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Shallow convective clouds are common, occurring over many areas of the world, and are an important component in the atmospheric radiation budget. In addition to synoptic and mesoscale meteorological conditions, [...]

ABSTRACT The Clouds, Aerosol, and Precipitation in the Marine Boundary Layer (CAP-MBL) deployment at Graciosa Island in the Azores generated a 21-month (April 2009December 2010) comprehensive dataset documenting clouds, aerosols, [...]

Accurately accounting for radiative energy balance between the incoming solar and the outgoing infrared radiative fluxes is very important in modeling the Earth's climate. Water vapor absorption plays a critical role in the radiative heating rate profile in the midtroposphere by strongly absorbing both infrared and solar radiation in several absorption bands throughout the electromagnetic spectrum. One of the most important of these absorption bands is in the far-infrared portion of the spectrum, where the far-infrared is defined here to be wavelengths longer than 15 microns. A large fraction (~40%) of the outgoing infrared flux is emitted by water vapor in the far-infrared. Errors in the radiative transfer models associated with the far-infrared and other strong water vapor absorption bands will therefore affect the calculation of the planet's total outgoing radiative flux and its vertical distribution of the radiant energy; these errors may result in inaccurate modeling of the general circulation of the planet. A set of field experiments, called the Radiative Heating in Underexplored Bands Campaigns (RHUBC), has been conducted as part of the Atmospheric Radiation Measurement (ARM) program. The RHUBC campaigns deployed spectrally resolved far-infrared spectrometers alongside other ARM observations in extremely dry environments to provide a robust and complete dataset that allows radiative transfer models to be evaluated in the far-infrared and other spectral regions where water vapor absorbs strongly. RHUBC I was conducted in February-March 2007 in Barrow, Alaska, and RHUBC II was conducted in August-October 2009 in the Atacama Desert region of Chile at an altitude of 5.3 kin. The motivation for and initial results from these experiments are described, as well as the implications for global climate models. DOI: 10.1175/2010BAMS2904.1

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.08.027 Byline: Sergey L. Gratiy (a), Deborah A. Levin (a), Andrew C. Walker (b) Keywords: Radiative transfer; Io Abstract: Validation of global numerical models of planetary atmospheres requires simulating images and spectra from the IR to UV spectral regions in order to compare them with remote observations. This paper describes Rassvet, a 3-D spherical-shell backward Monte Carlo radiative transfer model developed for such simulations. It utilizes a new methodology for calculating atmospheric brightness in scattered sunlight by introducing the concept of an 'effective emission source'. This allows for the accumulation of the scattered contribution along the entire path of a ray and the calculation of the atmospheric radiation when both scattered sunlight and thermal emission contribute to the remote measurement - which was not possible in previous models. A 'polychromatic' algorithm is extended for applications with the backward Monte Carlo method and implemented in the model. It allows for the calculation of radiative intensity for several wavelengths simultaneously, resulting in improved efficiency. The capabilities of the model are demonstrated by simulating remote measurements from the atmosphere of Io. Author Affiliation: (a) Department of Aerospace Engineering, Pennsylvania State University, University Park, PA 16802, USA (b) Department of Aerospace Engineering, University of Texas at Austin, Austin, TX 78712, USA Article History: Received 9 March 2009; Revised 14 August 2009; Accepted 31 August 2009

Consistency of upper-tropospheric water vapor measurements from a variety of state-of-the-art instruments was assessed using collocated Geostationary Operational Environmental Satellite-8 (GOES-8) 6.7-[micro]m brightness temperatures as a common benchmark during the Atmospheric Radiation Measurement Program (ARM) First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Water Vapor Experiment (AFWEX). To avoid uncertainties associated with the inversion of satellite-measured radiances into water vapor quantity, profiles of temperature and humidity observed from in situ, ground-based, and airborne instruments are inserted into a radiative transfer model to simulate the brightness temperature that the GOES-8 would have observed under those conditions (i.e., profile-to-radiance approach). Comparisons showed that Vaisala RS80-H radiosondes and Meteolabor Snow White chilled-mirror dewpoint hygrometers are systemically drier in the upper troposphere by ~30%-40% relative to the GOES-8 measured upper-tropospheric humidity (UTH). By contrast, two ground-based Raman lidars (Cloud and Radiation Test Bed Raman lidar and scanning Raman lidar) and one airborne differential absorption lidar agree to within 10% of the GOES-8 measured UTH. These results indicate that upper-tropospheric water vapor can be monitored by these lidars and well-calibrated, stable geostationary satellites with an uncertainty of less than 10%, and that correction procedures are required to rectify the inherent deficiencies of humidity measurements in the upper troposphere from these radiosondes. DOI: 10.1175/2009JAMC2250.1

The phase composition and microphysical structure of clouds define the manner in which they modulate atmospheric radiation and contribute to the hydrologic cycle. Issues regarding cloud phase partitioning and transformation come to bear directly in mixed-phase clouds, and have been difficult to address within current modeling frameworks. Ground-based, remote-sensing observations of mixed-phase clouds can contribute a significant body of knowledge with which to better understand, and thereby more accurately model, clouds and their phase-defining processes. Utilizing example observations from the Mixed-Phase Arctic Cloud Experiment (M-PACE), which occurred at the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program's Climate Research Facility in Barrow, Alaska, during autumn 2004, we review the current status of ground-based observation and retrieval methods used in characterizing the macrophysical, microphysical, radiative, and dynamical properties of stratiform mixed-phase clouds. In general, cloud phase, boundaries, ice properties, liquid water path, optical depth, and vertical velocity are available from a combination of active and passive sensors. Significant deficiencies exist in our ability to vertically characterize the liquid phase, to distinguish ice crystal habits, and to understand aerosol-cloud interactions. Further validation studies are needed to evaluate, improve, and expand our retrieval abilities in mixed-phase clouds.

Comparisons are presented between measured and predicted energy deposition spectra from neutron-nuclear events within a silicon semiconductor detector. Experiment data are from irradiations performed at the TRIUMF and TSL neutron beam facilities. Predictions are based on the Geant4 radiation transport toolkit, which is applied both to quantify the effects of interactions in the detector and scattering of the neutron source by other experiments upstream of the detector. The predictions highlight the importance of quantifying scattering effects in large experiment setups, the lower-energy neutrons in the continuum of the quasi-monoenergetic spectrum suffering greater scattering. The results are used to correct neutron-induced single event upset cross-section data. Index Terms--Atmospheric radiation, neutron irradiation, pulse-height energy deposition, single event effects, single event upsets.

A two-dimensional computational study is presented to unravel radial structure of a dielectric barrier discharge in atmospheric helium when the gas voltage exceeds slightly the breakdown voltage and the discharge current is low to retain a repitative dynamic pattern of one discharge event every half cycle of the applied voltage. Simulations result reveal that during each half cycle of the applied voltage gas breakdown occurs first in a central region and around the electrode axis.

The polarization difference [DELTA][T.sub.b] between the vertical and horizontal components of thermal radiation emitted by clouds was studied using 37- and 85-GHz radiometers. The measurements were conducted during the Alliance Icing Research Project in Ottawa, Canada, during the winter of 1999/2000. Polarization differences ([DELTA][T.sub.b]) greater than 0.1 K were observed in approximately 30% of the cloudy periods. Characteristic values of the polarization difference at 85 GHz were about 2 K with a maximum value of about 4.5 K. Polarization difference at 37 GHz usually did not exceed 2.5 K and was typically 2-6 times less than that at 85 GHz. Both positive and negative polarization differences were observed. It is suggested that the microwave polarization results from scattering of atmospheric thermal radiation by cloud ice particles. The observations were interpreted with a model of radiative transfer in mixed-phase clouds. The characteristic polarization difference observed during groundbased measurements was found to agree with predictions of the radiative transfer model for typical values of cloud liquid and ice water content.

The separation of the individual contributions of aerosols and gases to the total attenuation of radiation through the atmosphere has been the subject of much scientific investigation since remote sensing experiments first began. We describe a new scheme to account for the spectral variation of the aerosol extinction in the inversion of transmission data from occultation measurements. Because the spectral variation of the aerosol extinction is generally unknown, the inversion problem is underdetermined and cannot be solved without a reduction in the number of unknowns in the set of equations used to describe the attenuation at each wavelength. This reduction can be accomplished by a variety of methods, including use of a priori information, the parameterization of the aerosol spectral attenuation, and the specification of the form of the aerosol size distribution. We have developed and implemented a parameterization scheme based on existing empirical and modeled information about the microphysical properties of aerosols. This scheme employs the eigenvectors from an extensive set of simulations to parameterize the aerosol extinction coefficient for incorporation into the inversion algorithm. We exAmine the accuracy of our method using data sets containing over 24,000 extinction spectra and compare it with that of another scheme that is currently implemented in the Polar Ozone and Aerosol Measurement (POAM) satellite experiment. In simulations using 80 wavelengths in the UV--visible--near-IR spectral range of the Stratospheric Aerosol and Gas Experiment III (SAGE) instrument, we show that, for our optimal parameterization, errors below 1% are observed in 80% of cases, whereas only approximately 20% of all cases are as accurate as this in a quadratic parameterization employing the logarithm of the wavelength. OCIS codes: 010.1110, 010.1320, 280.1310, 290.2200, 010.1280.

On June 11, 2000, the first vicarious calibration experiment in support of the Multi-angle Imaging SpectroRadiometer (MISR) was conducted. The purpose of this experiment was to acquire in situ measurements of surface and atmospheric conditions over a bright, uniform area. These data were then used to compute top-of-atmosphere (TOA) radiances, which were correlated with the camera digital number output, to determine the in-flight radiometric response of the on-orbit sensor. The Lunar Lake Playa, NV, was the primary target instrumented by the Jet Propulsion Laboratory for this experiment. The airborne MISR simulator (AirMISR) on board a NASA ER-2 acquired simultaneous observations over Lunar Lake. The in situ estimations of top-of-atmosphere radiances and AirMISR measurements at a 20-km altitude were in good agreement with each other and differed by 9% from MISR measurements. The difference has been corrected by adjusting the gain coefficients used in MISR standard product generation. Data acquired simultaneously by other sensors, such as Landsat, the Terra Moderate-Resolution Imaging SpectroRadiometer (MODIS), and the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS), were used to validate this correction. Because of this experiment, MISR radiances are 9% higher than the values based on the on-board calibration. Semiannual field campaigns are planned for the future in order to detect any systematic trends in sensor calibration. Index Terms--Calibration, MISR radiometric calibration, vicarious calibration.

A hypothesis that the fractional scattering into the forward peak is related to solar zenith angle and single scattering albedo is proposed. Calculations show that this assumption can increase the accuracy of the [Delta]-Eddington approximation. For the scattering conservative case this method can improve the results in the region of thin optical depth. For the scattering nonconservative case this method can reduce the errors for reflection and absorption in the region of small solar zenith angle, where the incoming solar energy is most significant.

We have analysed the ability of the Angstrom power law to model the spectral aerosol optical depth, [[Tau].sub.a[Lambda]], for the 400-670 nm band, obtained from spectral direct irradiance measurements at normal incidence. The spectra were registered at ground level in Valencia, Spain, using a Li-cor 1800 spectroradiometer. The results obtained showed that the fitting method that introduces lower errors in the determination of the Angstrom power law coefficients is to adjust directly the spectral experimental data. In this way the errors obtained for the turbidity coefficient, [Beta], were about 0.004 and for the wavelength exponent, [Alpha], 0.07. The correlation coefficient was always greater than 0.95. These values of the correlation coefficient could be improved by parameterisation of [[Tau].sub.a[Lambda]] using an alternative function of the wavelength to the Angstrom power law. But this may not be justified for the turbidity values attained in Valencia.

The optical breakdown induced in air at atmospheric pressure by Nd:YAG Q-switched laser pulses is studied in terms of the spectral features of the emitted radiation in the wavelength range 180-850 nm during the first 200 ns after the laser pulse onset. During the plasma build up, radiation emission features intense, broadband, and structureless ultraviolet-visible spectra before the appearance of atomic lines on the microsecond scale. Also, the emitting plasma kernel, imaged during the buildup and decay stages in the early tens of nanoseconds, turns out to have a size of [approximately] 0.3 mm and a volume of [approximately] 0.02 [mm.sup.3]. The coupling of direct emission data and broadband absorption measurements allowed us to retrieve peak values of electron temperature above 100,000 K and of an optical depth of the order of unity, under the assumptions of local thermodynamic equilibrium and a homogeneous kernel. The simultaneous occurrence of such temporal, spatial, and spectral features of the plasma kernel suggests its exploitation as a pulsed, bright, and broadband ultraviolet-visible light source.

Beam transmittance, emittance, reflectance, and outgoing radiance are inferred from interferometric measurements in the infrared window region for 14 temperate continental and 12 subtropical cirrus cloud cases observed during FIRE II at Parsons, Kansas (37 [degrees] 18[minutes]N, 95 [degrees] 07[minutes]W), and the ASTEX at Porto Santo, Madeira (33 [degrees] 5[minutes]N, 16 [degrees] 21[minutes]W). Cirrus emittances were found to span nearly the entire range from 0 to 1 for cloud systems in each location. Spectrally averaged volume extinction coefficients of 0.19 and 0.62 [km.sup.-1] were found for the respective continental and subtropical samples. A delta-Eddington routine was incorporated into the inference technique to examine the sensitivity of the inferences to the upwelling surface and subcloud-layer emission reflected by the cloud assuming spherical and nonspherical cloud particles. Including reflectance had only a small effect on the spectrally averaged values of the radiative parameters; however, the slope of outgoing longwave radiation across the window region was altered with the introduction of smaller particles. The iterative method is structured in a manner that does not constrain the transmittances of the clear atmosphere to line-by-line model results. Inferred emittances and extinction coefficients are compared to previously published results.

Cloud drop size distributions, microphysics and simple equations enable the derivation of a new parameterization scheme for the optical thickness and for the shortwave radiative properties of water clouds in the solar spectrum. Delta-Eddington approximation helps determine the cloud bulk radiative characteristics. A dimensionless parameter connects transmissivity, flux reflectivity and absorptivity. Features of the new parameterization scheme are analyzed and are compared with other schemes.

The chord length distributions within a cloud and between clouds are considered. Such information is needed as input to certain statistical models of cloud-radiation interaction. Modeling the clouds as azimuthally symmetric ellipsoids, the authors find that the chord length distribution through a cloud of fixed size is proportional to the chord length. The proportionality constant depends upon the semiaxes of the ellipse as well as the angle of incidence of the radiation. This linear behavior is easily convolved over an arbitrary size distribution of the clouds to obtain the chord length distribution through a statistical mixture of different cloud sizes. The chord length distribution between clouds is also considered for an atmospheric layer of finite thickness. In this case, both analytic and numerical methods are needed to obtain results. In the limit of an infinite thickness atmospheric layer described by homogeneous statistics and fixed cloud chord lengths, our considerations reduce to a Markovian (exponentially distributed chord lengths) model for the intercloud spacing.

The possibility of using the Perez irradiance model for reconstructing the beam and diffuse components of solar radiation from measurements with a fixed multipyranometer system is investigated. It is shown that an iterative algorithm can be derived that enables determination of the direct beam component to about plus or minus 50 W/m squared without requiring any site-dependent adjustments. The standard error on the derived global irradiance on inclined surfaces of arbitrary orientation is at the plus or minus W/m squared level. It is concluded that the present degree of precision renders the multipyranometer method an attractive alternative to standard instrumentation for radiation surveys in remote regions. Possibilities for further improvements are discussed.

This paper presents an unsteady two-dimensional analysis of a flat-plate solar collector subjected to time varying insolation with considerable diffuse components as a better characterization of practical solar collectors. The analysis considers the thermal masses of the absorber, tube, glazing, and the working fluid in the system, as well as the different optical and thermodynamic properties of beam and diffuse radiation. Using two sets of insolation data, one for a near clear day and the other for an overcast day, an exergetic optimization of the system was carried out and flow rates which maximize the total exergy output (extracted and accumulated) were determined for flow update periods of once every 30 min, 1 h, and the entire daylight period. The instantaneous optimum flow rates were found to follow the insolation pattern. On a daily basis, the optimum exergetic efficiencies and optimum flow rates were almost independent of the choice of the interval of fluid update, but were about 30% and 10% respectively, higher for the clear day than for the overcast day.

This paper presents a new method for predicting the radiative transfer in an absorbing and isotropically scattering, non-gray plane-parallel atmosphere. The technique is based upon the F ('F-hat') concept and was developed to determine the radiative transfer at short wavelengths for the transparent insulation material, monolithic silica aerogel. The angular dependence of solar transmittance of diffuse and ground reflected radiation is evaluated along with the 'transmittance-absorptance product' of a collector. The F technique is also used to determine the IR transmission of the cover from which the overall loss coefficient for a flat-plate collector constructed with aerogel filled covers is determined. The performance of a system using aerogel collectors is compared to systems using other types of high performing collectors. It is shown that, at least for the system investigated, the decrease in solar transmittance of monolithic silica aerogel is more than compensated by the decrease in thermal losses.

The validity is investigated of the three discrete radiance components (TDRC) model, which was introduced by the author as a model for the instantaneous and time-averaged distribution of diffuse sky radiance. The TDRC model incorporates the cloudy sky geometry (CSG) model, a model proposed by the author for the distribution of clouds across the sky dome in which clouds are modelled as upright vertical cylinders of variable diameter and height, and in which the distribution of clouds is modelled as cylinders randomly spaced, but having an average separation distance in any azimuthal direction. The investigation involves a calibration of the TDRC model for the case of cumulus clouds, followed by a validation under time-averaged conditions of the calibrated model, in which predictions of the model are compared with luminance observations, for a wide range of cloudiness. This comparison showed good agreement, and supports the validity of the TDRC model. It is anticipated that the TDRC model will be of significance to researchers in several fields, particularly those in solar engineering.

The solar radiation climate of Beer Sheva, Israel, is reported upon in detail. The database utilized in this analysis consisted of global radiation on a horizontal surface, normal incidence beam radiation, and global radiation on a south-facing surface tilted at 40 degrees. Monthly-average hourly and daily values are reported for each of these three types of measured radiations, together with the calculated monthly-average daily values for the components of the global radiation, viz. the horizontal beam and diffuse radiations. The monthly-average hourly and daily clearness index values have also been calculated and analyzed. Monthly-average daily frequency distributions of the clearness index values are reported for each month. The solar radiation climate of Beer Sheva has also been compared to those reported for a number of countries in this region. The annual-average daily global radiation incident on a horizontal surface is 18.91 MJ/m squared and that for normal incidence beam radiation is 21.17 MJ/m squared. The annual-average daily fraction of the horizontal global radiation that is beam is 0.72. The annual-average daily value for the clearness index is 0.587 and the average frequency of clear days annually is 58.6%. We conclude, based upon the above analysis, that Beer Sheva and its environs are characterized by relatively high, average-daily irradiation rates, both global and beam, and a relatively high frequency of clear days.

Monthly-mean atmospheric precipitable water data for twenty six stations in continental Australia and neighboring islands have been analyzed to develop relations for estimating precipitable water at sites where it has not been measured. Monthly-mean precipitable water can be estimated for a site with an rms error of about 14%, using the monthly mean surface temperature and relative humidity. This rms error can be reduced to about 9% by using the systematic deviations of the estimated precipitable water from the measured precipitable water for stations in the region about the site where an estimate is to be made, to adjust the precipitable water estimate. The amount of sunlight scattered by the atmosphere is a function of the amount of precipitable water and other quantities.

Using in situ measurements of aerosol optical properties and ground-based measurements of aerosol optical thickness ([[Tau].sub.s]) during the Smoke, Clouds and Radiation--Brazil (SCAR-B) experiment, a four-stream broadband radiative transfer model is used to estimate the downward shortwave irradiance (DSWI) and top-of-atmosphere (TOA) shortwave aerosol radiative forcing (SWARF) in cloud-free regions dominated by smoke from biomass burning in Brazil. The calculated DSWI values are compared with broadband pyranometer measurements made at the surface. The results show that, for two days when near-coincident measurements of single-scattering albedo [[Omega.sub.0] and [[Tau].sub.s] are available, the root-mean-square errors between the measured and calculated DSWI for daytime data are within 30 W [m.sup.-2]. For five days during SCAR-B, however, when assumptions about [[Omega].sub.0] have to be made and also when [[Tau].sub.s] was significantly higher, the differences can be as large as 100 W [m.sup.-2]. At TOA, the SWARF per unit optical thickness ranges from -20 to -60 W [m.sup.-2] over four major ecosystems in South America. The results show that [[Tau].sub.s] and [[Omega].sub.0] are the two most important parameters that affect DSWI calculations. For SWARF values, surface albedos also play an important role. It is shown that [[Omega].sub.0] must be known within 0.05 and [[Tau.sub.s] at 0.55 [micro]m must be known to within 0.1 to estimate DSWI to within 20 W [m.sup.-2], The methodology described in this paper could serve as a potential strategy for determining DSWI values in the presence of aerosols. The wavelength dependence of [[Tau].sub.s] and [[Omega].sub.0] over the entire shortwave spectrum is needed to improve radiative transfer calculations. If global retrievals of DSWI and SWARF from satellite measurements are to be performed in the presence of biomass-burning aerosols on a routine basis, a concerted effort should be made to develop methodologies for estimating [[Omega].sub.0] and [[Tau].sub.s] from satellite and ground-based measurements.

Errors in measurements of sea-surface skin temperature (SSST) caused by inappropriate measurements of sky radiance are discussed; both model simulations and in situ data obtained in the Atlantic Ocean are used. These errors are typically caused by incorrect radiometer view geometry (pointing), temporal mismatches between the sea surface and atmospheric views, and the effect of wind on the sea surface. For clear-sky, overcast, or high-humidity atmospheric conditions, SSST is relatively insensitive ([is less than]0.1 K) to sky-pointing errors of [+ or -] 10 [degrees] and to temporal mismatches between the sea and sky views. In mixed-cloud conditions, SSST errors greater than [+ or -] 0.25 K are possible as a result either of poor radiometer pointing or of a temporal mismatch between the sea and sky views. Sea-surface emissivity also changes with sea view pointing angle. Sea view pointing errors should remain below 5 [degrees] for SSST errors of [is less than]0.1K. We conclude that the clear-sky requirement of satellite infrared SSST observations means that sky-pointing errors are small when one is obtaining in situ SSST validation data at zenith angles of [is less than] 40 [degrees]. At zenith angles greater than this, large errors are possible in high-wind-speed conditions. We recommend that high-resolution inclinometer measurements always be used, together with regular alternating sea and sky views, and that the temporal mismatch between sea and sky views be as small as possible. These results have important implications for the development of operational autonomous instruments for determining SSST for the long-term validation of satellite SSST. OCIS codes: 010.4450, 010.1290, 260.3060, 120.5630, 120.0280, 120.6780.

NASA's proposed Terrestrial Planet Finder, a space-based inter-ferometer, will eventually allow spectroscopic analyses of the atmospheres of extrasolar planets. Such analyses would provide information about the existence of life on these planets. One strategy in the search for life is to look for evidence of [O.sub.3] (and hence [O.sub.2]) in a planet's atmosphere; another is to look for gases that might be present in an atmosphere analogous to that of the inhabited early Earth. In order to investigate these possibilities, we have calculated synthetic spectra for several hypothetical terrestrial-type atmospheres. The model atmospheres represent four different scenarios. The first two, representing inhabited terrestrial planets, are an Earth-like atmosphere containing variable amounts of oxygen and an early Earth-type atmosphere containing methane. In addition, two cases representing Mars-like and early Venus-like atmospheres were evaluated, to provide possible 'false positive' spectra. The calculated spectra suggest that ozone could be detected by an instrument like Terrestrial Planet Finder if the [O.sub.2] concentration in the planet's atmosphere is [is greater than or equal to] 200 ppm, or [10.sup.-3] times the present atmospheric level. Methane should be observable on an early-Earth type planet if it is present in concentrations of 100 ppm or more. Methane has both biogenic and abiogenic sources, but concentrations exceeding 1000 ppm, or 0.1% by volume, would be difficult to produce from abiogenic sources alone. High methane concentrations in a planet's atmosphere are therefore another potential indicator for extraterrestrial life. Key Words: exobiology; extrasolar planets; terrestrial planets; spectroscopy; atmospheres, composition.