects of water temperature and solar radiation were nonlinear, and the interaction of these two major factors was significant. The model-based estimated chlorophyll a concentrations showed a shift from radiation dominance to temperature-positive effects along the temperature gradient. This might represent as yet neglected pattern of phytoplankton seasonal development in warm monomictic lakes worldwide.

Diffusion of turbulence following both stable and unstable step stratification perturbations

The evolution of a two-phase, air and unsaturated water vapor, time-decaying, shearless, and turbulent layer has been studied in the presence of both stable and unstable perturbations of the normal temperature lapse rate. The top interface between a warm vapor cloud and clear air in the absence of water droplets was considered as the reference dynamics. Direct, three-dimensional, and numerical simulations were performed within a 6 × 6-m-wide and 12-m-high domain, which was hypothesized to be located close to an interface between the warm cloud and clear air. The Taylor microscale Reynolds number was 250 inside the cloud portion. The squared Froude's number varied over intervals of [0.4; 981.6] and [−4.0; −19.6]. A sufficiently intense stratification was observed to change the mixing dynamics. The formation of a sublayer inside the shearless layer was observed. The sublayer, under a stable thermal stratification condition, behaved like a pit of kinetic energy. However, it was observed that kinetic energy transient growth took place under unstable conditions, which led to the formation of an energy peak just below the center of the shearless layer. The scaling law of the energy time-variation inside the interface region was quantified: this is an algebraic law with an exponent that depends on the perturbation stratification intensity. The presence of an unstable stratification increased the differences in statistical behavior among the longitudinal velocity derivatives, compared with the unstratified case. Since the mixing process is suppressed in stable cases, small-scale anisotropy is also suppressed.

Diatom-inferred centennial-millennial postglacial climate change in the Pacific Northwest of North America

A diatom record from Moss Lake, Washington, USA spans the last 14,500 cal year and revealed Holocene climate change in the Pacific Northwest (PNW), including evidence for periodicities related to ocean-atmosphere teleconnections and/or variations in solar output. Three main climate phases were identified: (i) Late Pleistocene to early Greenlandian (until 10,800 cal year BP, spanning GI-1, GS-1), with a cold climate and low diatom abundance; (ii) early Greenlandian to Northgrippian (10,800–7500 cal year BP), shifting to a warmer climate; and (iii) late Northgrippian and Meghalayan from 7500 cal year BP onwards, with a cooler, moist climate. These climate shifts are in good agreement with the pollen record from the same core and other regional studies. Fluctuations in Discostella pseudostelligera and Aulacoseira taxa suggest climate cycles of different frequency and amplitude throughout the record. Spectral and wavelet analyses revealed periodicities of approximately 1400 and 400–500 years. We interpret the ~ 1400-year and ~ 400–500-year cycles to reflect alternating periods of enhanced (and reduced) convective mixing in the water column, associated with increased (and decreased) storms, resulting from ocean–atmosphere teleconnections in the wider Pacific region. The ~ 1400-year periodicity is evident throughout the Late Pleistocene and late Northgrippian/Meghalayan, reflecting high-amplitude millennial shifts from periods of stable thermal stratification of the water column (weak wind intensity) to periods of convective mixing (high wind intensity). The millennial cycle diminishes during the Greenlandian, in association with the boreal summer insolation maximum, consistent with suppression of ENSO-like dynamics by enhanced trade winds. Ocean–atmosphere teleconnection suppression is recorded throughout the PNW, but there is a time discrepancy with other records, some that reveal suppression during the Greenlandian and others during the Northgrippian, suggesting endogenic processes may also modulate the Moss Lake diatom record. The large amplitude of millennial variability indicated by the lake data suggests that regional climate in the PNW was characterised over the longer term by shifting influences of ocean–atmosphere dynamics and that an improved understanding of the external forcing is necessary for understanding past and future climate conditions in western North America.

Thermal and fire characteristics of hydrogen jet flames in the tunnel at longitudinal ventilation strategies

Hydrogen leakage of vehicles in the tunnel is a great threat to the safety operation of the tunnel and longitudinal ventilation strategies have always been utilized to control the fire and smoke movement of rail transit, electric and fossil-fueled vehicles in the engineering field. It is in doubt whether the longitudinal ventilation strategy could still help to reduce the jet fire hazard of transportation with H2 power in the tunnel, considering the rapid development of the hydrogen energy. In present work, a numerical research on effects of longitudinal ventilation strategies on hydrogen jet flames in the tunnel is conducted. The results illustrate that longitudinal ventilation could affect the flame characteristics of jet flames greatly in the tunnel. The critical ventilation velocity increases firstly with the increase of hydrogen leakage rates and then changes little after a critical value. The predicted theoretical model of pool fires could well predict the critical ventilation velocity for hydrogen jet fires. With the increase of longitudinal ventilation velocity, maximum ceiling temperatures are decreased greatly. According to the heat releases, jet speeds and ventilation velocities, three kinds of flame bending characteristics of hydrogen jet fire could be observed due to different effects of the inertial force. At last, the stable thermal stratification could also be destroyed by large ventilation velocities but the corresponding ventilation velocity is far larger than the critical ventilation one. With the increase of longitudinal ventilation velocities, the height of thermal layer is reduced firstly and then maintained at a constant value.

Multi-spectral imaging for Thermochromic Liquid Crystal based Particle Image Thermometry: A proof of concept

In this contribution, a novel imaging approach for Thermochromic Liquid Crystal (TLC) based Particle Image Thermometry (PIT) is demonstrated. In contrast to state of the art approaches, a multi-spectral camera was used to record the color response of the Thermochromic Liquid Crystals seeding particles. An experiment with a transparent, water-filled, cylindrical cell as the central element was set up to investigate the novel approach. The temperature in the cell can be controlled by adjusting the temperature of the bottom and top plate. Calibration images at eleven different temperatures ranging from 18 ◦C to 21.6 ◦C, as well as images of a stable thermal stratification, were recorded. 90 percent of the calibration data was used to train a neural network (NN) to predict the temperature. The remaining 10 percent of the calibration data and the data of the stable thermal stratification were used to test the NN. The tests show that the deviation between predicted and ground truth temperature is mostly below 0.1 K and that the linear profile of the stable thermal stratification can be predicted with a maximum deviation of ≈ 0.15 K. This shows that multi-spectral imaging with neural networks for data processing is feasible and a promising concept.

Ice‐Covered Lakes of Tibetan Plateau as Solar Heat Collectors

AbstractThe Qinghai‐Tibet Plateau possesses the largest alpine lake system, which plays a crucial role in the land‐atmosphere interaction. We report first observations on the thermal and radiation regime under ice of the largest freshwater lake of the Plateau. The results reveal that freshwater lakes on the Tibetan Plateau fully mix under ice. Due to strong solar heating, water temperatures increase above the maximum density value 1–2 months before the ice break, forming stable thermal stratification with subsurface temperatures >6°C. The resulting heat flow from water to ice makes a crucial contribution to ice cover melt. After the ice breakup, the accumulated heat is released into the atmosphere during 1–2 days, increasing lake‐atmosphere heat fluxes up to 500 W m−2. The direct biogeochemical consequences of the deep convective mixing are aeration of the deep lake waters and upward supply of nutrients to the upper photic layer.

The Thermal Evolution of Planetesimals During Accretion and Differentiation: Consequences for Dynamo Generation by Thermally‐Driven Convection

AbstractThe meteorite paleomagnetic record indicates that differentiated (and potentially, partially differentiated) planetesimals generated dynamo fields in the first 5–40 Myr after the formation of calcium‐aluminum‐rich inclusions (CAIs). This early period of dynamo activity has been attributed to thermal convection in the liquid cores of these planetesimals during an early period of magma ocean convection. To better understand the controls on thermal dynamo generation in planetesimals, we have developed a one dimensional model of the thermal evolution of planetesimals from accretion through to the shutdown of convection in their silicate magma oceans for a variety of accretionary scenarios. The heat source of these bodies is the short‐lived radiogenic isotope 26Al. During differentiation, 26Al partitions into the silicate portion of these bodies, causing their magma oceans to heat up and introducing stable thermal stratification to the top of their cores, which inhibits dynamo generation. In “instantaneously” accreting bodies, this effect causes a delay on the order of >10 Myr to whole core convection and dynamo generation while this stratification is eroded. However, gradual core formation in bodies that accrete over >0.1 Myr can minimize the development of this stratification, allowing dynamo generation from ∼4 Myr after CAI formation. Our model also predicts partially differentiated planetesimals with a core and mantle overlain by a chondritic crust for accretion timescales >1.2 Myr, although none of these bodies generate a thermal dynamo field. We compare our results from thousands of model runs to the meteorite paleomagnetic record to constrain the physical properties of their parent bodies.

Characteristics and driving factors of thermal stratification evolution in Daheiting Reservoir

Thermal stratification which is common in water bodies is subject to such factors as the water depth of the water body (a lake or reservoir, for instance), the fluidity of the water and the local meteorological conditions. The stable thermal stratification in reservoirs will lead to changes in the physical and chemical properties of the water as well as distribution of aquatic creatures, hence leaving an impact on the water quality. The Daheiting Reservoir was taken as the research object in this study. Based on the continuous monitored water temperature data in the reservoir, the tempo-spatial change features of the water temperature structure in the reservoir were analyzed, and the driving factors of thermal stratification in the reservoir was studied. The research found that air temperature, wind speed, and hydrodynamic factors are the driving factors for the thermal stratification and corresponding water temperature change patterns in Daheiting Reservoir. Among these factors, air temperature is the fundamental precondition, the wind speed is the auxiliary precondition, and the hydrodynamic factors are the disturbance factors for thermal stratification in the Reservoir. All these factors act together to cause the thermal stratification pattern and evolution features in Daheiting Reservoir.

Decoupling of a Douglas fir canopy: a look into the subcanopy with continuous vertical temperature profiles

Abstract. Complex ecosystems such as forests make accurately measuring atmospheric energy and matter fluxes difficult. One of the issues that can arise is that parts of the canopy and overlying atmosphere can be turbulently decoupled from each other, meaning that the vertical exchange of energy and matter is reduced or hampered. This complicates flux measurements performed above the canopy. Wind above the canopy will induce vertical exchange. However, stable thermal stratification, when lower parts of the canopy are colder, will hamper vertical exchange. To study the effect of thermal stratification on decoupling, we analyze high-resolution (0.3 m) vertical temperature profiles measured in a Douglas fir stand in the Netherlands using distributed temperature sensing (DTS). The forest has an open understory (0–20 m) and a dense overstory (20–34 m). The understory was often colder than the atmosphere above (80 % of the time during the night, >99 % during the day). Based on the aerodynamic Richardson number the canopy was regularly decoupled from the atmosphere (50 % of the time at night). In particular, decoupling could occur when both u*<0.4 m s−1 and the canopy was able to cool down through radiative cooling. With these conditions the understory could become strongly stably stratified at night. At higher values of the friction velocity the canopy was always well mixed. While the understory was nearly always stably stratified, convection just above the forest floor was common. However, this convection was limited in its vertical extent, not rising higher than 5 m at night and 15 m during the day. This points towards the understory layer acting as a kind of mechanical “blocking layer” between the forest floor and overstory. With the DTS temperature profiles we were able to study decoupling and stratification of the canopy in more detail and study processes which otherwise might be missed. These types of measurements can aid in describing the canopy–atmosphere interaction at forest sites and help detect and understand the general drivers of decoupling in forests.

Characteristics of quasistationary near-wall turbulence subjected to strong stable stratification in open-channel flows

Turbulent open channel-flow under strongly stable thermal stratification is investigated. It is shown that the dominant effects of strong stable stratification on the characteristics of near-wall turbulence are transient. The budget of turbulent kinetic energy, the budget for the tangential Reynolds stress, and the relevant length scales are discussed. It is shown that near-wall turbulence at quasistationarity is approximately insensitive to the choice of upper thermal boundary condition.

Effect of temperature difference on the thermal mixing phenomenon in a T-junction under inflow pulsation

The effects of inlet temperature difference between the main and branch fluids on the flow and thermal mixing characteristics in the T-junction under inflow pulsation are numerically investigated by large eddy simulation (LES) turbulent model. The inlet velocities of both the main and branch pipes are assumed to be sinusoidal distribution, which is similar to the rolling condition in the ocean. Thermal mixing between hot and cold fluids with temperature differences of 15 K, 30 K and 45 K is carried out in the present study. The flow and mixing characteristics, including qualitative and quantitative analyses of temperature and velocity are studied. The results show that the reverse flow occurs in the unsteady thermal mixing processes due to the effect of inertia force, and the area of reverse flow decreases as the inlet temperature difference increases. Increasing inlet temperature difference induces a more stable thermal stratification flow with smaller thermal mixing length and lower temperature gradient. The maximum root mean square (RMS) of temperature fluctuation slightly decreases and shifts to the upper of main pipe as the inlet temperature difference increases. In addition, the spectral analysis of temperature fluctuation performed by Wavelet Transform (WT) indicates that the number of frequency peaks decreases as the temperature difference increases. Meanwhile, the main frequencies of wavelet power spectrum densities (WPSDs) for the lower points (points 1 ~ 3) decrease, while those for the upper points (points 5 ~ 7) increase as the temperature difference increases.

MELTING WITHIN HORIZONTAL H-SHAPED ENCLOSURE WITH ADIABATIC CURVED BOUNDARY AFFECTED BY INCLINATION, MONO/HYBRID NANOFLUIDS AND FINS

From an energy saving viewpoint, full melting of phase change material in thermal storage systems should be achieved. Constrained ice melting with natural convection inside a horizontal H-shaped capsule with adiabatic curved sidewalls is not completed because energy input from hot surfaces overheats the liquid phase on top while stable thermal stratification on bottom persists. Although 90° inclination of capsule engenders full melting of pure ice, the melting process is still sluggish due to low thermal conductivity of ice/water. Hence, heat transfer enhancement techniques using mono Cu, hybrid Ag/MgO nanoparticles, and 310 stainless steel fins are incorporated into system. Existing enthalpy-based lattice Boltzmann method with double distribution function model in single-phase framework is implemented. Insertion of Ag-MgO hybrid nanoparticles within horizontal H-shaped enclosure does not eradicate persistent thermal stratification. Full melting time inside 90° inclined capsule is diminished 13.6 and 24.5%, respectively, when the volume fraction of hybrid nanoparticles is increased from 0.0 to 0.01 and 0.02. While mono Cu nanoparticles give a better thermal performance in contrast to Ag-MgO hybrid nanoparticles, their price is double. Lower volume fraction (0.01) of mono Cu nanoparticles is prescribed since storage capacity is less decreased. Compared to pure PCM melting, partial internal fins mounted on bottom hot surface diminish full melting time 28.0%. However, magnitude of maximum velocity in molten PCM demonstrates that existence of fins considerably limits growth of natural convection flow.

ESTIMATES OF THERMAL DIFFUSIVITY IN DIMICTIC LAKES

Based on the results of long-term measurements of water temperature in three dimictic lakes, we obtained estimates of the average effective coefficients of vertical turbulent exchange for the period of formation of stable thermal stratification (July-August). Deep lakes were chosen as objects of this study: Onega (Russia), Kallavesi (Finland), Inari (Finland). For seasonal water temperature trends averaged over a multi-annual period the effects of horizontal heat transfer can be neglected, so the estimates were based on the analysis of the one-dimensional version of the heat transfer differential equation. The effective thermal diffusivity was calculated in three ways: based on changes in the heat content of the water column, local changes in water temperature, and also through the analysis of the amplitude and phase response of the deep layers temperature on quasi-periodic changes in the average temperature of the epilimnion. These estimates are two to four orders of magnitude greater than the value of the coefficient of molecular thermal diffusivity in water.