Homogeneous Cloud Research Articles

Comparison of cirrus optical depths derived from GOES 8 and surface measurements

Ground‐based passive radiometer measurements are used to validate satellite‐derived cirrus optical depths over the Atmospheric Radiation Measurement Program Southern Great Plains site during March 2000. Optical depths derived from direct beam measurements by a multifilter rotating shadow band radiometer were well correlated with those determined from the Geostationary Operational Environmental Satellite, especially in relatively homogenous cloud fields. Compared to the multifilter rotating shadow band radiometer (MFRSR) results, on average, the satellite retrieval overestimated optical depth by ∼0.67 (29%), even though 75% of the GOES values were within ±1.0 of the MFRSR results. Some of the bias is attributable to cloud inhomogeneities, mismatches in observed clouds, errors in the surface albedo, and possible errors in the ice crystal scattering phase function. The results demonstrate the potential for using MFRSR data, available over many parts of the globe, for validating satellite cloud retrievals in many different surface and atmospheric conditions.

A STUDY ON COMBINING IMAGE REPRESENTATIONS FOR IMAGE CLASSIFICATION AND RETRIEVAL

A flexible description of images is offered by a cloud of points in a feature space. In the context of image retrieval such clouds can be represented in a number of ways. Two approaches are here considered. The first approach is based on the assumption of a normal distribution, hence homogeneous clouds, while the second one focuses on the boundary description, which is more suitable for multimodal clouds. The images are then compared either by using the Mahalanobis distance or by the support vector data description (SVDD), respectively.The paper investigates some possibilities of combining the image clouds based on the idea that responses of several cloud descriptions may convey a pattern, specific for semantically similar images. A ranking of image dissimilarities is used as a comparison for two image databases targeting image classification and retrieval problems. We show that combining of the SVDD descriptions improves the retrieval performance with respect to ranking, on the contrary to the Mahalanobis case. Surprisingly, it turns out that the ranking of the Mahalanobis distances works well also for inhomogeneous images.

Models of gas-grain chemistry in interstellar cloud cores with a stochastic approach to surface chemistry

We present a gas-grain model of homogeneous cold cloud cores with time-independent physical conditions. In the model, the gas-phase chemistry is treated via rate equations while the diffusive granular chemistry is treated stochastically. The two phases are coupled through accretion and evaporation. A small network of surface reactions accounts for the surface production of the stable molecules water, formaldehyde, methanol, carbon dioxide, ammonia, and methane. The calculations are run for a time of 10 7 years at three different temperatures: 10 K, 15 K, and 20 K. The results are compared with those produced in a totally deterministic gas-grain model that utilizes the rate equation method for both the gas-phase and surface chemistry. The results of the different models are in agreement for the abundances of the gaseous species except for later times when the surface chemistry begins to affect the gas. The agreement for the surface species, however, is somewhat mixed. The average abundances of highly reactive surface species can be orders of magnitude larger in the stochastic-deterministic model than in the purely deterministic one. For non-reactive species, the results of the models can disagree strongly at early times, but agree to well within an order of magnitude at later times for most molecules. Strong exceptions occur for CO and H2CO at 10 K, and for CO2 at 20 K. The agreement seems to be best at a temperature of 15 K. As opposed to the use of the normal rate equation method of surface chemistry, the modified rate method is in significantly better agreement with the stochastic-deterministic approach. Comparison with observations of molecular ices in dense clouds shows mixed agreement.

Thermal radiative fluxes through inhomogeneous cloud fields: a sensitivity study using a new stochastic cloud generator

We analyze the effects of flat and bumpy top, fractional and internally inhomogeneous cloud layers on large area-averaged thermal radiative fluxes. Inhomogeneous clouds are generated by a new stochastic model: the tree-driven mass accumulation process (tdMAP). This model is able to provide stratocumulus and cumulus cloud fields with properties close to those observed in real clouds. A sensitivity study of cloud parameters is done by analyzing differences between 3D fluxes simulated by the spherical harmonic discrete ordinate method and three “standard” models likely to be used in general circulation models: plane-parallel homogeneous cloud model (PPH), PPH with fractional cloud coverage model (FCPPH) and independent pixel approximation model (IPA). We show that thermal fluxes are strong functions of fractional cloud coverage, mean optical depth, mean geometrical thickness and cloud base altitude. Fluctuations of “in-cloud” horizontal variability in optical depth and cloud-top bumps have negligible effects in the whole. We also showed that PPH, FCPPH and IPA models are not suitable to compute thermal fluxes of flat top fractional inhomogeneous cloud layer, except for completely overcast cloud. This implies that horizontal transport of photon at thermal wavelengths is important when cloudy cells are separated by optically thin regions.

Stochastic generation of subgrid‐scale cloudy columns for large‐scale models

AbstractTo use the Monte Carlo Independent Column Approximation method for computing domain‐average radiative fluxes in large‐scale atmospheric models (LSAMs), a method is needed for generating cloudy subcolumns within LSAM columns. Here, a stochastic cloud generator is introduced to produce the subcolumns. The generator creates a cloud field on a column‐by‐column basis using information about layer cloud fraction, vertical overlap of cloud fraction and cloud condensate for adjacent layers, and density functions describing horizontal variations in cloud water content. The performance of the generator is assessed using a single day's worth of data from an LSAM simulation that employed a low‐resolution two‐dimensional cloud‐resolving model (CRM) within each LSAM column (a total of ∼59 000 cloudy domains). Statistical characteristics of generated cloud fields are compared against original CRM data, and radiative‐transfer biases associated with the generator are evaluated. When the generator is initialized to the greatest extent possible with information obtained from the CRM fields, overall biases are small. For example, global‐mean total cloud fraction exhibits a bias of −0.004, as compared with −0.024 for maximum‐random overlap (MRO) and 0.047 for random overlap. Biases in radiative fluxes and heating rates are in general ¼ to ½ those for MRO with horizontally homogeneous clouds. Copyright © 2004 Royal Meteorological Society

The Angular Distribution of UV-B Sky Radiance under Cloudy Conditions: A Comparison of Measurements and Radiative Transfer Calculations Using a Fractal Cloud Model

Abstract In recent years, global warming concerns have focused attention on cloud radiative forcing and its accurate encapsulation in radiative transfer measurement and modeling programs. At present, this process is constrained by the dynamic movement and inhomogeneity of cloud structure. This study attempts to quantify the UV sky radiance distribution induced by a partial and overcast stratiform cloud field while addressing some of the inherent spatial and temporal errors resulting from cloud. For this purpose, high-quality azimuthally averaged 2-min measurements of erythemal UV-B sky radiance distribution were undertaken by a variable sky-view platform at Hobart, Australia (42.90°S, 147.33°E). Measurements were subsequently compared with Monte Carlo radiative transfer simulations using both a multifractal and plane-parallel homogenous (PPH) cloud field. Data were also compared with several empirical parameterizations. Results at solar zenith angles of 30° and 50° show that for overcast conditions, the multifractal model is superior to the PPH model. For broken cloud conditions, the radiance measurements are biased toward higher instances of direct-beam interruption by cloud. This tends to smooth the near-sun sky radiance field whereas the multifractal model under the same conditions continues to exhibit the circumsolar effect, indicating that its performance may be still valid for radiation modeling. An empirical parameterization of the same multifractal model produced similar sky radiance profiles, warranting its use in radiative transfer models.

The determination of cloud altitudes using GOME reflectance spectra: multilayered cloud systems

This paper is devoted to the application of the Semi-Analytical Cloud Retrieval Algorithm (SACURA) to the cloud-top height determination using data from the Global Ozone Measurement Experiment (GOME) instrument onboard Earth Remote Sensing satellite (ERS-2). In particular, measurements of the top-of-atmosphere reflectance in the oxygen absorption A-band are used. The technique is based on the asymptotic radiative transfer theory as applied to studies of the oxygen absorption bands in reflected light. Our approach is valid for optically thick clouds with cloud optical thickness larger than approximately 5. The accuracy of the algorithm is checked against independent retrieval techniques for completely cloudy pixels. In particular, the close coincidence with data obtained from the Along Track Scanning Radiometer (ATSR-2) onboard ERS-2 is found. Some results of retrievals using these different instruments disagree (up to 2 km). This is explained by the strong horizontal inhomogeneity of clouds under investigation, which is not accounted by the SACURA or, possibly, by well-known problems of infrared techniques as applied to low-level clouds. The effective cloud geometrical thickness l is also retrieved. This parameter is defined as the geometrical thickness of a vertically homogeneous cloud, which allows for the minimization of differences between observed and calculated top-of-atmosphere reflectance spectra. For inhomogeneous clouds, the value of l differs from a real cloud geometrical thickness, but it gives us an indication of the possible existence of the multilayered cloud system in the field of view of the optical instrument.

A method for molecular-line radiative-transfer computations and its application to a two-dimensional model for the starless core L1544

We present a numerical method and the URAN(IA) computer code for two-dimensional, axially symmetric radiative-transfer computations in molecular lines and spectral modeling. The algorithm is based on Monte Carlo computations of the mean radiation intensity and Accelerated Λ Iterations (ALI) to provide self-consistency between the radiation field and molecular excitation. The code is applied to the structure and kinematic properties of the starless core L1544, which is often considered to be the collapsing core of a molecular cloud. This object has been well studied, but none of the one-dimensional models obtained earlier has been able to provide a self-consistent picture of its structure and kinematics. We show that the spectral features of L1544 can be reproduced in a two-dimensional model in which the cloud has an axial ratio of 2: 1, a mean velocity of contraction (collapse) of Vr∼50 m/s, and a rotational velocity of up to Vφ ∼ 200 m/s. We construct the model of L1544 based on a continuous transition from an initially homogeneous cloud to the observed configuration. The velocity of the contraction is appreciably lower than is predicted by one-dimensional dynamical models. We discuss the problems of interpreting observed molecular-line profiles and prospects for developing self-consistent models for the chemical and dynamical evolution of molecular clouds.

Spectral discrimination of coarse and fine mode optical depth

The recognition that the aerosol particle size distribution (PSD) is effectively bimodal permits the extraction of the fine and coarse mode optical depths (τf and τc) from the spectral shape of the total aerosol optical depth (τa = τf + τc). This purely optical technique avoids intermediate computations of the PSD and yields a direct optical output that is commensurate in complexity with the spectral information content of τa. The separation into τf and τc is a robust process and yields aerosol optical statistics, which are more intrinsic than those, obtained from a generic analysis of τa. Partial (optical) validation is provided by (1) demonstrating the physical coherence of the simple model employed, (2) demonstrating that τc variation is coherent with photographic evidence of thin cloud events and that τf variation is coherent with photographic evidence of clear sky and haze events, and (3) showing that the retrieved values of τf and τc are well‐correlated, if weakly biased, relative to formal inversions of combined solar extinction and sky radiance data. The spectral inversion technique permitted a closer scrutiny of a standard (temporally based) cloud‐screening algorithm. Perturbations of monthly or longer‐term statistics associated with passive or active shortcomings of operational cloud screening were inferred to be small to occasionally moderate over a sampling of cases. Diurnal illustrations were given where it was clear that such shortcomings can have a significant impact on the interpretation of specific events; (1) commission errors in τf due to the exclusion of excessively high‐frequency fine mode events and (2) omission errors in τc due to the inclusion of insufficiently high‐frequency thin homogeneous cloud events.

Representation of a nonspherical ice particle by a collection of independent spheres for scattering and absorption of radiation: 2. Hexagonal columns and plates

A cloud of nonspherical ice particles may be represented in radiation models by a collection of spheres, in which the model cloud contains the same total volume of ice and the same total surface area as the real cloud but not the same number of particles. The spheres then have the same volume‐to‐area (V/A) ratio as the nonspherical particle. In previous work this approach was shown to work well to represent randomly oriented infinitely long circular cylinders for computation of hemispherical reflectance, transmittance, and absorptance. In this paper the results have been extended to hexagonal columns and plates using a geometric optics technique for large particles and finite‐difference‐time‐domain theory (FDTD) for small particles. The extinction efficiency and single‐scattering coalbedo for these prisms are closely approximated by the values for equal‐V/A spheres across the ultraviolet, visible, and infrared from 0.2 to 25 μm wavelength. Errors in the asymmetry factor can be significant where ice absorptance is weak, at visible wavelengths for example. These errors are greatest for prisms with aspect ratios close to 1. Errors in hemispheric reflectance, absorptance, and transmittance are calculated for horizontally homogeneous clouds with ice water paths from 0.4 to 200,000 g m−2 and crystal thicknesses of 1 to 400 μm, to cover the range of crystal sizes and optical depths from polar stratospheric clouds (PSCs) through cirrus clouds to surface snow. The errors are less than 0.05 over most of these ranges at all wavelengths but can be larger at visible wavelengths because of the ideal shapes of the prisms. The method was not tested for, and is not expected to be accurate for, angle‐dependent radiances.

Assessing 1D Atmospheric Solar Radiative Transfer Models: Interpretation and Handling of Unresolved Clouds

Abstract The primary purpose of this study is to assess the performance of 1D solar radiative transfer codes that are used currently both for research and in weather and climate models. Emphasis is on interpretation and handling of unresolved clouds. Answers are sought to the following questions: (i) How well do 1D solar codes interpret and handle columns of information pertaining to partly cloudy atmospheres? (ii) Regardless of the adequacy of their assumptions about unresolved clouds, do 1D solar codes perform as intended? One clear-sky and two plane-parallel, homogeneous (PPH) overcast cloud cases serve to elucidate 1D model differences due to varying treatments of gaseous transmittances, cloud optical properties, and basic radiative transfer. The remaining four cases involve 3D distributions of cloud water and water vapor as simulated by cloud-resolving models. Results for 25 1D codes, which included two line-by-line (LBL) models (clear and overcast only) and four 3D Monte Carlo (MC) photon transport ...

Assessment of multiple scattering and horizontal inhomogeneity in IR radiative transfer calculations of observed thin cirrus clouds

Outgoing longwave (LW) radiation (OLR) and infrared heating rate errors due to the neglect of multiple scattering by clouds and horizontal cloud inhomogeneity in infrared radiative transfer parameterizations are assessed with a modified version of the Fu and Liou [1992] radiative transfer model and field measurements of optically thin cirrus that were observed during the fall 1997 and spring 2000 intensive observation periods conducted at the Atmospheric Radiation Measurement Southern Great Plains site in Oklahoma. A rigorous method is first introduced for incorporating atmospheric and cloud optical and geometric properties derived from measurements in radiative transfer calculations and for ensuring the consistency of these calculations with observations. Part of that method is the use of optical depth derived from measurements of a Multi Filter Rotating Shadowband Radiometer (MFRSR) at 415 nm to determine the cloud ice water content for the longwave and shortwave radiative transfer calculations. The optical depth is corrected to account for forward scattering by cirrus crystals, which is dominant under the thin cirrus conditions observed. Surface downwelling fluxes from the radiative transfer calculations are compared against observations from the Baseline Surface Radiation Network for consistency, with primary emphasis placed on achieving good agreement between the observed and calculated direct normal components. For all cases considered, there is very good agreement in the solar direct irradiance and reasonably good agreement in the solar diffuse irradiance, but without the forward scattering correction the agreement for both would degrade; the diffuse agreement, in particular, would do so by 12 W m−2 (>30%). Very good agreement is achieved in the LW with an absolute mean difference of 0.02 W m−2. For the observed cases considered, neglecting multiple scattering of LW by thin cirrus overestimates OLR by 6–8 W m−2 or less (depending on cloud optical depth and particle size) because of exclusion of reflection of upwelling LW at the cloud base and results in heating rate errors of as much as 0.2 K d−1. The effect of horizontally homogeneous clouds is approximated by employing the half hourly mean of the derived optical and geometric properties in the calculation. The impact of the combined effect on OLR and LW heating rates is shown from these calculations to be as high as 30 to 35 W m−2, and heating rate errors are on the order of 0.5 to over 1 K d−1, which are very significant errors.

Solar radiative transfer for stratiform clouds with horizontal variations in liquid‐water path and droplet effective radius

AbstractEffects of horizontal variations in liquid‐water path (LWP) and droplet effective radius (re) on solar radiative‐transfer properties of stratiform clouds are addressed. To this end, a broadband Monte Carlo solar radiation code and in situ aircraft observations of cloud properties collected in the Radiation, Aerosol and Cloud Experiment are used. The experiments confirm that the radiative impact of cloud horizontal inhomogeneity is dominated by variations in LWP, which make cloud short‐wave albedo smaller than that for a corresponding plane‐parallel horizontally homogeneous cloud. When horizontal variations in re are accounted for, however, the impact due to horizontal variations in LWP is usually reduced. When re and LWP are highly correlated, as is typically the case, the impact on albedo due to LWP variations alone can be reduced easily by 30% when variations in re are included. Uncertainty factors involved in the calculations are also discussed. Copyright © 2003 Royal Meteorological Society.

Effect of multiple scattering on depolarization measurements with spaceborne lidars

An analytical model based on the integration of the scattering-angle and light-path manifold has been developed to quantify the effect of multiple scattering on cirrus measurements obtained with elastic polarization lidars from space. Light scattering by molecules and by a horizontally homogeneous cloud is taken into account. Lidar parameter, including laser beam divergence, can be freely chosen. Up to 3 orders of scattering are calculated. Furthermore, an inversion technique for the retrieval of cloud extinction profiles from measurements with elastic-backscatter lidars is proposed that explicitly takes multiple scattering into account. It is found that for typical lidar system parameters such as those of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) instrument multiple scattering does not significantly affect depolarization-ratio measurements in cirrus clouds with small to moderate optical depths. For all simulated clouds, the absolute value of the difference between measured and single-scattering volume depolarization ratio is < 0.006. The particle depolarization ratio can be calculated from the measured volume depolarization ratio and the retrieved backscatter ratio without degradation of accuracy; thus characterization of the various cirrus categories in terms of the particle depolarization ratio and retrieval of cloud microphysical properties is feasible from space. The results of this study apply to polar stratospheric clouds as well.

The influence of horizontal inhomogeneity on radiative characteristics of clouds: an asymptotic case study

The paper is devoted to analytical studies of the influence of horizontal inhomogeneity of clouds on their radiative properties in the framework of the asymptotic radiative transfer theory. It is assumed that a cloud field is optically thick. Thus, only overcast cloud fields are under consideration. The study is based on the independent column approximation, assuming Gamma distribution of cloud optical thickness (COT) in the cloud field under study. This paper confirms all essential findings of cloud optics, concerning the influence of horizontally inhomogeneous clouds on transmitted, reflected, and absorbed solar light. For instance, we found the decrease of light reflection and absorption (negative biases) and the increase of light transmission (positive biases) for inhomogeneous clouds as compared to the case of a homogeneous cloud field with the value of optical thickness equal to that of an average COT of an inhomogeneous cloud field. Analytical equations for biases of radiative characteristics (e.g. the reflection function, spherical albedo, transmittance, and absorptance) are derived. Also, for the first time, we established the relationships between biases of different radiative characteristics. Analytical equations proposed are simple and highly accurate for optically thick weakly absorbing cloud fields. They can be incorporated in large-scale atmospheric models for the simplification of radiation blocks both for homogeneous and inhomogeneous cloud fields in visible and near-infrared spectral regions.

Nonself-similar regimes of isothermal collapse of protostellar clouds

We consider the development of inhomogeneity in the isothermal collapse of protostellar clouds. The initial and boundary conditions correspond to the classical statement of the problem on the contraction of a homogeneous cloud from a given volume. A centered rarefaction wave is shown to propagate from the outer boundary of the cloud toward its center at the first collapse stage. Analysis reveals two possible regimes of isothermal collapse, depending on the relationship between the rarefaction wave focusing time t* and the cloud free-fall collapse time tff. For cold clouds, t*=t ff and the rarefaction wave is not reflected. In this case, as time elapses, the cloud collapse becomes self-similar with the characteristic density profile ρ∼r−2. In hot clouds, t*<t ff and the focusing can take place before the formation of an opaque core. Since the velocities of the rarefaction wave along and across magnetic field lines in a magnetized cloud are different, its front assumes a shape elongated along magnetic field lines. Depending on the initial conditions, based on analytical estimates, we investigate various possible scenarios for the collapse of magnetic protostellar clouds.

Consistency of ARESE II cloud absorption estimates and sampling issues

Data from three cloudy days (3, 21, and 29 March 2000) of the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment II (ARESE II) were analyzed and broadband absorptances were obtained for three sets of instruments. Grand (total flight) averages of fractional solar absorptances were ∼0.21–0.22, with the exception of 3 March when two sets of instruments gave values smaller by ∼0.03–0.04. The robustness of these values was evaluated by examining potential sampling problems with the aid of 500 nm spectral fluxes. The grand average of 500 nm apparent absorptance was statistically indistinguishable from zero for 21 and 29 March but acquired a large positive value of ∼0.10 on 3 March, which is not physically understood. We present results showing that each of the three days is unique in terms of cloud morphology and behavior of the absorptance time series. When the conditional sampling method is applied to the two days with reliable 500 nm absorptances (21 and 29 March), the resulting histogram of broadband absorptances is significantly narrower than the histogram constructed from the original time series. Corrections to the absorptance time series à la Cess et al. [1999] were successful for all three days because of the generally good correlation between broadband and 500 nm absorptances, removing the bulk of low‐end and high‐end extremes most likely associated with sampling and horizontal flux artifacts. The above two methods suggest an ARESE II range of broadband absorptances of ∼0.18–0.26. Finally, our study shows that the data set obtained on 29 March is the most self‐consistent and straightforward to analyze among the three, because this day fulfilled all the requirements of the ARESE II experimental design, namely the presence of thick, overcast, homogeneous clouds.

Inverse Problems Related to Photon Transport in an Interstellar Cloud

In this article we study a one-dimensional mathematical model of photon transport in a homogeneous interstellar cloud, occupying a convex region, with a source of photons inside. Existence, uniqueness and positivity of the solution are proved by using the theory of semigroups. The quasi-static solution is investigated and it is shown that it is a good approximation to the exact solution. Two inverse problems are studied: the computation of the cloud dimension, and the total cross section of the cloud itself.

High resolution simulations of star formation

In order to study the capabilities of SPH in self-gravitating hydrodynamical problems, we have performed a series of three-dimensional numerical simulations of the collapse of a rotating homogeneous cloud, varying the number of particles and the artificial viscosity prescription.

Accounting for Unresolved Clouds in a 1D Infrared Radiative Transfer Model. Part II: Horizontal Variability of Cloud Water Path

A 1D infrared radiative transfer model that handles clouds with subgrid-scale horizontal variability is developed and tested. It assumes that fluctuations in cloud absorptance optical depth κ across layers (and collections of layers) can be described by gamma distributions. Unlike homogeneous clouds, flux incident at a level inside a horizontally inhomogeneous cloud requires explicit computation of transmittance to all other levels in the cloud. Consequently, in addition to estimates of variability for each layer, variability between any two levels must be specified too. Scattering by hydrometeors and a general treatment of cloud overlap are included in this model. Solutions for isothermal and nonisothermal Planck source functions are presented. For the synthetic cloudy atmospheres used here, the new model produces errors for outgoing longwave radiation (OLR) and cloud cooling rates that are typically more than an order of magnitude smaller than those associated with the conventional homogeneous cloud model (as used in all GCMs at present). It is shown that up- and downwelling fluxes and cloud cooling rates can depend much on subgrid-scale variability. For high overcast clouds with realistic variability, OLR can be up to 20 W m−2 more than that predicted by a conventional homogeneous model using the same mean κ. At the same time, cooling rate errors at cloud top and cloud base due to the homogeneous assumption can be up to ±25%; the sign depending primarily on mean κ and magnitude of variability. For lower, thicker clouds, the homogeneous assumption leads primarily to errors in cloud-top cooling. The new code usually remedies these errors greatly. This model, and its solar counterpart, are used currently in the Canadian Centre for Climate Modelling and Analysis GCM.