Mean Heat Flux Research Articles

Numerical study of thermoacoustic Taconis oscillations

This paper studies quantitatively a thermoacoustic field by the Taconis oscillations with finite amplitude in a helium-filled, quarter-wavelength tube. Numerical simulations are performed based on the one-dimensional theory in the boundary-layer approximation developed in a previous paper [N. Sugimoto and D. Shimizu, Phys. Fluids 20, 104102 (2008)] by solving initial- and boundary-value problems for a smooth step temperature distribution. It is found that the variations in the density, temperature, and entropy are so significant that the mean values deviate from the respective values in quiescent state. The mean acoustic energy flux and mean heat flux are calculated not only in the main-flow region but also in the boundary layer. The mean convective heat flux appears locally in the main-flow region due to higher-order nonlinear effects. While the total heat flux into the gas vanishes per one period, the local heat flux flows into the gas over a middle part of the tube.

Interannual variability of latent and sensible heat fluxes in the South China Sea

The South China Sea (SCS) is significantly influenced by El Nino and the Southern Oscillation (ENSO) through ENSO-driven atmospheric and oceanic changes. We analyzed measurements made from 1960 to 2004 to investigate the interannual variability of the latent and sensible heat fluxes over the SCS. Both the interannual variations of latent and sensible heat fluxes are closely related to ENSO events. The low-pass mean heat flux anomalies vary in a coherent manner with the low-pass mean Southern Oscillation Index (SOI). Time lags between the heat flux anomalies and the SST anomalies were also studied. We found that latent heat flux anomalies have a minimum value around January of the year following El Nino events. During and after the mature phase of El Nino, a change of atmospheric circulation alters the local SCS near-surface humidity and the monsoon winds. During the mature phase of El Nino, the wind speed decreases over the entire sea, and the air-sea specific humidity difference anomalies decreases in the northern SCS and increases in the southern SCS. Thus, a combined effect of wind speed anomalies and air-sea specific humidity difference anomalies results in the latent heat flux anomalies attaining minimum levels around January of the year following an El Nino year.

Surface energy budget over the South Pole and turbulent heat fluxes as a function of an empirical bulk Richardson number

Routine radiation and meteorological data at South Pole Station are used to investigate historical discrepancies of up to 50 W m−2 in the monthly mean surface energy budget and to investigate the behavior of turbulent heat fluxes under stable atmospheric temperature conditions. The seasonal cycles of monthly mean net radiation and turbulent heat fluxes are approximately equal, with a difference of 40 W m−2 between summer and winter, while the seasonal cycle of subsurface heat fluxes is only a few W m−2. For an 8‐month period (the winter of 2001), we calculate two estimates of turbulent heat fluxes, one from Monin‐Obukhov (MO) similarity theory and one as the residual of the surface energy budget (i.e., subsurface heat fluxes minus net radiation, where all fluxes toward the snow surface are positive). The turbulent fluxes from MO theory agree well with the residual of the energy budget under lapse conditions. However, under stable conditions MO theory underestimates turbulent fluxes by approximately 40–60%. The relationship between turbulent heat fluxes as a residual of the energy budget, temperature inversion strength, and wind shear as a function of the bulk Richardson number (Rib) is examined under stable conditions (i.e., positive Rib). The Rib used here is calculated from 10‐m wind speeds and 0‐ to 2‐m temperature inversion strength. No critical value of Rib is found where the turbulent heat fluxes drop to zero. However, a threshold (Rib = 0.05) exists below which 70% of the turbulent energy fluxes can be explained by only the temperature inversion strength. For Rib > 0.05, the relationship between turbulent heat fluxes and temperature inversion strength decreases, while the importance of wind shear to turbulent heat transfer increases. Above Rib = 0.05, a growing linear correlation also exists between atmospheric temperature inversion strength and wind shear. Thus, inversion strength and wind shear are not independent predictors of turbulent heat flux for extremely stable conditions. The exact values of the correlation coefficients and Rib threshold are likely specific to the experimental conditions; however, their implications are probably valid for all stable flows. Knowledge of the time‐varying surface characteristics would help to generalize these parameters.

Estimation of sea ice thickness distributions through the combination of snow depth and satellite laser altimetry data

Combinations of sea ice freeboard and snow depth measurements from satellite data have the potential to provide a means to derive global sea ice thickness values. However, large differences in spatial coverage and resolution between the measurements lead to uncertainties when combining the data. High‐resolution airborne laser altimeter retrievals of snow‐ice freeboard and passive microwave retrievals of snow depth taken in March 2006 provide insight into the spatial variability of these quantities as well as optimal methods for combining high‐resolution satellite altimeter measurements with low‐resolution snow depth data. The aircraft measurements show a relationship between freeboard and snow depth for thin ice allowing the development of a method for estimating sea ice thickness from satellite laser altimetry data at their full spatial resolution. This method is used to estimate snow and ice thicknesses for the Arctic basin through the combination of freeboard data from ICESat, snow depth data over first‐year ice from AMSR‐E, and snow depth over multiyear ice from climatological data. Due to the nonlinear dependence of heat flux on ice thickness, the impact on heat flux calculations when maintaining the full resolution of the ICESat data for ice thickness estimates is explored for typical winter conditions. Calculations of the basin‐wide mean heat flux and ice growth rate using snow and ice thickness values at the ∼70 m spatial resolution of ICESat are found to be approximately one‐third higher than those calculated from 25‐km mean ice thickness values.

Forced-convection film condensation on a horizontal cylinder with wavy surface structure

The forced-convection film condensation on a horizontal cylinder with wavy surface structure was performed by boundary-layer-approximation. The local/mean heat fluxes were obtained for the effects of tube temperature, wave number, and wave amplitude. The mean heat flux increases with decreasing wavy amplitude and tube temperatures. Furthermore, when β = 20 and α = 0.005, the mean heat flux slightly increases from 1.1 to 3.6% compared with that of smooth tube, depending on tube temperature.

Comment on “Seasonal heat budgets of the Red and Black seas” by Matsoukas et al.

Abstract : Matsoukas et al. [2007] present a monthly analysis of heat fluxes in relation to heat budget in the Red and Black Seas to provide further insight for air-sea exchange processes in the small ocean basins. Components of net surface heat flux are illustrated during 1984-1995. In computing latent and sensible heat fluxes, Matsoukas et al. [2007] apply traditional bulk formulations. A heat balance method that is based on the available energy for evaporation flux is also presented to compare latent heat fluxes with those from the bulk formulations. All near-surface atmospheric variables, including wind speed at 10 m, used in the heat balance method are obtained from reanalysis of a numerical weather product (NWP). Initial input data for radiation flux calculations are at resolutions of 1.0-degrees and 2-degrees, depending on the availability. Monthly means of heat budget components are computed on the basis of monthly means of atmospheric variables during 1984-2000.

Vertical heat transport in eddying ocean models

The effect of mesoscale eddies on the vertical heat transport of the ocean is examined using two eddy‐resolving numerical models. The global heat transport by the mean flow and diffusion are both downwards and are balanced by an upward eddy heat flux. Mean and eddy advective heat fluxes dominate the subpolar regions, while diffusive flux is important primarily in the subtropics. In the subtropical abyss, the mean advective heat flux is balanced by a combination of eddy and diffusive fluxes and the classical Munk‐type advective‐diffusive heat balance must be modified. The Munk and Wunsch (1998) expression for the vertical turbulent diffusivity over‐estimates the diffusivity by as much as a factor of four near the base of the main thermocline. This implies that the mixing required to close the meridional overturning circulation determined by Munk and Wunsch (1998) may be an over‐estimate due to the neglect of mesoscale eddies.

Vertical Velocity and Vertical Heat Flux Observed within Loop Current Eddies in the Central Gulf of Mexico

Abstract Sixteen months of observations from a surface-to-bottom mooring in the central Gulf of Mexico show that acoustic Doppler current profilers (ADCPs) are useful for directly measuring the vertical velocity within mesoscale anticyclonic eddies, such as those shed from the Loop Current; and combining simultaneous temperature measurements, vertical heat flux can also be estimated (as a covariance of both variables). There is evidence of significant and coherent signals of vertical velocity ∼2–3 mm s−1 and vertical heat (temperature) transport ∼10−3 °C m s−1 during the presence of three anticyclones. A simple analysis shows downward flow near the eddies’ centers above 350 m and essentially upward flow in the peripheries, but below 700-m depth the pattern is indeed the opposite; however, further study is necessary to determine the eddies’ interior structures. The observations also suggest the existence of a vertical convergence of heat somewhere around 600-m depth, and estimations of adiabatic heat flux suggest that part of the converged heat, which is not recirculated within the eddy, must escape from the eddy and flow upward along the isopycnals up to the surface layers. This is in good agreement with previous results that have suggested that an excess heat gained by the Gulf in the intermediate levels through exchanges with the Caribbean Sea must be exported to the upper layers by an upward mean heat flux.

Quantification of land use/land cover changes in Pearl River Delta and its impact on regional climate in summer using numerical modeling

This study quantified land use/land cover (LULC) changes in Pearl River Delta (PRD) of South China and its impact on regional climate over the last two decades. The LULC change analyses were accomplished by applying a change detection method to a set of Landsat imagery and ancillary data acquired from 1970s to 2000. The results indicate that the urban expansion is the prevailing LULC change in the PRD. Impact of LULC change on regional climate was simulated by using a mesoscale climate model. Two different land cover datasets circa 1990 and 2000 were input to the model to investigate the impact of urbanization on regional weather and climate condition in summer 2005. The simulation results show that rapid urban expansion can substantially alter regional climate conditions in the PRD region including monthly mean temperature, precipitation, moisture, and surface heat fluxes.

Time dependence in 3‐D mantle convection models featuring evolving plates: Effect of lower mantle viscosity

Evolving plate configurations and dynamically determined plate velocities are featured in Cartesian geometry mantle convection simulations. The numerical model enables the evolution of plate shape and size by migrating idealized plate triple junctions. The motion of the model triple junctions responds to the time‐dependent velocities of the adjacent plates. Each calculation includes four high‐viscosity plates in a 3 × 3 × 1 solution domain. We analyze the effect of plate evolution on the time dependence of plate velocity and heat flux in three different models characterized by lower mantle to upper mantle viscosity ratios of 30, 90, and 300. We examine the difference in behavior between calculations featuring fixed and mobile plate boundaries for each viscosity model. When plates are permitted to evolve in response to the convective vigor of the system, plate positions and shapes can change considerably while features in the high‐viscosity lower mantle may change very little. In addition, plate velocities and surface heat flux can be highly time dependent. We find that when the contrast between lower mantle and upper mantle viscosity magnitude is a factor of 30, surface velocities may fluctuate by 75% of the mean value and heat flux by 60%. We also find that plate velocity evolution is characterized by periods of reorganization, punctuating more stable periods. When the lower mantle viscosity is increased to 90 times the upper mantle value, plate reorganization events also occur, but only when plate boundary motion is enabled. When the lower mantle to upper mantle viscosity contrast is increased to a factor of 300, we find that the mean surface velocity and heat flux become very steady in cases both with and without plate boundary evolution, despite the substantial migration of convergent plate boundaries in the former case.

Lake Huron climatology, inter-lake exchange and mean circulation

Selected key features of the physical limnology, hydrology and climate influencing Lake Huron is presented. The lake is deep (229 m), consists of four interconnected bodies of water and currently there are no diversions into or out of the lake. Long-term annual summaries of meteorological and hydrological variables show that the lake affects spatial distributions across the basin. An estimate of the long-term hydrologic budget shows the relative importance of the major inflows, outflows and net basin supply. Long-term monthly average water level is 176.6 m. Monthly mean total heat flux ranges from −155 W m− 2 in December to 154 W m− 2 in June. Light attenuation is higher in Saginaw Bay and the nearshore compared to the mid-lake. Ice extent can be an important factor in the Lake Huron system with values ranging from < 10% to > 90% depending on winter severity. Surface and mean lake temperature for shallow North Channel are similar, however, large volumes of main Lake Huron and Georgian Bay results in mean temperatures nearly 10°C less than surface temperatures in summer. Inter-lake exchange at the Straits of Mackinac is about 2 cm s− 1 but fluctuating flows can be up to 30 times larger. The averaged summer and winter circulation shows cyclonic patterns for Lake Huron.

Influence of the boundary layer height on the global air–sea surface fluxes

Results from large-eddy simulations and field measurements have previously shown that the velocity field is influenced by the boundary layer height, z i , during close to neutral, slightly unstable, atmospheric stratification. During such conditions the non-dimensional wind profile, φ m , has been found to be a function of both z/L and z i /L. At constant z/L, φ m decreases with decreasing boundary layer height. Since φ m is directly related to the parameterizations of the air–sea surface fluxes, these results will have an influence when calculating the surface fluxes in weather and climate models. The global impact of this was estimated using re-analysis data from 1979 to 2001 and bulk parameterizations. The results show that the sum of the global latent and sensible mean heat fluxes increase by 0.77 W m−2 or about 1% and the mean surface stress increase by 1.4 mN m−2 or 1.8% when including the effects of the boundary layer height in the parameterizations. However, some regions show a larger response. The greatest impact is found over the tropical oceans between 30°S and 30°N. In this region the boundary layer height influences the non-dimensional wind profile during extended periods of time. In the mid Indian Ocean this results in an increase of the mean annual heat fluxes by 2.0 W m−2 and an increase of the mean annual surface stress by 2.6 mN m−2.

Impact of swell on simulations using a regional atmospheric climate model

When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared with under wind-sea conditions. Using measurements from the OØstergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A 3-year simulation and two case studies using the wavefield from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2°C annually on average) whereas the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg-1). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space.

Numerical study of circulation and temperature-salinity distributions in the Bras d’Or Lakes

The Bras d’Or Lakes (BdOL) are a large, complex and virtually land-locked estuary in central Cape Breton Island of Nova Scotia and one of Canada’s charismatic ecosystems, sustaining ecological and cultural communities unique in many aspects. The BdOL comprise two major basins, many deep and shallow bays, several narrow channels and straits and a large, geologically complex watershed. Predictive knowledge of the water movement within the estuary is a key requirement for effective management and sustainable development of the BdOL ecosystem. A three-dimensional (3D) primitive-equation ocean circulation model is used to examine the estuary’s response to tides, winds and buoyancy forcing associated with freshwater runoff in a series of numerical experiments validated with empirical data. The model results generate intense, jet-like tidal flows of about 1 m s−1 in the channels between the basins and connecting them to the ocean and relatively weak tidal currents in other regions, which agrees well with previous observations and numerical results. Wind forcing and buoyancy forcing associated with river runoff play important roles in generating the significant sub-tidal circulations in the estuary, including narrow channels, deep basins and shallow bays. The circulation model is also used to reconstruct the 3D circulation and temperature-salinity distributions in the summer months of 1974, when current and hydrographic measurements were made at several locations. The sub-tidal circulation in the estuary produced by the model is characterised by wind and barometric set-up and set-down in different sections of the system, and a classic two-layer estuarine circulation in which brackish, near-surface waters flow seaward from the estuary into the Atlantic Ocean, and deep salty waters flow landward through the major channel. The model results reproduce reasonably well the overall features of observed circulation and temperature-salinity fields made in the BdOL in 1974 but generally underestimate the observed currents and density stratification. The model discrepancies reflect the use of spatially mean wind forcing and spatially and monthly mean surface heat flux and the inability of the coarse model horizontal resolution (∼500 m) to resolve narrow channels and straits.

Thermal convection in ice-I shells of Titan and Enceladus

Cassini–Huygens observations have shown that Titan and Enceladus are geologically active icy satellites. Mitri and Showman [Mitri, G., Showman, A.P., 2005. Icarus 177, 447–460] and McKinnon [McKinnon, W.B., 2006. Icarus 183, 435–450] investigated the dynamics of an ice shell overlying a pure liquid-water ocean and showed that transitions from a conductive state to a convective state have major implications for the surface tectonics. We extend this analysis to the case of ice shells overlying ammonia-water oceans. We explore the thermal state of Titan and Enceladus ice-I shells, and also we investigate the consequences of the ice-I shell conductive–convective switch for the geology. We show that thermal convection can occur, under a range of conditions, in the ice-I shells of Titan and Enceladus. Because the Rayleigh number Ra scales with δ 3 / η b , where δ is the thickness of the ice shell and η b is the viscosity at the base of the ice-I shell, and because ammonia in the liquid layer (if any) strongly depresses the melting temperature of the water ice, Ra equals its critical value for two ice-I shell thicknesses: for relatively thin ice shell with warm, low-viscosity base (Onset I) and for thick ice shell with cold, high-viscosity base (Onset II). At Onset I, for a range of heat fluxes, two equilibrium states—corresponding to a thin, conductive shell and a thick, convective shell—exist for a given heat flux. Switches between these states can cause large, rapid changes in the ice-shell thickness. For Enceladus, we demonstrate that an Onset I transition can produce tectonic stress of ∼ 500 bars and fractures of several tens of km depth. At Onset II, in contrast, we demonstrate that zero equilibrium states exist for a range of heat fluxes. For a mean heat flux within this range, the satellite experiences oscillations in surface heat flux and satellite volume with periods of ∼ 50 – 800 Myr even when the interior heat production is constant or monotonically declining in time; these oscillations in the thermal state of the ice-I shell would cause repeated episodes of extensional and compressional tectonism.

Role of the inclination of an inverted‐V upper plate on the heat and flow behavior of trapped gases in a modified Rayleigh–Bénard cavity

AbstractThermal buoyant air inside a modified Rayleigh–Bénard (RB) cavity bounded by a lower flat plate and an inverted‐V upper plate has been investigated numerically using the finite‐volume method. The second‐order‐accurate QUICK and SIMPLE schemes were used for the discretization of the convective terms and the pressure–velocity coupling in the set of conservation equations, respectively. The problem under study is controlled by two parameters: (1) the Rayleigh number ranging from 103 to 106 and (2) the relative height of the vertical sidewalls d. In reference to the latter, it varies from one limiting case corresponding to the standard RB cavity (a rectangle with d = 1) to another limiting case represented by an isosceles triangular cavity where d = 0. The numerical results for the velocity and temperature fields are presented in terms of streamlines, isotherms, local and mean heat fluxes. An additional effort was devoted to determine the critical Ra values characterizing the transition from symmetrical to asymmetrical buoyant airflow responsive to incremental changes in Ra. For purposes of engineering design, a general correlation equation for the Nusselt number in terms of the pertinent Ra and d was constructed using nonlinear multiple regression theory. Copyright © 2007 John Wiley & Sons, Ltd.

Airflow and heat flux through the vertical opening of buoyancy-induced naturally ventilated enclosures

Characteristics of the mean and turbulent airflow and heat flux through the vertical opening of a buoyancy-induced naturally ventilated full-scale enclosure with upper and lower vents on one of the sidewalls were studied experimentally. The effect of the interaction between the mixing and the displacement ventilation modes on the airflow through the upper vent is explored. Measurements include vertical profiles of mean and turbulent air velocity and temperature through the upper opening using a three-dimensional sonic anemometer. The airflow appears to be inclined to the horizontal plane due to the effect of buoyancy. The level of the neutral plane at the upper vent, defined here as the plane separating between inflow and outflow, can be identified by the vertical profiles of both mean flow and turbulence intensity, with good agreement between the two approaches. The contribution of the turbulent to the total (mean and turbulent) heat flux through the vent decreases as ventilation transforms from the mixing to the displacement mode.

Bora event variability and the role of air‐sea feedback

A two‐way interacting high resolution numerical simulation of the Adriatic Sea using the Navy Coastal Ocean Model (NCOM) and Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) was conducted to improve forecast momentum and heat flux fields, and to evaluate surface flux field differences for two consecutive bora events during February 2003. (COAMPS® is a registered trademark of the Naval Research Laboratory.) The strength, mean positions and extensions of the bora jets, and the atmospheric conditions driving them varied considerably between the two events. Bora 1 had 62% stronger heat flux and 51% larger momentum flux than bora 2. The latter displayed much greater diurnal variability characterized by inertial oscillations and the early morning strengthening of a west Adriatic barrier jet, beneath which a stronger west Adriatic ocean current developed. Elsewhere, surface ocean current differences between the two events were directly related to differences in wind stress curl generated by the position and strength of the individual bora jets. The mean heat flux bias was reduced by 72%, and heat flux RMSE reduced by 30% on average at four instrumented over‐water sites in the two‐way coupled simulation relative to the uncoupled control. Largest reductions in wind stress were found in the bora jets, while the biggest reductions in heat flux were found along the north and west coasts of the Adriatic. In bora 2, SST gradients impacted the wind stress curl along the north and west coasts, and in bora 1 wind stress curl was sensitive to the Istrian front position and strength. The two‐way coupled simulation produced diminished surface current speeds of ∼12% over the northern Adriatic during both bora compared with a one‐way coupled simulation.

Crop Coefficients for Microsprinkler-Irrigated, Clean-Cultivated, Mature Citrus in an Arid Climate

Crop evapotranspiration (ETc) was measured over a clean-cultivated, mature navel orange orchard with microsprinkler irrigation located near Lindsay, California. Hourly mean latent heat flux density was determined as the residual of the energy balance equation with measured net radiation, soil heat flux density and sensible heat flux density estimated using the surface renewal method. The ETc was compared with ETo calculated using hourly weather data and the ASCE-EWRI Penman-Monteith equation. Following pruning and topping of the trees in the spring of 2001, the Kco values slowly increased as the canopy developed in the following season. An average Kco=0.82 was observed. In the following year, the mean summertime value increased to about Kco=0.95, and in 2003 and 2004, the summertime value averaged near Kco=1.00, which is somewhat higher than observed for drip irrigated trees in southwestern Arizona and considerably higher than reported in the widely used Food and Agricultural Organization of the United Nations publications that were based on infrequent surface irrigation.

여수항의 평균 열플럭스

Based on the monthly weather report of Korea Meteorological Administration (KMA) and daily sea surface temperature (SST) data from National Fisheries Research and Development Institute (NFRDI) (1995-2004), mean heat fluxes were estimated at the port of Yeosu. Net heat flux was transported from the air to the sea surface during February to September, and it amounts to <TEX>$205 Wm^{-2}$</TEX> in daily average value in May. During October to January, the transfer of net heat flux was conversed from the sea surface to the air with <TEX>$-70 Wm^{-2}$</TEX> in minimum of daily average value in December. Short wave radiation was ranged from <TEX>$167 Wm^{-2}$</TEX> in December to <TEX>$300 Wm^{-2}$</TEX> in April. Long wave radiation (Sensible heat) was ranged from <TEX>$27 (-14) Wm^{-2}$</TEX> in July to <TEX>$90 (79) Wm^{-2}$</TEX> in December. Latent heat showed <TEX>$42 Wm^{-2}$</TEX> with its minimum in July and <TEX>$104 Wm^{-2}$</TEX> with its maximum in October in daily average value.