The thermal structure of small lakes: The influence of a modified wind speed

Abstract. In thermally stratified lakes, the greatest annual methane emissions typically occur during thermal overturn events. In July of 2012, Greenland experienced significant warming that resulted in substantial melting of the Greenland Ice Sheet and enhanced runoff events. This unusual climate phenomenon provided an opportunity to examine the effects of short-term natural heating on lake thermal structure and methane dynamics and compare these observations with those from the following year when temperatures were normal. Here, we focus on methane concentrations within the water column of 5 adjacent small lakes on the ice-free margin of Southwest Greenland under open-water and ice-covered conditions from 2012–2014. Enhanced warming of the epilimnion in the lakes under open-water conditions in 2012 led to strong thermal stability and the development of anoxic hypolimnions in each of the lakes. As a result, during open-water conditions, mean dissolved methane concentrations in the water column were significantly (p

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Wind Induced Circulation in Lakes

Hydrology Research ◽

10.2166/nh.1978.0007 ◽

1978 ◽

Vol 9 (2) ◽

pp. 75-94 ◽

Cited By ~ 9

Author(s):

Lars Bengtsson

Keyword(s):

Wind Speed ◽

Turbulent Flow ◽

Drag Coefficient ◽

Large Scale ◽

Field Observations ◽

Small Lakes ◽

Current Pattern ◽

Induced Currents ◽

Large Scale Flow ◽

Wind Currents

In most lakes the wind is the most important flow generating mechanism. In this paper the problem of wind generated circulation - directly wind induced currents and seiches - in small lakes is reviewed. Many field observations are presented and discussed. In the thermocline and the hypolimnion forced seiche currents are shown to dominate the directly induced wind currents. Different kind of non-convective mathematical lake models are discussed and applied to different small lakes. Comparisons of observed and calculated currents show that lake models can be used to reproduce the currents of the upper 3–4 metres in a lake. The interaction between large-scale flow and turbulent flow is yet unknown, and therefore it is not possible to explain the physical current pattern and density anomalies at greater depth. In respect to the limited knowledge on turbulent processes in lakes, it is acceptable to apply a quadric relationship between wind stress and wind speed with a drag coefficient of about 1.0.10−3..

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Acoustical Measurement of Nonlinear Internal Waves Using the Inverted Echo Sounder

Journal of Atmospheric and Oceanic Technology ◽

10.1175/2009jtecho652.1 ◽

2009 ◽

Vol 26 (10) ◽

pp. 2228-2242 ◽

Cited By ~ 23

Author(s):

Qiang Li ◽

David M. Farmer ◽

Timothy F. Duda ◽

Steve Ramp

Keyword(s):

Surface Roughness ◽

Wind Speed ◽

Internal Waves ◽

Thermal Structure ◽

Sea Surface ◽

Model Calculations ◽

Nonlinear Internal Waves ◽

Instrument Performance ◽

Inverted Echo Sounder ◽

Echo Sounder

Abstract The performance of pressure sensor–equipped inverted echo sounders for monitoring nonlinear internal waves is examined. The inverted echo sounder measures the round-trip acoustic travel time from the sea floor to the sea surface and thus acquires vertically integrated information on the thermal structure, from which the first baroclinic mode of thermocline motion may be inferred. This application of the technology differs from previous uses in that the wave period (∼30 min) is short, requiring a more rapid transmission rate and a different approach to the analysis. Sources of error affecting instrument performance include tidal effects, barotropic adjustment to internal waves, ambient acoustic noise, and sea surface roughness. The latter two effects are explored with a simulation that includes surface wave reconstruction, acoustic scattering based on the Kirchhoff approximation, wind-generated noise, sound propagation, and the instrument’s signal processing circuitry. Bias is introduced as a function of wind speed, but the simulation provides a basis for bias correction. The assumption that the waves do not significantly affect the mean stratification allows for a focus on the dynamic response. Model calculations are compared with observations in the South China Sea by using nearby temperature measurements to provide a test of instrument performance. After applying corrections for ambient noise and surface roughness effects, the inverted echo sounder exhibits an RMS variability of approximately 4 m in the estimated depth of the eigenfunction maximum in the wind speed range 0 ≤ U10 ≤ 10 m s−1. This uncertainty may be compared with isopycnal excursions for nonlinear internal waves of 100 m, showing that the observational approach is effective for measurements of nonlinear internal waves in this environment.

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Modelling hourly rates of evaporation from small lakes

Hydrology and Earth System Sciences ◽

10.5194/hess-15-267-2011 ◽

2011 ◽

Vol 15 (1) ◽

pp. 267-277 ◽

Cited By ~ 49

Author(s):

R. J. Granger ◽

N. Hedstrom

Keyword(s):

Wind Speed ◽

Water Temperature ◽

Land Surface ◽

Open Water ◽

Turbulent Fluxes ◽

Water Evaporation ◽

Small Lakes ◽

Lake Evaporation ◽

Temperature Contrast ◽

Land Water

Abstract. The paper presents the results of a field study of open water evaporation carried out on three small lakes in Western and Northern Canada. In this case small lakes are defined as those for which the temperature above the water surface is governed by the upwind land surface conditions; that is, a continuous boundary layer exists over the lake, and large-scale atmospheric effects such as entrainment do not come into play. Lake evaporation was measured directly using eddy covariance equipment; profiles of wind speed, air temperature and humidity were also obtained over the water surfaces. Observations were made as well over the upwind land surface. The major factors controlling open water evaporation were examined. The study showed that for time periods shorter than daily, the open water evaporation bears no relationship to the net radiation; the wind speed is the most significant factor governing the evaporation rates, followed by the land-water temperature contrast and the land-water vapour pressure contrast. The effect of the stability on the wind field was demonstrated; relationships were developed relating the land-water wind speed contrast to the land-water temperature contrast. The open water period can be separated into two distinct evaporative regimes: the warming period in the Spring, when the land is warmer than the water, the turbulent fluxes over water are suppressed; and the cooling period, when the water is warmer than the land, the turbulent fluxes over water are enhanced. Relationships were developed between the hourly rates of lake evaporation and the following significant variables and parameters (wind speed, land-lake temperature and humidity contrasts, and the downwind distance from shore). The result is a relatively simple versatile model for estimating the hourly lake evaporation rates. The model was tested using two independent data sets. Results show that the modelled evaporation follows the observed values very well; the model follows the diurnal trends and responds to changes in environmental conditions.

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Surfactant Effects on the Free Surface Thermal Structure and Subsurface Flow in a Wind-Wave Tunnel

Journal of Fluids Engineering ◽

10.1115/1.2234781 ◽

2006 ◽

Vol 128 (5) ◽

pp. 913-920 ◽

Cited By ~ 4

Author(s):

Supathorn Phongikaroon ◽

K. Peter Judd

Keyword(s):

Free Surface ◽

Wind Speed ◽

Surface Temperature ◽

Thermal Structure ◽

Subsurface Flow ◽

Surface Velocity ◽

Ir Detector ◽

Near Surface ◽

Wind Speeds ◽

Surfactant Effects

In this study, the dynamic effects of surfactant (oleyl alcohol) on the surface temperature and the near surface velocity field of a wind driven free surface are investigated. Different surfactant concentrations and wind speeds were examined to elucidate the flow physics. The water surface was imaged with an infrared (IR) detector and the subsurface flow was interrogated utilizing digital particle image velocimetry (DPIV). The IR imagery reveals the presence of a Reynolds ridge that demarcates the boundary between clean (hot) fluid and contaminated (cold) fluid. The clean region was found to be composed of laminae structures known as fishscales. A “wake region” which is an intermediate temperature region resulting from mixing of the near surface fluid layers develops behind the ridge. Experimental results from infrared imagery indicate that the fishscales in the clean region become elongated and narrowed as the wind speed increases. In addition, the results reveal that higher wind speed is required to form a Reynolds ridge in the presence of higher surfactant concentration. The plots of the surface temperature probability density functions reveal that these thermal structures undergo the same evaporative process while the increase in wind speed enhances this process. DPIV results reveal that the growth of a subsurface boundary layer for the contaminated case is more pronounced than that for the clean case.

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Response of water temperatures and stratification to changing climate in three lakes with different morphometry

Hydrology and Earth System Sciences ◽

10.5194/hess-21-6253-2017 ◽

2017 ◽

Vol 21 (12) ◽

pp. 6253-6274 ◽

Cited By ~ 43

Author(s):

Madeline R. Magee ◽

Chin H. Wu

Keyword(s):

Wind Speed ◽

Surface Area ◽

Air Temperature ◽

Thermal Structure ◽

Surface Heat ◽

Heat Fluxes ◽

Wind Speeds ◽

Changing Climate ◽

Air Temperatures ◽

Schmidt Stability

Abstract. Water temperatures and stratification are important drivers for ecological and water quality processes within lake systems, and changes in these with increases in air temperature and changes to wind speeds may have significant ecological consequences. To properly manage these systems under changing climate, it is important to understand the effects of increasing air temperatures and wind speed changes in lakes of different depths and surface areas. In this study, we simulate three lakes that vary in depth and surface area to elucidate the effects of the observed increasing air temperatures and decreasing wind speeds on lake thermal variables (water temperature, stratification dates, strength of stratification, and surface heat fluxes) over a century (1911–2014). For all three lakes, simulations showed that epilimnetic temperatures increased, hypolimnetic temperatures decreased, the length of the stratified season increased due to earlier stratification onset and later fall overturn, stability increased, and longwave and sensible heat fluxes at the surface increased. Overall, lake depth influences the presence of stratification, Schmidt stability, and differences in surface heat flux, while lake surface area influences differences in hypolimnion temperature, hypolimnetic heating, variability of Schmidt stability, and stratification onset and fall overturn dates. Larger surface area lakes have greater wind mixing due to increased surface momentum. Climate perturbations indicate that our larger study lakes have more variability in temperature and stratification variables than the smaller lakes, and this variability increases with larger wind speeds. For all study lakes, Pearson correlations and climate perturbation scenarios indicate that wind speed has a large effect on temperature and stratification variables, sometimes greater than changes in air temperature, and wind can act to either amplify or mitigate the effect of warmer air temperatures on lake thermal structure depending on the direction of local wind speed changes.

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Characterizing the convective heat exchange with plastic shading nets under natural arid conditions

International Journal of Energy Technology ◽

10.32438//ijet.11415 ◽

2019 ◽

pp. 11-20

Author(s):

Ahmed M Abdel-Ghanya ◽

Ibrahim M Al-Helal

Keyword(s):

Wind Speed ◽

Thermal Radiation ◽

Convective Heat ◽

Convective Heat Transfer Coefficient ◽

Convective Heat Exchange ◽

Radiative Properties ◽

Wooden Frame ◽

Arid Conditions ◽

Air Temperatures ◽

Radiation Fluxes

Plastic nets are extensively used for shading purposes in arid regions such as in the Arabian Peninsula. Quantifying the convection exchange with shading net and understanding the mechanisms (free, mixed and forced) of convection are essential for analyzing energy exchange with shading nets. Unlike solar and thermal radiation, the convective energy, convective heat transfer coefficient and the nature of convection have never been theoretically estimated or experimentally measured for plastic nets under arid conditions. In this study, the convected heat exchanges with different plastic nets were quantified based on an energy balance applied to the nets under outdoor natural conditions. Therefore, each net was tacked onto a wooden frame, fixed horizontally at 1.5-m height over the floor. The downward and upward solar and thermal radiation fluxes were measured below and above each net on sunny days; also the wind speed over the net, and the net and air temperatures were measured, simultaneously. Nets with different porosities, colors and texture structures were used for the study. The short and long wave’s radiative properties of the nets were pre-determined in previous studies to be used. Re and Gr numbers were determined and used to characterize the convection mechanism over each net. The results showed that forced and mixed convection are the dominant modes existing over the nets during most of the day and night times. The nature of convection over nets depends mainly on the wind speed, net-air temperature difference and texture shape of the net rather than its color and its porosity.

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