Radiative Flux Divergence Research Articles

Statistics of shallow convection on Mars based on large-eddy simulations. Part 1: shearless conditions

Results of large-eddy simulations of shallow, quasi-steady, shear-less convection in the Martian boundary layer are presented and discussed. In the considered three cases, turbulence is forced by the radiative flux divergence, prescribed as given functions of height, and the strength of the surface heat flux. It is constrained by the temperature inversion at the boundary-layer top. The resulting convective boundary layer exhibits horizontal cellular structures. The presence of radiative heating causes dimensionless statistics of turbulence to depend on the parameter F, defined in terms of the integrated radiative and turbulent heating rates in the boundary layer.

Convective damping of buoyancy anomalies and its effect on lapse rates in the tropical lower troposphere

Abstract. In regions of the tropics undergoing active deep convection, the variation of lower tropospheric lapse rates (2.0 km to 5.2 km) with height is inconsistent with both reversible moist adiabatic and pseudoadiabatic assumptions. It is argued that this anomalous behavior arises from the tendency for the divergence of a convective buoyancy anomaly to be primarily offset by the collective divergence of other updrafts and downdrafts within one Rossby radius of deformation. Ordinarily, convective mass flux divergences are at least partially offset by an induced radiative mass flux divergence in the background atmosphere. If mass flux divergences from lower tropospheric convection are balanced mainly by those of neighboring updrafts/downdrafts, it would force the vertical clear sky radiative mass flux of the background atmosphere to be weakly dependent on height. This is observed at several radiosonde locations in the Western Tropical Pacific between 2.0 and the 5.2 km melting level. At tropical locations where SST's exceed 27°C over a region whose horizontal extent exceeds the local Rossby radius, this condition on the vertical variation of the background radiative mass flux partially constrains the range of physically allowed mean temperature and moisture profiles in the lower troposphere.

A Statistical Model of Cloud Vertical Structure Based on Reconciling Cloud Layer Amounts Inferred from Satellites and Radiosonde Humidity Profiles

Abstract To diagnose how cloud processes feed back on weather- and climate-scale variations of the atmosphere requires determining the changes that clouds produce in the atmospheric diabatic heating by radiation and precipitation at the same scales of variation. In particular, not only the magnitude of these changes must be quantified but also their correlation with atmospheric temperature variations; hence, the space–time resolution of the cloud perturbations must be sufficient to account for the majority of these variations. Although extensive new global cloud and radiative flux datasets have recently become available, the vertical profiles of clouds and consequent radiative flux divergence have not been systematically measured covering weather-scale variations from about 100 km, 3 h up to climate-scale variations of 10 000 km, decadal inclusive. By combining the statistics of cloud layer occurrence from the International Satellite Cloud Climatology Project (ISCCP) and an analysis of radiosonde humidity profiles, a statistical model has been developed that associates each cloud type, recognizable from satellite measurements, with a particular cloud vertical structure. Application of this model to the ISCCP cloud layer amounts produces estimates of low-level cloud amounts and average cloud-base pressures that are quantitatively closer to observations based on surface weather observations, capturing the variations with latitude and season and land and ocean (results are less good in the polar regions). The main advantage of this statistical model is that the correlations of cloud vertical structure with meteorology are qualitatively similar to “classical” information relating cloud properties to weather. These results can be evaluated and improved with the advent of satellites that can directly probe cloud vertical structures over the globe, providing statistics with changing meteorological conditions.

Spectral Radiative Heat Transfer Analysis of the Planar SOFC

Thermo-mechanical failure of components in planar-type solid oxide fuel cells (SOFCs) depends strongly on the local temperature gradients at the interfaces of different materials. Therefore, it is of paramount importance to accurately predict the temperature fields within the stack, especially near the interfaces. Because of elevated operating temperatures (of the order of 1000K or even higher), radiation heat transfer could become a dominant mode of heat transfer in the SOFCs. In this study, we extend our recent work on radiative effects in solid oxide fuel cells [J. Power Sources, 124, No. 2, pp. 453–458] by accounting for the spectral dependence of the radiative properties of the electrolyte material. The measurements of spectral radiative properties of the polycrystalline yttria-stabilized zirconia electrolyte we performed indicate that an optically thin approximation can be used for treatment of radiative heat transfer. To this end, the Schuster–Schwartzchild two-flux approximation is used to solve the radiative transfer equation for the spectral radiative heat flux, which is then integrated over the entire spectrum using an N-band approximation to obtain the total heat flux due to thermal radiation. The divergence of the total radiative heat flux is then incorporated as a heat sink into a three-dimensional thermo-fluid model of a SOFC through the user-defined function utility in the commercial FLUENT computational fluid dynamics software. The results of sample calculations are reported and compared against the base line cases when no radiation effects are included and when the spectrally gray approximation is used for treatment of radiative heat transfer.

Remotly Sensed Sea Surface Temperature from Aircraft

The experiment was conducted around south of the Azores islands, in the middle eastern part of the northern-Atlantic basin. The experimental area was situated between 31°N-38°N and 21°W-28°W. The experiment was performed in order to improve our knowledge of ocean-atmosphere interactions from the local-scale to the meso-scale. The sea surface temperature remotely measured by aircraft was corrected considering the radiation flux divergence. From the descending IR flux in function of the difference between the surface temperatures measured from the ship and the aircraft (SSTshipSSTaircraft), the difference decreases when the IR flux increases. The sea surface temperature was corrected of the radiation flux divergence of the atmosphere between the sea surface and the flight level. The cloudness plays an important role in this correction. The comparison between SST of the ship and the SST of the aircraft shows a difference of 1°C on average which is affected of the descending infrared flux (0.25°C per 100 m)

Heat Balance in the Nocturnal Boundary Layer during CASES-99

Abstract A unique set of nocturnal longwave radiative and sensible heat flux divergences was obtained during the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). These divergences are based on upward and downward longwave radiation measurements at two levels and turbulent eddy correlation measurements at eight levels. In contrast to previous radiation divergence measurements obtained within 10 m above the ground, radiative flux divergence was measured within a deeper layer—between 2 and 48 m. Within the layer, the radiative flux divergence is, on average, comparable to or smaller than the sensible heat flux divergence. The horizontal and vertical temperature advection, derived as the residual in the heat balance using observed sensible heat and radiative fluxes, are found to be significant terms in the heat balance at night. The observations also indicate that the radiative flux divergence between 2 and 48 m was typically largest in the early evening. Its magnitude depends on how fast the gr...

Importance of Turbulence-Radiation Interactions in Turbulent Diffusion Jet Flames

Traditional modeling of radiative transfer in reacting flows has ignored turbulence-radiation interactions (TRI). Radiative fluxes, flux divergences and radiative properties have been based on mean temperature and concentration fields. However, both experimental and theoretical work have suggested that mean radiative quantities may differ significantly from those predictions based on the mean parameters because of their strongly nonlinear dependence on the temperature and concentration fields. The composition PDF method is able to consider many nonlinear interactions rigorously, and the method is used here to study turbulence-radiation interactions. This paper tries to answer two basic questions: (1) whether turbulence-radiation interactions are important in turbulent flames or not; and (2) if they are important, then what correlations need to be considered in the simulation to capture them. After conducting many flame simulations, it was observed that, on average, TRI effects account for about 1/3 of the total drop in flame peak temperature caused by radiative heat losses. In addition, this study shows that consideration of the temperature self correlation alone is not sufficient to capture TRI, but that the complete absorption coefficient-Planck function correlation must be considered.

Radiation heat transfer analysis of the monolith type solid oxide fuel cell

In this study, a modeling framework for heat and mass transport is established for a unit monolith type SOFC, with emphasis on quantifying the radiation heat transfer effects. The Schuster–Schwartzchild two-flux approximation is used for treating thermal radiation transport in the optically thin yttria-stabilized-zirconia (YSZ) electrolyte, and the Rosseland radiative thermal conductivity is used to account for radiation effects in the optically thick Ni–YSZ and LSM electrodes. The thermal radiation heat transfer is coupled to the overall energy conservation equations through the divergence of the local radiative flux. Commercially available FLUENT™ CFD software was used as a platform for the global thermal-fluid modeling of the SOFC and the radiation models were implemented through the user-defined functions. Results from sample calculations show significant changes in the operating temperatures and parameters of the SOFC with the inclusion of radiation effects.

Radiative and turbulent fluxes in the nocturnal boundary layer

This study examines the relative contributions of turbulent and radiative flux divergences to nocturnal cooling by analyzing data from the 60-m flux tower in CASES99 and using an offline model of the radiative flux. The sensitivity of radiative cooling to structure of the atmosphere near the surface is first examined in terms of artificial profiles. In general, the radiative cooling is proportional to the negative curvature of the temperature profile, magnitude of the specific humidity and the deficit of the ground surface temperature. For the days examined, the radiative flux divergence appears to control the initial formation of the surface inversion at the beginning of the night when the turbulence collapses.

Radiative and turbulent fluxes in the nocturnal boundary layer

This study examines the relative contributions of turbulent and radiative flux divergences to nocturnalcooling by analyzing data from the 60-m flux tower in CASES99 and using an offline model of theradiative flux. The sensitivity of radiative cooling to structure of the atmosphere near the surface is firstexamined in terms of artificial profiles. In general, the radiative cooling is proportional to the negativecurvature of the temperature profile, magnitude of the specific humidity and the deficit of the groundsurface temperature. For the days examined, the radiative flux divergence appears to control the initialformation of the surface inversion at the beginning of the night when the turbulence collapses.

Cases-97: Late-Morning Warming And Moistening Of The Convective Boundary Layer Over The Walnut River Watershed

Aircraft, radiosonde, surface-flux, and boundary-layer windprofiler data from the Cooperative Atmosphere Surface Exchange Study's 1997 field project, CASES-97, are combined with synoptic data to study the evolution of the vertically-averaged mixed-layerpotential temperature [Θ]and mixing-ratio [Q] onthree nearly-cloudless days from 1000 CST to 1200CST (local noon is approximately 1230 CST). This was achieved through examination of the terms in the time-tendency (`budget')equations for [Θ]and [Q]. We estimate three of the terms –local time rate of change, vertical flux divergence, andhorizontal advection. For the [Q]-budget, vertical flux divergence usually dominates, buthorizontal advection is significant on one of the three days. The [Q]-budget balances for two of the three days to within the large experimental error. For the Θ-budget,vertical flux divergence accounts for most of the morningwarming, with horizontal advection of secondary importance.The residual in the Θ-budget has the same sign for all three days, indicating that not all the heating is accounted for. We can balance the [Θ]-budgets to within experimental error on two of the three days by correcting the vertical-flux divergence for apparent low biases in the flux measurements of one of the aircraft and in the surface fluxes, and accounting for direct heating of the mixed layer by radiative flux divergence allowing for the effects of carbonaceous aerosols. The [Θ];-budget with these corrections also balances on the third day if horizontal gradients from synoptic maps are used to estimate the horizontal advection. However, the corrected budget for this day does not balance if the horizontal gradient in the advection term is estimated using CASES-97aircraft and radiosondes; we suggest that persistent mesoscale circulations led to an overestimate of the horizontal gradient andhence horizontal advection.

RADIATION ELEMENT METHOD FOR TRANSIENT HYPERBOLIC RADIATIVE TRANSFER IN PLANE-PARALLEL INHOMOGENEOUS MEDIA

In this study the radiation element method is formulated to solve transient radiative transfer with light radiation propagation effect in scattering, absorbing, and emitting media with inhomogeneous property. The accuracy of the method is verified by good agreement between the present calculations and Monte Carlo simulations. The sensitivity of the method against element size, ray emission number, and time increment size is examined. The transient effect of radiation propagation is essential in short-pulse laser radiation transport when the input pulse width is not considerably larger than the system radiation propagation time. The transient characteristics of radiative transfer are investigated in the media subject to collimated laser irradiation and/or diffuse irradiation withtemporal Gaussian and/or square profiles. The inhomogeneous profile of extinction coefficient of the medium affects strongly the transient radiative flux divergence inside the medium.

Short-wave radiation flux divergence in arctic cirrus: a case study

Radiation and particle measurements have been performed with an aircraft in deep cirrus cloud fields near the island of Svalbard. The data of 12 March 1993, when measurements at 10 different levels could be obtained, are used in a comparative study with radiative transfer calculations. In a first analysis, the cirrus cloud field was assumed to be horizontally homogeneous and invariable during the time of measurements (frozen properties). Calculations of the up and downward radiative flux densities showed root mean square differences of 9 Wm −2 from the measurements. To estimate the possible effect of changes of the optical properties of cirrus with time, the flux densities in the upper part (6000–8500 m) and the lower part (3000–5500 m) of the cirrus cloud were analyzed separately. In these simulations, the optical thickness in the lower (upper) part was increased (decreased) by 50%. By this treatment, most of all calculated flux densities were within one standard deviation of the natural variability in each leg. Finally, the effect of inhomogeneities in the cloud field on the solar flux density has been simulated using a Monte Carlo method, since the upper part of the cirrus field has indeed been very inhomogeneous. This paper is a result of a collaborative effort between the MRI in Tsukuba, Japan, and the GKSS in Geesthacht, Germany.

Radiative heat transfer in inhomogeneous, nongray, and anisotropically scattering media

Radiative heat transfer in three-dimensional inhomogeneous, nongray and anisotropically scattering participating media was investigated by using REM2 method. The accuracy of the method was verified by benchmark comparisons against the solutions of Monte Carlo and YIX methods in the case of three-dimensional inhomogeneous media and in the case of three-dimensional media composed of nongray CO2 gas and carbon particles with strong anisotropically scattering. The ray effect and the influences of radiation element division and spectral discretization were examined. The present predictions of heat flux, flux divergence and emissive power were found to be in good agreement with those by Monte Carlo and YIX methods. As an example of an application of engineering interest, radiative heat transfer in a boiler model with non-isothermal, nongray, and anisotropically scattering media is numerically studied. The distributions of radiative heat flux and flux divergences in the boiler furnace are obtained. It is found that larger heat fluxes appear at the wall near the flame and larger heat flux divergences exist around the boundary of the flame. The effect of particle radiation is small in the region downstream the flame.

On meridional Circulation in Stars

Even though the existence of meridional currents in stars has been known for quite a long time (Eddington 1925, Vogt 1925), its exact structure as well as its influence on stellar evolution is still unclear. Some authors concentrated on finding the exact shape of meridional circulation in a rotating star, while others tried to model its effect on the chemical distribution in the interior. In all studies performed so far however, meridional circulation is considered in an asymptotic regime in which the advection of entropy by the meridional currents is supposed to balance exactly the source term of the non-zero radiative flux divergence. Other terms could however be added to that asymptotic regime which could turn out to dominate the transport of chemicals. We wish to present here preliminary results of 3D numerical simulation attempted to tackle this problem.

Dynamic simulation of thermal radiation in participating media by means of mode reduction

The modeling of combined radiative and conductive heat transfer in three-dimensional enclosures with participating media is challenging since the problem is always implicit in temperature, as the heat conduction equation has the divergence of radiative flux as a source term and the radiative transfer equation has the black body intensity as a source. The degree of freedom or the number of equations in these systems is tremendously large especially when the radiation field is resolved by the S 4 method which is known to yield reliable results for the whole range of optical thickness. But one must reduce the number of equations to be solved before implementing on these systems modern techniques of estimation of parameters or inverse problem which requires repeated computation of governing equations. In the present paper, we apply the Karhunen–Loève Galerkin procedure to reduce the governing equations of these systems to a low-dimensional model which predicts the performance of these systems with the accuracy of S 4 method at a drastically reduced computational cost. This technique is expected to facilitate the implementation of many techniques of estimation of parameters and the solution of inverse problems of the radiative heat transfer.

Including Radiative Effects in an Entrainment Rate Formula for Buoyancy-Driven PBLs

The effect of longwave radiative cooling at the planetary boundary layer (PBL) top in determining the entrainment rate was examined in this study. The entrainment rate equation that accounts for longwave radiative cooling can be formally derived from the following steps: 1) Derive the mean buoyancy budget in the thin layer between the averaged entrainment flux (i.e., the minimum buoyancy flux) level and the top of the entrainment zone where turbulent fluxes vanish. 2) Use the Deardorff entrainment closure assumption that the entrainment buoyancy flux is proportional to the vertically averaged buoyancy flux over the whole PBL, which is a generalized form of a widely accepted entrainment closure for the surface-heated convective PBL. This leads to an entrainment velocity that depends linearly on both the inverse of the interfacial Richardson number and the radiative flux divergence above the entrainment buoyancy flux level. The relative importance of these two terms was examined through large eddy simulations (LESs) of several smoke-cloud-topped PBLs with various radiative forcings, radiative properties, temperature inversion strengths, and numerical advective schemes. These PBLs are driven only by cloud-top radiative cooling. The LESs were performed with a fine-grid nesting layer in the entrainment zone where the grid size is about 16 m and 8 m in the horizontal and vertical, respectively, which should be sufficient to resolve entrainment processes, at least for cases with weak capping inversions. The LESs showed that the contribution to entrainment rate from the radiative flux divergence term was either larger than or about equal to that of the interfacial Richardson number term. The LESs also showed that this radiative flux divergence occurs within cloudy regions below the local cloud tops. The analysis was extended to the stratocumulus-topped PBL by using a stratocumulus-like smoke layer, and the result showed that the radiative flux contribution to the entrainment rate was still significant. Based on the physical understanding that this portion of the radiative flux divergence occurs within the upper half of cloud (or smoke) hummocks, the authors were able to analytically derive a relationship that links the radiative flux divergence to the cloud-top fluctuations, which were then empirically related to the interfacial Richardson number.

Summary of the Lövånger International Workshop on Turbulence and Diffusion in the Stable Planetary Boundary Layer

A workshop on the stable planetary boundary layer (PBL) was held on 21–24 October, 1997 at Lovanger, a small town about 80 km north of Umea, Sweden. Thirty-five scientists representing eight countries participated in the meeting, which was arranged by the U.S. Army Research Office, the Swedish Defence Research Establishment, the U.S. National Oceanic and Atmospheric Administration's Air Resources Laboratory, and the Meteorology Department of Uppsala University. Topics addressed included the very stable boundary layer, gravity wave/turbulence interactions, modeling the stable boundary layer, future observations and new measurement techniques, the role of condensation (fog) and radiative flux divergence, and atmospheric diffusion. Invited papers appear in this special issue. Workshop discussions, informal presentations, and specific recommendations are summarized. Workshop participants and organizers are presented in Appendix A.

静穏晴天夜間に裸地面直上で観測された大気の放射加熱について

The radiative flux in the lowest three meters above a bare-soil surface was directly measured on calm nights with little cloud cover. Although divergence of upward radiative flux occurred above 1m, convergence was often observed between 0.2m and 1m all through the night. Almost the same results were obtained for the net flux except that the transitional height between divergence and convergence was some tens of centimeters, which means that radiative warming occurred just above the bare-soil surface during the night. This phenomenon can be explained by postulating that cold air is produced by conduction at the surface of small heat-insulated projections (HIPs) such as soil grains on the ground surface, while the ground releases the heat stored during the day by radiation through the pores between HIPs and warms the air immediately above the surface at night. This “HIP hypothesis” can also account for the so-called “raised minimum (RM)” phenomenon.

Interpretation of Galileo Probe Data and Implications for Jupiter's Dry Downdrafts

The Galileo probe found the jovian abundance of H2S to be 30% solar at the 8 bar level, while the abundance of water was less than 3% solar at 12 bars. From 8 to 20 bars, H2S increased to three times solar, and water apparently increased as well. Since H2S and water condense at 2 and 5 bars, respectively, the probe probably entered a dry downdraft, wherein dry air above 2 bars is advected to 12 bars or deeper (Owenet al.1996,Eos(Spring Suppl.) 77, S171). This is consistent with the fact that the probe entered the south edge of a 5-μm hot spot, a local region of Jupiter's atmosphere known from spectral modeling to be unusually low in cloud abundance (Ortonet al.1996,Science272, 839).We use basic physical constraints to address three problems raised by Galileo probe data. First, it is unclear how the hypothesized downdraft remains dry, since simple models of convection preclude dessication below the 2- and 5-bar condensation levels. We suggest that to suppress moist plumes from below, the downdraft must be of low density below 5 bars and hence thermally indirect, requiring mechanical forcing from other parts of the atmosphere. Second, if geostrophic balance holds, the Galileo probe winds imply that the hot spot (north of the probe site) contains a stable layer from 1 to 5 bars; this is inconsistent with a downwelling, since downwellings should be adiabatic below 2 bars due to the low radiative flux divergence. We show that when the centripetal acceleration of curving parcel trajectories is included in the force balance, however, a variety of density profiles is possible within the hot spot (depending on the radius of curvature of the winds). The most plausible profile implies that the hot spot is nearly dry adiabatic and that the equatorial zone south of the probe site is stable from 2 to 6 bars, suggesting moist adiabatic upwellings with a water abundance of 1–2 times solar. This is consistent with Galileo and Voyager images suggesting upwelling at the equator. The profile further implies that from 1 to 5 bars the hot spot is denser than the equatorial zone south of the probe site. Third, probe data indicate that NH3increased with depth below 1 bar and became constant by 8 bars, H2S began increasing below 8 bars and leveled off by 16 bars, while water only began increasing below 12 bars and was still increasing with depth at 20 bars. We propose that lateral mixing along isopycnals (surfaces of constant potential density) could produce the observed pattern; alternatively, the downwelling might consist of column stretching, so that the NH3, NH4SH, and water lifting condensation levels were pushed to 8, 16, and >20 bars, respectively. In either case, the simplest form of this model requires the downdraft to be less dense than the surroundings from 0.5 to 20 bars. In its simplest form, this model is therefore incompatible with our favored interpretation of the winds; more detailed studies will be necessary to resolve the problem.