Radiative Flux Divergence Research Articles

Effects of radiation on clouds

The effects of radiation on the growth of individual cloud particles and on the structure of clouds are reviewed. It is shown that the rate of growth of both water drops and ice crystals may be changed significantly by effects of radiation either near the edges of cloud sheets or below cloud, although the overall effect on particle size depends on the period spent in such regions. Sub-cloud evaporation of ice particles may be reduced by radiative cooling of the crystals giving rise to persistent fall streaks. The role of radiative transfer in the formation of radiation fog is well established. It is shown, however, that the nature of the radiative process is crucial in explaining the observed distribution of liquid water in fogs. The way in which radiation fogs may be dissipated due to changes in the radiation balance are also described. Layer clouds contain regions of radiative flux divergence which give rise to localised heating or cooling. In low-level water clouds and in some higher-level ice clouds this gives rise to destabilisation of the cloud layer leading to the generation of turbulent kinetic energy. This may be enhanced by evaporation of cloud particles into air entrained into the cloud layer. The sensitivity of the cloud structure to the radiative processes may give rise to a pronounced diurnal variation in the structure and to a dependence of the structure on latitude. The turbulent motions generated, in part, by radiative processes feed back on the cloud particle size distribution either by increasing the time spent in the cloud by a small fraction of the particles or by modifying the coupling between those clouds regions in which particles form with those regions where growth occurs. The review brings together theoretical studies and observations. It is concluded that while the effects of radiation on the evolution of stratocumulus cloud sheets are comparatively well understood, some details remain unresolved. However, cirrus clouds are less fully understood owing to the limited observations and the greater coupling between radiative, dynamical and microphysical processes.

An Observational Study of Stratocumulus Entrainment and Thermodynamics

The stratocumulus (SC) entrainment and thermodynamics are studied using data from the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS) experiment. The rate of entrainment of air from the free troposphere into the cloud-topped PBL is estimated using a technique based on the measurement of ozone flux and mean distribution. The average measured value of entrainment rate was found to be 3.0 mm/s with a range of 1.0 to 5.0 mm/s for cloudy cases. Thermodynamic budgets are constructed for eight DYCOMS cases. It was found that the divergence of the solar radiative flux is an important component of the boundary-layer energetics during midday, the factor which must be accounted for in mixed-layer models. The net longwave radiative flux profiles show good agreement with theoretical models, but an unambiguous partitioning of the flux divergence between inversion and mixed layers could not be established.

Combined Convection–Conduction–Radiation Boundary Layer Flows Using Optimal Control Penalty Finite Elements

Numerical solutions for combined convection and radiation in a laminar boundary layer on an isothermal wall are obtained using optimal control penalty (OCP) finite elements. The integro-differential energy equation is solved without any limitation of optical thickness. The expression for the divergence of radiation flux containing integral terms is written in terms of a one-dimensional radiation field for a flat plate geometry. The radiation interaction effect on the temperature distribution in the boundary layer is described. The solution of the integro-differential energy equation is then compared with known solutions in the limits of optical thickness.

Radiation-Induced Buoyancy-Driven Flow in Rectangular Enclosures: Experiment and Analysis

Experiments have been performed to study the rate of internal radiative heating on the natural convective motion in a vertical rectangular enclosure irradiated from the side. A Mach–Zehnder interferometer has been used to determine the temperature field, and a fluorescing dye injection technique was employed to illustrate the flow structure with water as the working fluid. A theoretical model is developed for predicting the absorption of thermal radiation and the subsequent buoyancy-driven flow. Predictions based on spectral calculations for the radiation flux divergence agree well with the experimental data.

Analysis of radiation-induced natural convection in rectangular enclosures

A model has been developed for predicting the volumetric deposition rate of radiant energy from an external source in a vertical fluid layer and the subsequent buoyancy-driven flow and heat transfer. The model calls for solution of the coupled two-dimensional equations of continuity, momentum, and energy with a one-dimensional radiative transfer model. After experimental validation of the model, parametric calculations were performed to determine the effect of the modified Rayleigh number, fluid Prandtl number, fluid layer opacity, cavity aspect ratio, opaque wall reflectivity, and convective heat loss from the transmitting wall. Natural convective motion and heat transfer are quite different from that found in cavities with differentially heated side walls. Results are presented in the form of temperature distributions, local heat transfer, contours of the stream function, and profiles of the predicted radiative flux divergence. 20 references.

Observations of Saharan Aerosols: Results of ECLATS Field Experiment. Part II: Broadband Radiative Characteristics of the Aerosols and Vertical Radiative Flux Divergence

The results presented in this paper are a part of those obtained during the ECLATS experiment The broadband radiative characteristics of the Sahelian aerosol layer and the vertical radiative flux divergence within the dust layer were determined both from in situ measurements and Mie calculations. In situ measurements of the aerosol layer's reflectances and transmittances of solar radiation led to aerosol single-scattering albedos close to ωA∼0.95. Measurements of the 8–14 μm radiances led to an optied depth by unit of volume of dust in a vertical column CA∼0.34 μm−1. Mie calculations assuming the aerosol refractive index published by Carlson and Benjamin for solar radiation and that measured by Volz for the atmospheric window, showed good agreement with observations. The ratio of infrared to visible optical thickness was δA(8–14 μm)/δA (0.55 μm)∼0.1, instead of 0.3 as calculated by Carlson and Benjamin. This discrepancy is attributable to differences in size distributions assumed. The radiative budget of the Sahelian aerosol layer was determined for clear and dusty conditions. The additional aerosol shortwave heating was as much as 5 K day−1 for δA(0.55 μm) = 1.5 and with the sun overhead, whereas the additional cooling was close to 1 K day−1. As a consequence of the large temperature discontinuity at the surface, important infrared heating at the surface layer was observed. The rather large differences between the aerosol optical properties reported here and those previously reported in the literature are due to different aerosol size distributions; therefore the present paper stresses the importance of careful determination of the size distributions.

Heat transfer analysis of non-gray gas in a flow system. 1st Report. The case of small temperature difference between gas and a heat absorbing surface.

This study aims at establishing a heat transfer analysis of non-gray gas in a flow system. An integro-differential equation of energy conservation is established for the laminar channel flow of non-gray gas, which includes the multi-band feature of radiative gas. The numerical analysis was conducted and the results are compared with the results obtained from the analysis based on the conventional gray gas assumption. From the comparison of these results, it is made clear that the radiative heat flux qr obtained from the gray analysis is greater than that of the non-gray analysis, and convective heat flux qc is vice versa. By comparing the divergence of radiative heat flux and temperature profile, it is made clear that the gray gas analysis estimates excessively small absorption of radiation in the cold region near the heat absorbing surfaces. This results in the large qr and small qc compared with the non-gray analysis.

盆地における夜間冷却に関する熱収支的研究

Heat budget observations were carried out at Akaigawa, Hokkaido, to investigate the mechanism of extreme nocturnal cooling in a basin. The results of vertically integrated heat budget studies of the NBL (nocturnal boundary layer) show that when the general current above the NBL is weak, about 80% of the observed cooling of the whole NBL is attributed to horizontal advective cooling which is due to downslope cold air from surrounding mountains. Cooling due to sensible heat flux divergence accounts for less than 10% of the observed cooling and can be dominant only in the lowest layer. However, when the wind speed of the general current exceeds a critical value (about 10m/s at a height of 1000m), advective cooling decreases and turbulent cooling begins to dominate the observed cooling. Cooling due to atmospheric radiative heat flux divergence remains unchanged during the night and varies little from night to night. It accounts for about 10% of the observed cooling under weak wind conditions and its relative importance increases under strong wind conditions when the observed cooling is reduced.

大気のエーロゾルの有効複素屈折率の推定とその有用PartII.観測

Effective absorption properties of atmospheric aerosols were estimated from five aircraft measurements of reflectance using a method described in our previous paper. Results indicate that the imaginary terms of the effective refractive indices vary within the range from 0 to 0.013 showing a tendency to increase in lower layer corresponding to an increase in industrial aerosols. The method is based on Mie theory and this may introduce additional uncertainties since shapes of real atmospheric particles are nat spherical in general. Determination of this effect on reflectance is vey difficult. But observed and calculated radiative flux divergences in the atmosphere are compared for rough investigation of above uncertainties. The results of the comparisons indicate that they are in good agreement.

Parameterization of the radiative flux divergence in the 9.6 μm O 3 band

A parameterization of the cooling rate of the Earth's atmosphere in the height region of approximately 20–75 km due to radiation transfer in the 9.6 μm O 3 band is proposed. A general random band model is used to calculate the radiation transmission. The parameterization takes into consideration a peculiar feature of the radiative transfer—the height step increases with growing distance from the level for which cooling is calculated. A simple method of accounting for the variation of the vertical profile of the absorbing gas is proposed and utilized. The procedure proposed is multiparametric: 22 parameters are prescribed for calculating cooling at a given level. The procedure can be recommended for calculating radiative cooling for any temperature and ozone profile not having a small- or meso-scale structure. It can be used, in particular, for modelling of the middle atmospheric circulation.

Radiative cooling near the top of a cloudy mixed layer

The role of radiation above a cloudy mixed layer is examined. We find that whereas turbulent entrainment can be identified with a very sharp temperature jump at the cloud top, the effect of radiative flux divergence above the cloud leads to a much more gradual temperature increase. With adequate observations the two can easily be distinguished. The development of the boundary layer can be described in terms of equations in which above-cloud radiation plays no role.

Radiative cooling near the top of a cloudy mixed layer

AbstractThe role of radiation above a cloudy mixed layer is examined. We find that whereas turbulent entrainment can be identified with a very sharp temperature jump at the cloud top, the effect of radiative flux divergence above the cloud leads to a much more gradual temperature increase. With adequate observations the two can easily be distinguished. The development of the boundary layer can be described in terms of equations in which above‐cloud radiation plays no role.

Numerical Modeling of Diurnal Convergence Oscillations Above Sloping Terrain

Abstract We document the development and Sensitivity testing of a numerical model designed to predict diurnal cycles of boundary-layer flows of synoptic horizontal scale above sloping terrain. This application requires detailed vertical resolution of low-level jets, mixed layers and sharp inversions above terrain with large pointwise variations. The truncation errors of the pressure gradient term are of special concern. This term is computed using a thermodynamic state carried as a deviation from a standard state. Turbulent mixing is based upon a simplified diagnostic treatment. The final model produces rather stable solutions because of the rather careful pressure gradient calculation and strong physical dissipation. Sensitivity testing indicates that the model prediction of diurnal convergence cycles depends upon soil parameters. Results are also sensitive to absolute rotation and mixing parameterizations, but not equally sensitive to longwave radiative flux divergence. Applications over complex North A...

The role of radiation in an inversion-capped planetary boundary layer

In an inversion-capped planetary boundary layer (PBL), the structure of the turbulent fluxes as well as the height of the inversion are determined by the interaction of turbulent mixing in the PBL, large-scale subsidence above the PBL and radiational cooling. Here the sensitivity of the inversion height and of the turbulent fluxes due to radiational processes is investigated with the aid of a three-layered model for a well mixed PBL. For an example of the Trade-Wind region, the inversion height (i.e., the difference between surface pressure and pressure at the inversion level) varies between 46 and 257 mb and the surface flux of moist static energy between 417 and 99 W m-2, if the (mean) radiative net flux divergence for both the inversion and the well-mixed layer is changed over a reasonable range of values. None of the parameterization schemes existing in the literature is able to describe these radiational effects in an appropriate way. This is due to the fact that these parameterizations are either not or not flexibly enough linked to the thermodynamical model parameter. Therefore the demand for an adequate parameterization of the radiational influence in a well-mixed PBL under a subsidence inversion is obvious.

Radiative Cooling Effects within and above the Nocturnal Boundary Layer

For representative tropospheric profiles of water vapor, CO2 and temperature we have calculated in situ longwave radiative flux divergence for use in a simplified second-order closure model of nocturnal boundary-layer evolution. The time evolution of “bulk” boundary-layer parameters is little affected by radiational cooling, as seems to be the cast for stress, velocity variance and diffusivity profiles within this layer. In contrast the w′θ′ profile adjusts itself in response to radiative flux divergence which, within the surface layer, depends on surface emissivity, surface temperature and boundary-layer humidity. Under conditions of strong radiative cooling near the surface an elevated minimum in w′θ′ (at heights up to 0.1 h, h being boundary-layer height) exists, and the cooling produces significant effects on the nondimensional, surface-layer gradients ΦH and ΦM, particularly ΦM. Thermodynamically the boundary layer develops a three-layer structure—in the lowest (0.1 h thick) and uppermost (0.2 h thick) layers radiative cooling dominates the total cooling, while in the central layer occupying most of the boundary layer (0.7 h thick) turbulent cooling dominates. At h itself radiative cooling is a significant fraction of the surface cooling rate. Radiative effects are greatest above the boundary layer where large gradient Richardson numbers am generated. Consequently, turbulence in this region decays rapidly after transition, while in the absence of such effects a much slower decay occurs.

Marine Stratocumulus Convection. Part I: Governing Equations and Horizontally Homogeneous Solutions

Abstract A coupled, convective-radiative, boundary-layer model of marine stratocumulus clouds is presented. The model, which is a slight generalization of the cloud-topped, mixed-layer model of Lilly (1968), has as dependent variables the cloud-top height, cloud-base height, mixed-layer moist static energy and total water content, the turbulent fluxes of moist static energy and total water, the cloud-top jumps of moist static energy and total water, the cloud-top temperature, and the net radiative flux divergence at cloud top and in the mixed layer. Under horizontally homogeneous steady-state conditions the governing equations reduce to a system of algebraic equations which is easily solved. This system has been solved for sea surface temperatures between 13 and 18°C and large-scale divergences between 1×10−6 and 6×10−6 s−1. These calculations have been performed for the case when all the radiative cooling is confined to the cloud-top jump condition and for the case when some of the cooling is allowed to ...

Calculations of free atmospheric shortwave spectral characteristics over the desert (from the CAENEX-70 data)

This report presents a summary of a joint Soviet/American exchange program to compare calculations with observations in the real atmosphere, in order to determine and study the diabatic processes that are important for tropospheric energetics. The primary purpose of the present investigation was to determine the effect of aerosols upon radiation characteristics in the real atmosphere. Spectral measurements of the radiation field as a function of height have been made in 0.01 ?m increments from 0.4 ?m to 0.9 ?m along with aerosol optical properties. Using these data, radiative transfer calculations have been performed using the 4-term expansion of the spherical harmonics method in order to provide an independent check on the accuracy of the previous measurements as well as provide recommendations for improved observations. The effect of a small amount of highly absorbing material, such as hematite, has been demonstrated. Extensive data are presented giving spectral shortwave radiation flux and flux divergence values as a function of height in the atmosphere. Because no “best fit” overall spectral regions were obtained, further studies were performed to test the sensitivity of the theoretical results to small variations in the input parameters. The results indicate that the major reason for discrepancies between measured and calculated radiation fields is difficulties with the radiative and aerosol measurements, rather than shortcomings of the calculational scheme. DOI: 10.1111/j.2153-3490.1979.tb00878.x

The effect of radiative flux divergence on entrainment of a saturated convective boundary layer

The effect of radiative flux divergence on entrainment of a saturated convective boundary layer

The effect of radiative flux divergence on entrainment of a saturated convective boundary layer

The effect of radiative flux divergence on entrainment of a saturated convective boundary layer

Intensity, radiative flux and flux divergence of an exponential-wide band

Exact and asymptotic results are obtained for the total intensity, forward radiative flux and forward radiative flux divergence in a plane-parallel layer of a homogeneous medium whose absorption coefficient is given by the envelope of the exponential-wide band model. These are compared to an empirical correlation.