Homogeneous Cloud Research Articles

String membranes in higher dimensions

We shall study homogeneous and isotropic clouds formed of String membranes. These membranes are considered in extra dimensions of a higher dimensional Bianchi-I space-time. Some solutions are found for the field equations of the system.

Cloud scattering optical depth and local surface albedo in the Antarctic: Simultaneous retrieval using ground‐based radiometry

We have used solar irradiance measurements from a ground‐based multichannel radiometer system deployed at Palmer Station, Antarctica (64°46′S, 64°04′W), during spring 1991 to simultaneously estimate cloud scattering optical depth and surface albedo. Irradiance measurements at 410 and 630 nm, in conjunction with a discrete ordinate radiative transfer (RT) model, enable this simultaneous retrieval by exploiting the wavelength dependence in Rayleigh scattering strength. The RT model is used in an inverse mode to find the values of surface albedo and cloud optical depth that match calculated and measured irradiances at both wavelengths. Under the homogeneous stratiform cloud cover for which the technique applies, surface albedo at 630 nm was consistently retrieved at above 0.9. For most homogeneous, overcast conditions, cloud optical depth (at 630 nm) is found to be in the range 20–50, with a most probable value of 25. This measurement and retrieval technique should be useful for compiling high‐latitude cloud opacity and surface albedo climatologies of interest for global change and photobiology research.

Radiative Properties of Finite Inhomogeneous Cirrus Clouds: Monte Carlo Simulations

Abstract The Monte Carlo method is used to study the impact of various cloud morphologies (roughness, voids, waves, and horizontal spreading) on radiative properties of finite, thin, model cirrus clouds. The cloud-top reflectance is calculated for various cloud-top structures and is compared to reflectance of finite homogeneous cloud of the same ice crystal size and ice water content. Cloud roughness, voids, and waves generally decrease cloud reflectance as well as cloud absorption. Although the local horizontal variations in the reflectance can be quite large (several hundred percent), variation in the total reflectance, integrated over the top surface of a cloud, is in the range of a few percent. For overhead incident radiation, the decrease in cloud reflectance due to the considered morphological changes remains under 5%, as compared to a finite homogeneous cloud. A comparable reduction in cloud reflectivity is achieved by about a 5% increase of the effective size of ice crystals. The reflectivity of a...

Lidar Multiple Scattering in Water Droplet Clouds: Toward an Improved Treatment

Although it has long been recognized that the effects of photon multiple scattering generally need to be accounted for in the analysis of lidar cloud returns, this is a difficult problem and current approaches are still rudimentary. The multiple scattering process is controlled by the size of the lidar beamwidth and the distance to the cloud, which jointly determine the lidar footprint, but cloud microphysical content (i.e., particle size, concentration, and shape) exerts a strong influence on the range distribution and depolarization of the returned energy. Since clouds are inherently inhomogeneous with height, it is our premise that vertically homogeneous cloud simulations based on idealized particle size distributions lead to misleading results. We offer a more realistic approach based on the contents of growing water droplet clouds predicted by a sophisticated adiabatic cloud model, which are offered for use as new standard vertically-inhomogeneous cloud models. Lidar returned signal and depolarization profiles derived from our analytical double-scattering method are given for inter-comparison purposes.

Geometrical Thickness, Liquid Water Content, and Radiative Properties of Stratocumulus Clouds over the Western North Pacific

Abstract An algorithm was developed for retrieving cloud geometrical thickness from a measured liquid water path and equivalent width of 0.94-µm water vapor absorption band. The algorithm was applied to aircraft observations obtained by a microwave radiometer and a spectrometer in the winter of 1991 over the western North Pacific Ocean. Retrieved values of the cloud geometrical thickness are apt to be smaller than those observed by eye, especially for horizontally inhomogeneous clouds. Measured cloud albedos in the visible and near-infrared spectral region were also compared with calculated values. For homogeneous clouds there exists a single droplet size distribution that satisfies both spectral regions. However, for inhomogeneous clouds no single size distribution exists that satisfies the albedo observed in both spectral regions.

Simple parameterizations of the radiative properties of cloud layers: a review

Current atmospheric general circulation models parameterize cloud-radiation effects through relationships that are based on the assumption that a model grid is partly clear and the remaining area is filled with a homogeneous cloud layer. The optical properties of this homogeneous layer are related to cloud microphysical quantities such as the liquid water path and effective drop radius which in turn are diagnosed from model variables. In the future it will be necessary to consider distributions of liquid water paths within a grid and relate the effective radius to the type of cloud formed since the grid mean radiative properties are very sensitive to these parameters. There is also the need for a computationally efficient technique to model overlapping liquid and vapor absorption in the solar near infrared.

Radiative Transfer in Cloud Fields with Random Geometry

Abstract Numerical results are given to estimate the importance of effects associated with the stochastic geometry of cloud fields. These results show the important of treating radiative transfer in broken clouds as a statistical problem. In the case of the statistically homogeneous cloud fields, the mean fluxes and brightness fields depend weakly on the slide of cloud base. It is shown that mean fluxes and mean path lengths depend nonlinearly on cloud freedom.

Magnetogravitational instability of a rotating homogeneous gas cloud with radiation

Magnetogravitational instability of an infinite, homogeneous rotating hot plasma cloud associated with radiative processes has been studied with the help of relevant MHD equations using normal mode analysis. Rotation is taken parallel and perpendicular to the magnetic field for both, the longitudinal and transverse modes of propagation. The Jeans espression of instability is modified to give the stabilizing effect of radiation pressure. The stabilizing effect of magnetic field is observed only for transverse mode of propagation whereas rotation stabilizes only along the magnetic field for transverse mode. The stabilizing effect of rotation is comparatively more effective.

On the thermochemical stability of the pregalactic gas. 1: The equilibrium states

view Abstract Citations (5) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Thermochemical Stability of the Pregalactic Gas. I. The Equilibrium States Rosenzweig, Patricia ; Parravano, Antonio ; Ibanez S, Miguel H. ; Izotov, Yu. I. Abstract The thermochemical equilibrium states of pregalactic homogeneous clouds have been calculated as functions of the particle number density. Attention is mainly focused on the dependence of these equilibrium states upon five free parameters: the external flux level, its spectral shape, the mass of the cloud, the net cosmic-ray ionization rate, and the abundance by number of He nuclei. The cloud is assumed to be constituted by a gas with nine chemical species: H, H2, H(-), e(-), H(+), H2(+), He, He(+), and He(++). A total of 24 chemical reactions are considered. These reactions include processes of photoionization, photodissociation, recombination, and charge exchange. The equilibrium is calculated considering heating rates due to photodissociation and photoionization, and cooling rates due to collisional excitation of atomic and molecular species. In the range of density under consideration, there are at least two zones where the gas pressure decreases as the density increases, and where, in general, there are many thermochemical equilibrium states for the same density. Depending on the adopted values for the free parameters, self-shielding of the dissociating radiation produces a third many-valued zone between the two mentioned above. The physical relevance of the equilibrium states is discussed. In particular, the results suggest that for M approximately 109 solar mass, the onset of the gravitational collapse could be triggered by hydrogen recombination effects. It is also shown that a cloud of 107 solar mass cooling isobarically from a high-temperature, high-ionization state reaches thermochemical equilibrium in a relatively short period of time (less than 3 x 107 yr) if the pressure is below a critical value. Otherwise, a gravitationally unstable state is reached. These time-dependent calculations show that the results for the thermochemical equilibrium states help us to infer the final state toward which nonequilibrium configurations evolve. Publication: The Astrophysical Journal Pub Date: September 1994 DOI: 10.1086/174588 Bibcode: 1994ApJ...432..485R Keywords: Chemical Equilibrium; Galactic Evolution; Hydrodynamics; Intergalactic Media; Thermochemical Properties; Thermochemistry; Thermodynamic Equilibrium; Atomic Excitations; Charge Exchange; Helium; Hydrogen; Interstellar Matter; Molecular Excitation; Photodissociation; Photoionization; Recombination Reactions; Astrophysics; ATOMIC PROCESSES; GALAXIES: FORMATION; HYDRODYNAMICS; INSTABILITIES; ISM: GENERAL full text sources ADS |

Analysis of the POLDER polarization measurements performed over cloud covers

The POLDER instrument is designed to provide wide field of view bidimensional images in polarized light. During campaigns of the airborne version of the instrument, images of homogeneous cloud fields were acquired in polarized bands centered at 450 and 850 nm. The polarization of these images is analyzed. The bidirectional polarization distribution function measured in the 850 nm band is shown to make evident the liquid phase of the cloud droplets, by the large characteristic polarization of the cloudbows detected in backward scattering directions. The sensitivity of this feature to cloud parameters is discussed. On the contrary, for observation directions at about 90/spl deg/-100/spl deg/ from the Sun, the cloud polarization is negligible. In these directions, the polarized light observed in the 450 nm band is characteristic of the molecular scattering about the cloud, which allows the cloud top altitude to be derived. The feasibility of the method is analyzed and is tested on cloud pictures acquired at different altitudes above cloud fields. >

Interactions and collisions of bubbles in thermocapillary motion

A calculation of the motion of two bubbles in a constant temperature gradient is presented for the limit of creeping flow and vanishing Péclet number. The results are valid for undeforming, spherical bubbles of arbitrary size and orientation with respect to the temperature gradient, and for separations up to near touching. Trajectories of two bubbles, as they approach one another in an infinite fluid, are then determined from these instantaneous velocity calculations. The limiting trajectories, where the bubbles ‘‘just miss’’ each other, are used to tabulate collision efficiencies over a wide range of bubble size ratios. On the basis of the two-bubble results, a model for the evolution of a statistically homogeneous cloud of bubbles with a given initial size distribution is formulated in terms of a discrete stochastic collection equation. Using this simple model, the dependence of the rate of coalescence in a bubble cloud upon material properties, the bubble volume fraction, the average temperature gradient, and other characteristics of the system, is demonstrated. A significant conclusion is that the bubble collision rate increases with the standard deviation of the bubble size distribution.

Cirrus Microphysics and Radiative Transfer: Cloud Field Study on 28 October 1986

The radiative properties of cirrus clouds present one of the unresolved problems in weather and climate research. Uncertainties in ice particle amount and size and, also, the general inability to model the single scattering properties of their usually complex particle shapes, prevent accurate model predictions. For an improved understanding of cirrus radiative effects, field experiments, as those of the Cirrus IFO of FIRE, are necessary. Simultaneous measurements of radiative fluxes and cirrus microphysics at multiple cirrus cloud altitudes allows the pitting of calculated versus measured vertical flux profiles; with the potential to judge current cirrus cloud modeling. Most of the problems in this study are linked to the inhomogeneity of the cloud field. Thus, only studies on more homogeneous cirrus cloud cases promises a possibility to improve current cirrus parameterizations. Still, the current inability to detect small ice particles will remain as a considerable handicap.

Homogeneous and isotropic clouds of membranes in extra dimensions

A model of a homogeneous and isotropic cloud formed by multidimensional membranes evolving in a spacetime with extra dimensions is presented. The dynamical behaviour of homogeneous and isotropic universes with membranes of several dimensions located in extra spaces of different sizes are studied in some detail. Some general features that depend on the relative dimensions involved in these type of universes are found.

Light Scattering by Dust Particles in the Outer Solar System

AbstractPhotometric observations of the zodiacal light performed by Pioneer 10 indicated that there may be very little scattering by dust in the outer solar system. To shed more light on this problem we formulate explicit expressions for interpreting the brightness observed by a spacecraft travelling inside or outside a finite homogeneous cloud of scattering particles. An application is made to the ecliptic zodiacal light brightness as observed by Pioneer 10 and tabulated by Toller and Weinberg (1985). A satisfactory interpretation of these data as well as earthbound observations can be given by means of a model having a particle density distribution or mean scattering cross section which vanishes beyond 2.8 - 3.7 AU. Some implications for the nature and spatial distribution of the interplanetary dust are discussed.

Discrete angle radiative transfer: 1. Scaling and similarity, universality and diffusion

In order to facilitate study of very inhomogeneous optical media such as clouds, the difficult angular part of radiative transfer calculations is simplified by considering systems in which scattering occurs only in certain directions. These directions are selected in such a way that the intensity field decouples into an infinite number of independent (e.g., orthogonal) families in direction space, each coupled only within its family. Further discretization, this time in space, lends itself readily to both analytical renormalization approaches (part 2) and to numerical calculations (part 2). We are particularly interested in scaling systems in which the optical density field has no characteristic size over a wide range of scales; these include internally homogeneous media of any shape but are more generally internally inhomogeneous and better described as fractals or multifractals. In this case, the albedo and transmission obey power laws in the thick cloud limit if scattering is conservative. By deriving powerful discrete angle (DA) similarity relations, we show that the scaling exponents that characterize these laws are “universal” in the sense that they are independent of the DA phase functions. We argue that these universality classes may be generally expected to extend beyond DA to include standard (continuous angle) phase functions and transfer equations. By comparing the DA equations with the diffusion equation, we show that in general the thick cloud limits of the two will be different: the thick cloud regime will only be “diffusive” in very homogeneous clouds, hence the term “universality class” is more appropriate. The DA similarity relations indicate that in scaling systems spatial variability is of primary importance, this suggests that far more research should be made to realistically model the spatial variability and to investigate its effect on radiative response, even if the angular aspect of the transfer process is made much less sophisticated than is possible in the classical plane‐parallel type medium.

Discrete angle radiative transfer: 2. Renormalization approach for homogeneous and fractal clouds

The discrete angle radiative transfer systems discussed in part 1 readily lend themselves to approximation schemes in which simple scaling systems with known radiative transfer properties can be doubled in size yielding analytic expressions relating the transfer coefficients corresponding to the initial and doubled scale. This “real space renormalization” method can be viewed as a generalization of conventional invariant imbedding techniques to scaling systems. Analytic nonlinear doubling mappings are obtained for homogeneous square, cubic and triangular systems, as well as for a simple fractal system with both open and cyclic horizontal boundary conditions. The doubling mappings have both thick and thin cloud fixed points; to which the transmission and albedoes are respectively algebraically attracted and repelled, with universal (phase function independent) exponents we estimate analytically. The method is approximate since it systematically neglects small‐scale intensity gradients; however, the results are qualitatively correct, arid it therefore establishes the connection between the scaling of the cloud optical density field and the scaling of the corresponding transfer coefficients. We also discuss the limitations of the method; in part 3 we compare it with a numerical approach.

Discrete angle radiative transfer: 3. Numerical results and meteorological applications

In the first two installments of this series, various cloud models were studied with angularly discretized versions of radiative transfer. This simplification allows the effects of cloud inhomogeneity to be studied in some detail. The families of scattering media investigated were those whose members are related to each other by scale changing operations that involve only ratios of their sizes (“scaling” geometries). In part 1 it was argued that, in the case of conservative scattering, the reflection and transmission coefficients of these families should vary algebraically with cloud size in the asymptotically thick regime, thus allowing us to define scaling exponents and corresponding “universality” classes. In part 2 this was further justified (by using analytical renormalization methods) for homogeneous clouds in one, two, and three spatial dimensions (i.e., slabs, squares, or triangles and cubes, respectively) as well as for a simple deterministic fractal cloud. Here the same systems are studied numerically. The results confirm (1) that renormalization is qualitatively correct (while quantitatively poor), and (2) more importantly, they support the conjecture that the universality classes of discrete and continuous angle radiative transfer are generally identical. Additional numerical results are obtained for a simple class of scale invariant (fractal) clouds that arises when modeling the concentration of cloud liquid water into ever smaller regions by advection in turbulent cascades. These so‐called random “β models” are (also) characterized by a single fractal dimension. Both open and cyclical horizontal boundary conditions are considered. These and previous results are contrasted with plane‐parallel predictions, and measures of systematic error are defined as “packing factors” which are found to diverge algebraically with average optical thickness and are significant even when the scaling behavior is very limited in range. Several meteorological consequences, especially concerning the “albedo paradox” and global climate models, are discussed, and future directions of investigation are outlined. Throughout this series it is shown that spatial variability of the optical density field (i.e., cloud geometry) determines the exponent of optical thickness (hence universality class), whereas changes in phase function can only affect the multiplicative prefactors. It is therefore argued that much more emphasis should be placed on modeling spatial inhomogeneity and investigating its radiative signature, even if this implies crude treatment of the angular aspect of the radiative transfer problem.

Gravitational instability of a homogeneous gas cloud with radiation

The problem of the gravitational instability of an inviscid plasma cloud is investigated by taking into account radiative effects. It is shown that, on given conditions, the radiation stabilizes the system.

IRAS observations and local properties of interplanetary dust

Thermal emission from the interplanetary dust cloud has been confirmed by IRAS to be the most prominent component of the sky brightness in the 10–25 μm range. It is indeed a severe foreground for astronomical observations. It is also a major source of information on the local properties of the dust grains. Unlike the solar scattered light (zodiacal light), the thermal emission of the grains is isotropic; even more progress can therefore be expected from the inversion of IRAS integrated brightnesses. The pre-IRAS understanding of the interplanetary dust cloud is reviewed. Recent results obtained through inversion methods are presented. In the ecliptic near 1 AU, the heliocentric gradient of temperature is about −0.33, i.e. conflicts with a greybody assumption, while the gradient of albedo is about −0.7, i.e. conflicts with a homogeneous cloud assumption. Out of the ecliptic, the dust distribution is still open to dispute. It is however neither smooth (dust bands and trails, warped symmetry surface), nor homogeneous (location dependence of local polarization or albedo). Such results imply that the mixing ratios of the various interplanetary dust sources (comets, asteroids, β meteoroids) strongly depend upon location in the cloud.

Broken Cloud Biases in Albedo and Surface Insolation Derived from Satellite Imagery Data

Abstract Radiative transfer calculations for a one-dimensional column model of the atmosphere containing a plane-parallel, homogeneous cloud are used to show that the common procedure of assuming that fields of view for high resolution satellite imagers are either overcast or cloud-free will lead to biases in the infrared albedo and surface insulation for typical (250 km)2 weekly to monthly mean climate scales. The biases arise because cloud fields on the (1–8 km)2 scale typical of satellite imager resolution are often broken rather than overcast and because the anisotropy of the radiance held for overcast regions differs from that for cloud-flee regions. Furthermore, the anisotropy for overcast regions is a nonlinear function of cloud optical depth; consequently, there is no equivalent overcast system that will reproduce the anisotropy of the radiance field reflected by broken clouds. The results of the calculations indicate that the biases are a function of the sun-earth-satellite viewing geometry. For ...