Non‐closure of the surface energy balance is a frequently observed phenomenon of hydrometeorological field measurements, when using the eddy‐covariance method, which can be ascribed to an ...underestimation of the turbulent fluxes. Several approaches have been proposed in order to adjust the measured fluxes for this apparent systematic error. However, there are uncertainties about partitioning of the energy balance residual between the sensible and latent heat flux and whether such a correction should be applied on 30‐min data or longer time scales. The data for this study originate from two grassland sites in southern Germany, where measurements from weighable lysimeters are available as reference. The adjusted evapotranspiration rates are also compared with joint energy and water balance simulations using a physically based distributed hydrological model. We evaluate two adjustment methods: the first one preserves the Bowen ratio and the correction factor is determined on a daily basis. The second one attributes a smaller portion of the residual energy to the latent heat flux than to the sensible heat flux for closing the energy balance for every 30‐min flux integration interval. Both methods lead to an improved agreement of the eddy‐covariance based fluxes with the independent lysimeter estimates and the physically based model simulations. The first method results in a better comparability of evapotranspiration rates, and the second method leads to a smaller overall bias. These results are similar between both sites despite considerable differences in terrain complexity and grassland management. Moreover, we found that a daily adjustment factor leads to less scatter than a complete partitioning of the residual for every half‐hour time interval. The vertical temperature gradient in the surface layer and friction velocity were identified as important predictors for a potential future parameterisation of the energy balance residual.
Characteristics of CO2 and heat fluxes over a mangrove forest are investigated during dry and wet seasons in southern Thailand. Fluxes are estimated by the modified gradient method presented in Monji ...et al. (2002). Turbulent fluxes show considerable differences between wet and dry seasons. When it rains the latent heat flux exceeds the net radiation considerably and the sensible heat flux shows large negative values. CO2 flux during the rain is slightly upward. The relationship between global solar radiation and CO2 flux was not significantly different between dry and wet seasons. CO2 stored inside the canopy atmosphere during the night was found to be consumed in the morning by its photosynthesis. The storage rate is large when wind is low and its magnitude is not negligible in the CO2 exchange process.
ABSTRACT
A parameterization approach of effective roughness length was introduced into the Surface Energy Balance System (SEBS) model to account for subgrid‐scale topographical influences. Regional ...distribution of land surface heat flux values (including net radiation flux, ground heat flux, sensible heat flux, and latent heat flux) was estimated on the Tibetan Plateau (TP) based on the SEBS model, and utilizing remote sensing products and reanalysis datasets. We then investigated annual trends in these fluxes for the period 2001–2012. It was found that land surface net radiation flux increased slightly, especially in high, mountainous regions and the central TP, and was influenced by glacial retreat and topsoil wetting, respectively. Sensible heat flux decreased overall, especially in the central and northern TP. In the Yarlung Zangbo River (YZR) Basin, the sensible heat flux increased because of a rise in the ground‐air temperature difference. The latent heat flux increased over the majority TP, except for areas in the YZR Basin. This can be attributed to increases in precipitation and vegetation greening.
Comparison between observed and remote sensing‐based land surface albedo and land surface temperature (LST) at the QOMS and Namco stations. RMSE is the root‐mean‐square error, MB is mean bias, and R is the correlation coefficient.
The aim of the present work is to propose a methodology for the use of thermoelectric modules (TEM) as an alternative to conventional heat flux sensors, for estimating convective and radiative heat ...flux. In this experimental study, the performance of TEMs as heat flux sensors is compared to that of the heat flux sensor (HFS), a type of conventional heat flux sensor, based on the premise that the HFS has been proven to perform acceptably in heat flux measurement. The interest of this comparison arises from the cost and sensitivity of the TEM with respect to the HFS. A simple measurement device is proposed, consisting of heat flux and temperature sensors with a general formulation for decoupling the convective and radiative parts. This methodology is implemented in two cases (low and high thermal stress). The radiative part is found to be the same by using the TEM and the HFS in both cases. However, the convective part measured by the TEM is found to be about 2.5 times larger than the HFS measurements in case of low thermal stress, and 1.6 times larger for tin case of high thermal stress. To explain this difference, the extended surface approximation was employed. This approximation indicates that the convective heat flux estimated from the TEM is always expected to be 1.6 and 1.4 times larger than that from the HFS for the low and high thermal stresses respectively, when their thermoelectric properties and geometry are taken into account.
Ocean‐to‐ice heat flux (OHF) is important in regulating the variability of sea ice mass balance. Using surface drifting buoy observations, we show that during winter in the Arctic Ocean's Beaufort ...Gyre region, OHF increased from 0.76 ± 0.05 W/m2 over 2006–2012 to 1.63 ± 0.08 W/m2 over 2013–2018. We find that this is a result of thinner and less‐compact sea ice that promotes enhanced winter ice growth, stronger ocean vertical convection, and subsurface heat entrainment. In contrast, Ekman upwelling declined over the study period, suggesting it had a secondary contribution to OHF changes. The enhanced ice growth creates a cooler, saltier, and deeper ocean surface mixed layer. In addition, the enhanced vertical temperature gradient near the mixed layer base in later years favors stronger entrainment of subsurface heat. OHF and its increase during 2006–2018 were not geographically uniform, with hot spots found in an upwelling region where ice was most seasonally variable.
Plain Language Summary
Heat transferred from the ocean to the sea ice influences the extent to which sea ice melts or freezes. It is unclear how variable this heat transfer is during winter. Using multiple drifting instruments, our results reveal that ocean‐to‐ice heat transfer almost doubled from 2006–2012 to 2013–2018. The enhanced heat transfer is a result of thinner and looser sea ice that leads to enhanced ice growth during winter. This enhanced ice growth causes stronger mixing within the ocean and so larger transfers of heat from the ocean to the ice. Changes in the extent to which water is physically pushed upwards had a secondary role in enhanced ocean‐to‐ice heat transfer over the study period. As a result of the increased ice growth, the pool of water directly in contact with the ice cover is cooler, saltier, and deeper over 2013–2018 compared with 2006–2012. Changes in ocean temperature at depth additionally favor stronger subsurface heat entrainment during 2013–2018. Ocean‐to‐ice heat transfer and its increase during 2006–2018 was not geographically uniform, with hot spots found where ice was most seasonally variable.
Key Points
The winter ocean‐to‐ice heat flux increased by 0.87 ± 0.09 W/m2 from 2006–2012 to 2013–2018 in the Beaufort Gyre region
2013–2018, compared with 2006–2012, had stronger convection associated with thinner ice, and increased ice mobility and lead fraction
Changes to Ekman upwelling had a secondary role in enhanced heat flux and entrainment of subsurface heat
•This review explores passive enhancement of pool boiling using surface modification.•Examined are macrosacle, microscale, nanoscale and multiscale (hybrid-scale) techniques.•Addressed are ...performance goals of inhibiting incidence hysteresis and increasing both nucleate boiling heat transfer coefficient and CHF.•Shown is that micro and nano surface features are susceptible to blockage and changes in performance over time.
This paper provides a comprehensive review of published articles addressing passive enhancement of pool boiling using surface modification techniques. They include macroscale, microscale, and nanoscale surfaces, as well as multiscale (hybrid-scale), and hybrid-wettability techniques. Different enhancement methods are assessed in terms of underlying fluid routing mechanisms and ability to achieve three distinct heat transfer goals: eliminating incipient boiling hysteresis, increasing nucleate boiling heat transfer coefficient, and ameliorating critical heat flux (CHF), especially for inert dielectric coolants that are both highly wetting and possess relatively poor thermophysical properties. While different enhancement scales are shown to provide different degrees of success in achieving the three goals, it is shown that both microscale and nanoscale surface features are susceptible to blockage, resulting in deterioration of the enhancement over time. This review also points to scarcity of sufficiently sized databases for a given enhancement scheme in terms of fluid type, surface material, size, and orientation, enhancement shape, pattern, and scale, and operating pressure. This renders available findings less-than-adequate tools for design of practical cooling systems.
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•Discrepancies in reported CHF values on smooth copper surfaces are investigated.•A total of 54 data points from 47 sources from literature are analyzed.•Roughness and sample ...fabrication type statistically significantly influence CHF.•Thermal conductivity and gradient calculation method influence heat flux up to 30%.•Recommendations for reduction and fair evaluation of uncertainty are made.
This study investigates the effect of experimental setup design factors on pool boiling CHF, compares spatial temperature gradient calculation methods and analyzes the uncertainty of heat flux and surface superheat. Reported CHF values on smooth copper surfaces, measured for saturated pool boiling of water at atmospheric pressure on flat horizontal samples, are highly scattered, which cannot be explained solely by the measurement uncertainty or the randomness of the boiling process. CHF data for 54 experiments from 47 publications is analyzed using regression analysis and ANOVA to determine which experimental setup design factors influence the CHF value. Methods for estimating the axial temperature gradient in a heating stem are compared using the Monte Carlo method and analytical nonlinear gradients. Heat flux values calculated using temperature measurements in a cylindrical copper heating stem together with either constant or temperature-dependent thermal conductivity and various temperature gradient calculation methods are compared. Overall heat flux and surface superheat measurement uncertainties are analyzed and the impact of contributing uncertainties including that of the thermal conductivity, temperature measurement and distance between thermocouples is reported.
The thermal properties and boundary conditions of real (not theoretical) thermal systems are imprecise or uncertain. As a consequence, the temperatures and heat fluxes exhibit variability which is ...usually expressed in terms of a standard deviation. Generally uncertain system parameters are assumed to be stochastic. However, in real life it is highly probable that they are better described by fuzzy variables. This paper describes how to compute the variability for parameters which are stochastic, fuzzy, or both. It is shown that, for the problem considered here, a first order perturbation analysis gives reasonably accurate estimates of the system variability for both types of parameters.
There is no consensus regarding the change of magnitude of urban overheating during HW periods, and possible interactions between the two phenomena are still an open question, despite the increasing ...frequency and impacts of Heatwaves (HW). The purpose of this study is to explore the interactions between urban overheating and HWs in Sydney, which is under the influence of two synoptic circulation systems. For this purpose, a detailed analysis has been performed for the city of Sydney, while considering an urban (Observatory Hill), in the Central Business District (CBD), and a non-urban station in Western Sydney (Penrith Lakes). Summer 2017 was considered as a study period, and HW and Non-Heatwave (NHW) periods were identified to explore the interactions between urban overheating and HWs. A strong link was observed between urban overheating and HWs, and the difference between the peak average urban overheating magnitude during HWs and NHWs was around 8 °C. Additionally, the daytime urban overheating effect was more pronounced during the HWs when compared to nighttime. The advective flux was found as the most important interaction between urban overheating and HWs, in addition to the sensible and latent heat fluxes.
The correct partitioning of the energy between the sensible and latent heat fluxes at the bare soil and computing its ground temperature is very important in designing the surface scheme for modeling ...the processes in the land-air exchange. However, using different bare soil evaporation schemes in land surface parameterization an error in Bowen ratio and computing the ground temperature can be introduced. In parameterization of evaporation from bare soil surface in resistance representation the "α" and "β" approaches are commonly used in corresponding formulas where α and β are functions of soil water content. The performance of different schemes within these approaches are shortly discussed. For that purpose the six schemes (Philip 1957, Deardorff 1978, Barton 1979, Jacquemin and Noilhan 1990, Lee and Pielke 1992, Mihailović et al. 1993) based on different dependence α or β on volumetric soil moisture content and its saturated value are used. The latent and sensible fluxes and ground temperature outputs were obtained from the numerical tests using the foregoing schemes. The tests were based on time integrations by the bare soil parameterization scheme (BARESOIL) using real data. A dataset of 24 June 1982 obtained at the experimental site in Rimski Šančevi, Yugoslavia, on chernozem soil was used. The obtained values of Bowen ratio and ground temperature were compared with the observed ones. Finally, their variability was considered using a root-mean-square analysis.
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