Unresolved Clouds Research Articles

Accounting for unresolved clouds in a 1-D solar radiative-transfer model

SUMMARY New methods for the treatment of solar radiative transfer through overlapping and inhomogeneous clouds are presented. First, a new approach to cloud overlap is shown. For the adjacent cloud blocks, the traditional maximum overlap can be relaxed to a mixture of maximum and random overlap treatments for layers that are adjacent but not fully correlated. Second, a new radiative-transfer algorithm has been developed to deal with these various cloud overlap circumstances that is simple enough for implementation in a general-circulation model (GCM). When compared to appropriate benchmark calculations, we find that this new method can produce accurate results in heating rates and fluxes with relative errors generally less than 8%. Third, a new and very simple approach to treating radiative transfer through a cloud with horizontal subgrid-scale inhomogeneities is developed. This approach uses an optical-depth scaling technique to represent the subgrid-scale inhomogeneity. Finally, by combining all of the above elements, we provide a new algorithm for the combined treatment of cloud overlap and inhomogeneity and we show that it yields very reasonable accuracies for heating rates and fluxes. Through benchmark comparisons, we show that this new algorithm provides significant improvement over existing schemes in GCMs.

Chemical and dynamical effects on cloud droplet number: Implications for estimates of the aerosol indirect effect

Most aerosol‐cloud‐climate assessment studies use empirical aerosol number/droplet number relationships, which are subject to large variability. Historically, this variability has been attributed to unresolved variations in updraft velocity. We revisit this postulation and assess the effects of both updraft velocity and chemical composition on this variability. In doing so we utilize an inverse modeling approach. Using a detailed numerical cloud parcel model and published aerosol characteristics, with published correlations of cloud droplet versus sulfate and cloud droplet versus aerosol number as constraints, we determine a most probable size distribution and updraft velocity for polluted and clean conditions of cloud formation. A sensitivity analysis is then performed to study the variation in cloud droplet number with changes in aerosol chemistry and updraft velocities. This addresses the need to estimate the importance of chemical effects on spatial scales relevant for global climate models. Our analysis suggests that the effect of organic surfactants can introduce as much variability in cloud droplet number as the effect of expected variations in updraft velocity. In addition, the presence of organics seems to further enhance the sensitivity of droplet concentration to vertical velocity variability. The variability from organic surfactants is seen to be insensitive to variations in aerosol number concentration, implying that such effects can affect cloud droplet number consistently over large spatial scales. Our findings suggest that organics can be as important to the aerosol indirect effect as the effect of unresolved cloud dynamics, and they illustrate the potential and complex role of chemical effects on aerosol‐cloud interactions.

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 ...

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.

Accounting for Unresolved Clouds in a 1D Infrared Radiative Transfer Model. Part I: Solution for Radiative Transfer, Including Cloud Scattering and Overlap

Abstract Various aspects of infrared radiative transfer through clouds are investigated. First, three solutions to the IR radiative transfer equation are presented and assessed, each corresponding to a different approximation for the Planck function. It is shown that the differences in results between solutions with linear and exponential dependence of the Planck source function are small for typical vertical resolutions in climate models. Second, a new perturbation-based approach to solving the IR radiative transfer equation with the inclusion of cloud scattering is presented. This scheme follows the standard perturbation method, and allows one to identify the zeroth-order equation with the absorption approximation and the first-order equation as including IR scattering effects. This enables one solution to accurately treat cloudy layers in which cloud scattering is included, and allows for an improved and consistent treatment of absorbing aerosol layers in the absence of cloud by using the zeroth-order ...

Large‐Scale CiEmission from Molecular Clouds with Associated Ultraviolet Sources

We present large-scale observations of the 3P1→3P0 transition of neutral carbon, as well as observations of 12CO, 13CO, and C18O, in four molecular clouds with nearby ultraviolet sources: W3, L1630/NGC 2024, S140, and Cep A. The observations cover approximately 30'×30' regions with a 3' beam. A typical C0 column density is about 1.6×1017 cm-2. The overall extent and morphology of the C I emission is similar to that of the 12CO and 13CO J=2→1 emission. There is a strong correlation of C I and 13CO line intensities. The column densities and line strengths of the C I lines imply that these lines arise in gas at the edge of the molecular cloud that is dissociated by ultraviolet radiation. The correlation of C I and 13CO intensity can arise as a result of a combination of column density variations, volume density variations, and unresolved cloud structure. In the latter case, however, any unresolved structures containing both C I and CO need not be more than a few times smaller than the telescope beam. Taken together with C+ observations, the C I data imply that only about half of the gas-phase carbon in molecular clouds is in CO. The observations indicate that the large 13CO/C18O abundance ratio (~20) seen at the edges of molecular clouds results from isotope-selective photodissociation of C18O.

気象衛星NOAAのsplit windowデータを用いた陸域の可降水量の見積り法

A new algorithm to estimate precipitable water over land using the split-window data from the NOAA satellite is proposed. The estimation is based on the ratio of the split window channel brightness temperature difference and the ratio of the split window channel brightness temperature variance. The algorithm involves two main steps. The first is to extract high-confidence pixels which do not contain a large influence due to unresolved clouds, and to calculate the first-guess of precipitable water using the split-window variance ratio technique proposed by Jedlovec (1990). The second step corrects the first-guess of precipitable water using the difference between the mean-air temperature of the model atmosphere and the mean-air temperature calculated from the satellite data.By using this algorithm, the precipitable water over an area of about 33km×33km can be estimated, while the influences due to unresolved clouds and unknown temperature profiles decrease. Compared with four radiosonde observations during August of 1992, the satellite-derived precipitable water data are in good agreement with the radiosonde data.

An evaluation of various forms of VAS retrievals in the analysis of a preconvective environment

Quantitative retrievals derived from VISSR Atmospheric Sounder (VAS) radiances are combined with conventional surface and radiosonde data to evaluate the impact of the higher time and space resolution geosynchronous satellite soundings on the diagnosis of a preconvective environment over the central United States on 20 July 1981. Retrievals of temperature, dewpoint temperature, equivalent potential temperature, total column precipitable water, and lifted index, all derived at 60 km resolution over approximately three-hourly intervals, are shown to be physically consistent in space and time and to compare well with available radiosonde data. When VAS fields are used to augment qualitative VAS imagery and analyses from conventional data sources mesoscale regions with convective instability are more clearly delineated prior to the development of convection. The analysis of the VAS retrievals identifies significant spatial gradients and temporal changes in the thermal and moisture fields, especially at times and locations between radiosonde observations. Direct retrievals of vertically integrated “bulk” precipitable water and lifted index are particularly useful in this case since they correspond well to features in the VAS imagery and provide a strong preconvective signature. The nature of the VAS instrument dictates that these bulk parameters should display more consistency in space and time than the level-specific parameters due to the poor vertical resolution of the VAS instrument. The detailed analyses also point to limitations in using VAS data. Even with nearly optimal conditions for passive remote sounding (generally clear skies, minimal orographic effects, and a rapidly changing moisture field), the VAS retrievals were still degraded in some regions by VAS instrument noise and calibration errors and unresolved cloud contamination. Another problem is the cloud-free nature of the instrument data set, which biases the results toward a drier environment. In spite of these and other limitations, the analyses demonstrate that the geosynchronous VAS can be used in a case study mode to produce high-resolution spatial and temporal measurements which are useful for the quantitative analysis of a cloud-free preconvective environment.

The dependence of target discriminability on systematic and random variations in recorded radiance

Abstract Radiance reflected or emitted from a ground target exhibits an angular anisotropy which is wavelength dependent. While some empirical or modelling corrections may be made to reduce the impact of systematic effects upon target discrimination, no correction may be made for random variations in ground reflectance or emittance, nor for random variations in irradiance on the ground (optical reflective case) and in atmospheric transmission and backscallcr. A simple case in the optical-reflective region is considered, in order to demonstrate the dependence on scan angle (for a given Sun angle) of the effect of random variations on the discriminability of two ground targets partly obscured by a varying amount of unresolved cloud.

Effects of random and systematic variations in unresolved cloud on recorded radiance and on target discriminability

The purpose of this investigation has been to study the systematic and random effects of unresolved (i.e., smaller than instrument IFOV) cloud and haze on the ability to discriminate, e.g., vegetated targets using digital recorded radiance in preselected bandpasses. Calculations have, for the sake of example, centered on the discrimination of wheat and of soybeans from targets consisting of soybeans at various levels of stress severity. Calculations have mainly centered on nadir values (the only value of view angle for which reflectance data are available for various levels of disease stress severity), but some calculations of target discriminability for various view angles have been made to serve as examples. While means may be found to determine and to correct for systematic dependence of recorded radiance on view angle, it will not be possible to correct for random variations. Thus, while studies of systematic variations of target radiance with scan angle will lead to data calibration (to improve target discriminability), studies of random variation will indicate the limits imposed by fluctuations in physical factors on target differentiation and quantification. It is suggested that a future aim must be to determine which bandpasses or combinations of bandpasses may best be used to minimize random variations in target radiance, so as to optimize the discrimination of selected targets, in this case stressed from unstressed vegetation.

Sea surface temperature estimation using the NOAA 6 satellite advanced very high resolution radiometer

A procedure for estimating sea surface temperature was developed by using the near and thermal infrared data available from full resolution (1.1 km field of view) daytime passes of the NOAA 6 satellite Advanced Very High Resolution Radiometer. It is demonstrated that spatially unresolved clouds and variable amounts of water vapor in the atmosphere are the major error sources. The procedure therefore incorporates a means of identifying radiometer fields of view which are virtually cloud‐free, by comparing measured solar radiation at 1 μm, backscattered from the atmosphere, against that expected from a simple scattering model. A set of 82 cloud‐free fields of view is then assembled, spatially and temporally nearly coincident with research quality (±0.2°C) in situ ship and buoy sea surface temperature reports collected during 1979 and 1980. The set is then used to develop and evaluate a simple model to reduce variable water vapor effects by relating in situ sea surface temperature to the infrared brightness temperatures measured by the radiometer at 3.7 and 11 μm. Residual disagreement between satellite estimates and in situ measurements of sea surface temperature is about 0.6°C for the data set which extends from the equator to 50°N in the northeast Pacific. The procedure is then applied to 10 satellite passes in the northeast Pacific during April 1981 to obtain monthly mean sea surface temperature and temperature anomaly (departure from climatological norm) maps. These maps are compared against similar ones produced from routine ship reports, mostly consisting of engine injection temperature data, typically good to no better than 1°C. The maps agree to ±0.8°C (1 sigma), with the level of agreement clearly limited by the poor accuracy of the ship reports. It now appears that the radiometer data, when properly handled, can be used as a more accurate substitute for routine ship reports of sea surface temperature in applications such as climate variability studies requiring 0.5–1.0°C accuracy.

Echelle spectrometry of the Seyfert galaxies NGC 3783 and Markarian 509

view Abstract Citations (74) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Echelle spectrometry of the Seyfert galaxies NGC 3783 and Markarian 509. Atwood, B. ; Baldwin, J. A. ; Carswell, R. F. Abstract Echelle spectrometry of two Seyfert galaxies, NGC 3783 and Markarian 509, is used to investigate the narrow and broad emission-line profiles. In NGC 3783 it is found that while the narrow emission lines have smooth profiles, they must be the summation of a number of components having different electron densities and/or temperatures. In Markarian 509 the forbidden S II and N II line profiles are similar to those of forbidden O III, suggesting that the physical conditions as a function of velocity are more homogeneous than for NGC 3783. The broad H-alpha and H-beta lines in both galaxies have approximately identical profiles. For Mrk 509, a cross-correlation of the two profiles indicates that the number of optically thin unresolved clouds needed to make up the emission line region is greater than about 50,000. Publication: The Astrophysical Journal Pub Date: June 1982 DOI: 10.1086/160011 Bibcode: 1982ApJ...257..559A Keywords: Emission Spectra; Galactic Nuclei; Seyfert Galaxies; Spectral Line Width; Spectrophotometry; Continuous Spectra; Electron Density (Concentration); Electron Energy; Forbidden Bands; H Lines; High Resolution; Molecular Clouds; Spectrum Analysis; Astronomy full text sources ADS | data products NED (3) SIMBAD (2)