For over a decade, several research groups have been developing air-sea heat flux information over the global ocean, including latent (LHF) and sensible (SHF) heat fluxes over the global ocean. This ...paper aims to provide new insight into the quality and error characteristics of turbulent heat flux estimates at various spatial and temporal scales (from daily upwards). The study is performed within the European Space Agency (ESA) Ocean Heat Flux (OHF) project. One of the main objectives of the OHF project is to meet the recommendations and requirements expressed by various international programs such as the World Research Climate Program (WCRP) and Climate and Ocean Variability, Predictability, and Change (CLIVAR), recognizing the need for better characterization of existing flux errors with respect to the input bulk variables (e.g. surface wind, air and sea surface temperatures, air and surface specific humidities), and to the atmospheric and oceanic conditions (e.g. wind conditions and sea state). The analysis is based on the use of daily averaged LHF and SHF and the associated bulk variables derived from major satellite-based and atmospheric reanalysis products. Inter-comparisons of heat flux products indicate that all of them exhibit similar space and time patterns. However, they also reveal significant differences in magnitude in some specific regions such as the western ocean boundaries during the Northern Hemisphere winter season, and the high southern latitudes. The differences tend to be closely related to large differences in surface wind speed and/or specific air humidity (for LHF) and to air and sea temperature differences (for SHF). Further quality investigations are performed through comprehensive comparisons with daily-averaged LHF and SHF estimated from moorings. The resulting statistics are used to assess the error of each OHF product. Consideration of error correlation between products and observations (e.g., by their assimilation) is also given. This reveals generally high noise variance in all products and a weak signal in common with in situ observations, with some products only slightly better than others. The OHF LHF and SHF products, and their associated error characteristics, are used to compute daily OHF multiproduct-ensemble (OHF/MPE) estimates of LHF and SHF over the ice-free global ocean on a 0.25°×0.25° grid. The accuracy of this heat multiproduct, determined from comparisons with mooring data, is greater than for any individual product. It is used as a reference for the anomaly characterization of each individual OHF product.
•Establishing reference input dataset maximizing the use of remotely sensed data•Performing a cross-comparison of different heat flux algorithms and approaches•Generating an ensemble of turbulent fluxes, including multiple approaches•Evaluating the quality and consistency of ensemble realizations•Exploiting integral heat constraints at local, regional and global scales
•Unlike the traditional routine, the Cattaneo–Christov heat flux in formulation is employed.•Variable thermal conductivity is accounted.•Surface with variable thickness is considered.•Double ...stratification is analyzed.•Nonlinear stretching phenomenon is imposed.
Here temperature dependent thermal conductivity in stagnation point flow toward a nonlinear stretched surface with variable thickness is considered. Heat flux in formulation is based upon Cattaneo–Christov theory. Double stratification and chemical reaction effects are further retained. Convergent series solution for flow of Jeffrey fluid and heat and mass transfer are developed. Residual errors are calculated for the velocity, temperature and concentration equations and results are discussed through graphs. Influences of skin friction coefficient is also studied. It is observed that temperature profile decreases for higher thermal relaxation parameter.
This paper gives an overview of 20 years of research on the energy balance closure problem. It will be shown that former assumptions that measuring errors or storage terms are the reason for the ...unclosed energy balance do not stand up because even turbulent fluxes derived from documented methods and calibrated sensors, net radiation, and ground heat fluxes cannot close the energy balance. Instead, exchange processes on larger scales of the heterogeneous landscape have a significant influence. By including these fluxes, the energy balance can be approximately closed. Therefore, the problem is a scale problem and has important consequences to the measurement and modeling of turbulent fluxes.
In a future climate, the Hadley cell and associated trade easterlies are projected to expand poleward. This projected change in the atmospheric circulation is expected to impact the ocean through ...changes in the mean sea surface temperature (SST). We also expect implications for the large‐scale SST variability, because near‐surface wind is directly related to two drivers of the SST, that is, turbulent heat flux and anomalous wind‐driven Ekman heat flux. Previous studies show that in the subtropics, anomalous turbulent and Ekman heat fluxes oppose each other, acting to reduce SST variability, whereas, in the midlatitudes, they reinforce each other and enhance SST variability. Through analysis of reanalysis products and Coupled Model Intercomparison Project simulations, we find that the subtropical regions where the fluxes oppose each other are projected to expand poleward in a future climate, following the poleward expansion of the Hadley cell, with potential implications for the amplitude of subtropical SST variability.
Plain Language Summary
Atmospheric surface wind drives upper ocean circulations. Hence, a change in the surface wind pattern causes a change in ocean circulation. There is evidence that climate change alters the spatial pattern and strength of the surface winds. In particular, easterly winds in the subtropics are projected to expand poleward. The poleward shift of the easterly wind belt also has potential implications for the sea surface temperature (SST) variations because two important drivers of the SST are related to the surface wind: air‐sea heat exchange and the advection of ocean temperature by the wind‐driven ocean current. In the subtropical region, these two mechanisms typically oppose each other, acting to reduce the amplitude of SST variations. We show that the projected poleward expansion of the easterly wind belt will subsequently expand the subtropical region where these two mechanisms oppose each other toward the poles, with potential implications for subtropical SST variability.
Key Points
Anomalous Ekman heat flux opposes and reinforces the anomalous turbulent heat flux in the subtropics and midlatitudes, respectively
The latitudinal boundary where the role of anomalous Ekman heat flux changes is modulated by the Hadley cell edge
A poleward shift of the Hadley boundary in a future climate drives a poleward shift in the latitude where Ekman heat flux changes its role
Serving as the successor to the ERA‐Interim dataset, a new global reanalysis (i.e., ERA5) has been released by the European Centre for Medium‐Range Weather Forecasts. However, evaluation of the ...performance of ERA5 in relation to the Tibetan Plateau (TP) remains lacking. Using in situ eddy covariance observations from eight stations located on the TP, this study evaluated the uncertainties of both ERA5 and ERA‐Interim. Results showed that ERA5 effectively corrects the overestimation of both sensible heat flux (H) and latent heat flux (LE) over the TP, which is severe in ERA‐Interim. In most seasons, the bias errors (BEs) of H and LE are generally smaller in ERA5 than in ERA‐Interim, although the BEs of wintertime H and LE in spring and autumn are not decreased significantly from ERA‐Interim to ERA5. The spatiotemporal relationships with the Global Land Data Assimilation System (GLDAS) and the Global Land Evaporation Amsterdam Model (GLEAM) reanalysis data were investigated. Analysis of correlation coefficients and root mean squared errors revealed large inconsistency in H between ERA5/ERA‐Interim and GLDAS, but good agreement in LE between ERA5/ERA‐Interim and GLEAM. Overall, the level of agreement of ERA5 with GLEAM was better than that of ERA‐Interim. Anomaly correlation coefficients (ACCs) between the two ERA datasets (ERA5 and ERA‐Interim) and the two reference datasets (GLDAS and GLEAM) were also investigated. The ACC results further confirmed that ERA5 has better agreement than ERA‐Interim with GLEAM in terms of LE over the TP.
1. Obvious reduction of uncertainties in surface heat flux is achieved from ERA‐Interim to ERA5 over Tibetan Plateau (TP).
2. There are good agreements between ERA‐reanalysis and GLEAM but inconsistencies between ERA‐reanalysis and GLDAS over TP.
3. Some studies about climate of surface heating over TP based on earlier reanalysis (such as NCEP) are needed to be re‐examined.
Heat removal from high heat flux devices such as computer chips, laser diodes, and other electronic devices and components has been a significant issue with the advances in micro- and nanofabrication ...capabilities. High heat fluxes suggest phase-change heat transfer modes including flow boiling to accomplish effective cooling of miniature devices. In contrast to pool boiling, flow boiling heat transfer is a more applied mode of heat transfer and has been most widely used in high heat flux cooling systems. There are two general approaches for improving flow boiling heat transfer: active and passive methods. In this review, all of the active and passive methods are covered, and almost all of the literature related to high heat flux is included. The other useful parts of this review are the tables, where almost all of the related literature, including studies on various working fluids (particularly refrigerants), channel sizes, and improvement methods to achieve high heat flux conditions are summarized so that scientists and engineers working in the field could greatly benefit from them. At the end, the review presents important developments in the field broad conclusion as well as suggestions and future research directions for future studies.
Using daily reanalysis data from 1979 to 2015, this paper examines the impact of winter Ural blocking (UB) on winter Arctic sea ice concentration (SIC) change over the Barents and Kara Seas (BKS). A ...case study of the sea ice variability in the BKS in the 2015/16 and 2016/17 winters is first presented to establish a link between the BKS sea ice variability and UB events. Then the UB events are classified into quasi-stationary (QUB), westward-shifting (WUB), and eastward-shifting (EUB) UB types. It is found that the frequency of the QUB events increases significantly during 1999–2015, whereas the WUB events show a decreasing frequency trend during 1979–2015.
Moreover, it is shown that the variation of the BKS-SIC is related to downward infrared radiation (IR) and surface sensible and latent heat flux changes due to different zonal movements of the UB. Calculations show that the downward IR is the main driver of the BKS-SIC decline for QUB events, while the downward IR and surface sensible heat flux make comparable contributions to the BKS-SIC variation for WUB and EUB events. The SIC decline peak lags the QUB and EUB peaks by about 3 days, though QUB and EUB require lesser prior SIC. The QUB gives rise to the largest SIC decline likely because of its longer persistence, whereas the BKS-SIC decline is relatively weak for the EUB. The WUB is found to cause a SIC decline during its growth phase and an increase during its decay phase. Thus, the zonal movement of the UB has an important impact on the SIC variability in BKS.
•Three direct cooling techniques are presented and compared in detail.•The thermal and flow parameters which have influence on performances are investigated.•The spray cooling technique is ...characterized by the most promising thermal performances.
Most advanced, high power technologies require a large amount of heat to be dissipated from the limited surface area or space. Solutions to such problems are vital, among others, in the field of computer microchips, where promising designs of future high power processing components can reach heat fluxes up to 500Wcm−2 in the background or even 1000Wcm−2 at the hot-spots. Such high requirements can be satisfied by so-called Direct Cooling Techniques, which are heat removal techniques that apply porous media, microchannel heat sinks and spray cooling. The paper presents an exhaustive comparison of the aforementioned techniques with respect to the media type, operating fluids and flow character, maximal achievable heat flux dissipation and heat transfer coefficient, pressure drop, the Reynolds number, other selected thermal and/or flow parameters. Special attention is paid to the spray cooling technique, which is the most effective direct cooling technology. For that reason, several parameters, such as: spray and fluid types, maximum achieved heat flux, heat transfer coefficient, the Sauter Mean Diameter and flow rate are studied and compared in detail.
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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.
Increased heat-wave frequency across the United States has led to the need for improved predictability of heat-wave events. A detailed understanding of land–atmosphere interactions and the ...relationship between soil moisture and temperature extremes could provide useful information for prediction. This study identifies, for many locations, a threshold of soil moisture below which there is an increase in the sensitivity of atmospheric temperature to declining soil moisture. This shift to a hypersensitive regime causes the atmosphere to be more susceptible to atmospherically driven heat-wave conditions. The soil moisture breakpoint where the regime shift occurs is estimated using segmented regression applied to observations and reanalysis data. It is shown that as the soil gets drier, there is a concomitant change in the rate of decrease in latent heat flux and increase in sensible heat flux leading to a strong positive feedback of increased air temperature near the surface, which further dries out the soil. Central, southwestern, and southeastern parts of the United States seem to have regions of clear regime shifts, while the eastern part of the United States generally does not get dry enough to reveal significant breakpoints. Sensible heat flux is seen to be a primary driver of this increased temperature sensitivity aided by the drop in latent heat flux. An investigation of flux tower sites verifies the breakpoint–flux relationships found in reanalysis data. Accurate estimation of these breakpoints can contribute to improved heat-wave prediction.
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