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Optical Depth (optical + depth)
Selected AbstractsGlobal statistical analysis of MISR aerosol data: a massive data product from NASA's Terra satelliteENVIRONMETRICS, Issue 7 2007Tao Shi Abstract In climate models, aerosol forcing is the major source of uncertainty in climate forcing, over the industrial period. To reduce this uncertainty, instruments on satellites have been put in place to collect global data. However, missing and noisy observations impose considerable difficulties for scientists researching the global distribution of aerosols, aerosol transportation, and comparisons between satellite observations and global-climate-model outputs. In this paper, we fit a Spatial Mixed Effects (SME) statistical model to predict the missing values, denoise the observed values, and quantify the spatial-prediction uncertainties. The computations associated with the SME model are linear scalable to the number of data points, which makes it feasible to process massive global satellite data. We apply the methodology, which is called Fixed Rank Kriging (FRK), to the level-3 Aerosol Optical Depth (AOD) dataset collected by NASA's Multi-angle Imaging SpectroRadiometer (MISR) instrument flying on the Terra satellite. Overall, our results were superior to those from non-statistical methods and, importantly, FRK has an uncertainty measure associated with it that can be used for comparisons over different regions or at different time points. Copyright © 2007 John Wiley & Sons, Ltd. [source] Synoptic climatological influences on the spatial and temporal variability of aerosols over North AmericaINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2006Helen C. Power Abstract The spatial and temporal variability of atmospheric aerosols is not well understood, as most studies have been constrained to data sets that include few stations and are of short duration. Furthermore, all methods for quantifying atmospheric turbidity suffer from a major constraint in that they require cloudless sky conditions. This restriction produces gaps in the turbidity record and sampling bias, which has led to questionable inferences about the variability of aerosols. In this research, we address these concerns via analyses at scales broader than all previous studies. We analyzed the spectral aerosol optical depth at 500 nm (,a5) and Ångström's wavelength exponent (,), which represents the relative size distribution of aerosols. A total of 27 sites, with a mean period of record of 7.3 years, are included. Beyond seasonal and spatial summaries of aerosol variability, we have divided observations by synoptic condition, utilizing the Spatial Synoptic Classification (SSC). Our results show that atmospheric turbidity across North America is greatest over the east. Seasonality of both parameters was shown, most notably a greater ,a5 during summertime. Utilizing the SSC, we have uncovered significant differences across weather types. Moist weather types, especially moist tropical, display considerably higher turbidity, while the colder, drier dry polar weather type is associated with low aerosol optical depth. Certain weather types show considerable seasonal variability; the dry tropical weather type is associated with relatively low values in winter, but high values in summer, when convection is significant. Cluster analyses of stations yielded three general regions, each with similar synoptic variability: a western cluster with low aerosol optical depth and minimal synoptic variability, an eastern cluster with higher turbidity and variability, and a cluster located on the periphery of the eastern cluster, associated with moderate levels of turbidity but very high variability, suggesting a varied influence of nearby industrial areas. Copyright © 2006 Royal Meteorological Society. [source] The first broad-band X-ray study of the Supergiant Fast X-ray Transient SAX J1818.6,1703 in outburstMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2009L. Sidoli ABSTRACT The Supergiant Fast X-ray Transient (SFXT) SAX J1818.6,1703 underwent an outburst on 2009 May 6 and was observed with Swift. We report on these observations which, for the first time, allow us to study the broad-band spectrum from soft to hard X-rays of this source. No X-ray spectral information was available on this source before the Swift monitoring. The spectrum can be deconvolved well with models usually adopted to describe the emission from HMXB X-ray pulsars, and is characterized by a very high absorption, a flat power law (photon index ,0.1,0.5) and a cut-off at about 7,12 keV. Alternatively, the SAX J1818.6,1703 emission can be described with a Comptonized emission from a cold and optically thick corona, with an electron temperature kTe= 5,7 keV, a hot seed photon temperature, kT0, of 1.3,1.4 keV and an optical depth for the Comptonizing plasma, ,, of about 10. The 1,100 keV luminosity at the peak of the flare is 3 × 1036 erg s,1 (assuming the optical counterpart distance of 2.5 kpc). These properties of SAX J1818.6,1703 resemble those of the prototype of the SFXT class, XTE J1739,302. The monitoring with Swift/XRT reveals an outburst duration of about 5 d, similar to other members of the class of SFXTs, confirming SAX J1818.6,1703 as a member of this class. [source] Microlensing by cosmic stringsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Konrad Kuijken ABSTRACT We consider the signature and detectability of gravitational microlensing of distant quasars by cosmic strings. Because of the simple image configuration such events will have a characteristic lightcurve, in which a source would appear to brighten by exactly a factor of 2, before reverting to its original apparent brightness. We calculate the optical depth and event rate, and conclude that current predictions and limits on the total length of strings on the sky imply optical depths of , 10,8 and event rates of fewer than one event per 109 sources per year. Disregarding those predictions but replacing them with limits on the density of cosmic strings from the cosmic microwave background fluctuation spectrum, leaves only a small region of parameter space (in which the sky contains about 3 × 105 strings with deficit angle of the order of 0.3 milli-seconds) for which a microlensing survey of exposure 107 source years, spanning a 20,40-year period, might reveal the presence of cosmic strings. [source] Ly, leaks and reionizationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2008Longlong Feng ABSTRACT Ly, absorption spectra of QSOs at redshifts z, 6 show complete Gunn,Peterson absorption troughs (dark gaps) separated by tiny leaks. The dark gaps are from the intergalactic medium (IGM) where the density of neutral hydrogen are high enough to produce almost saturated absorptions, however, where the transmitted leaks come from is still unclear so far. We demonstrate that leaking can originate from the lowest density voids in the IGM as well as the ionized apatches around ionizing sources using semi-analytical simulations. If leaks are produced in lowest density voids, the IGM must already be highly ionized, and the ionizing background should be almost uniform; in contrast, if leaks come from ionized patches, the neutral fraction of IGM should be still high, and the ionizing background is significantly inhomogeneous. Therefore, the origin of leaking is crucial to determining the epoch of inhomogeneous-to-uniform transition of the ionizing photon background. We show that the origin could be studied with the statistical features of leaks. Actually, Ly, leaks can be well defined and described by the equivalent width W and the full width of half-area WH, both of which are less contaminated by instrumental resolution and noise. It is found that the distributions of W and WH of Ly, leaks are sensitive to the modelling of the ionizing background. We consider four representative models: uniform ionizing background (model 0), the photoionization rate of neutral hydrogen ,H i and the density of IGM are either linearly correlated (model I), or anticorrelated (model II), and ,H i is correlated with high-density peaks containing ionizing sources (model III). Although all of these models can match to the mean of the observed effective optical depth of the IGM at z, 6, the distributions of W and WH are very different from each other. Consequently, the leak statistics provides an effective tool to probe the evolutionary history of reionization at z, 5,6.5. Similar statistics will also be applicable to the reionization of He ii at z, 3 [source] Stellar contents and star formation in the young star cluster Be 59MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008A. K. Pandey ABSTRACT We present UBV Ic CCD photometry of the young open cluster Be 59 with the aim to study the star formation scenario in the cluster. The radial extent of the cluster is found to be ,10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between E(B,V) , 1.4 to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as 3.7 ± 0.3. The distance of the cluster is found to be 1.00 ± 0.05 kpc. Using near-infrared (NIR) colours and slitless spectroscopy, we have identified young stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs range between <1 and ,2 Myr, whereas the mean age of the massive stars in the cluster region is found to be ,2 Myr. There is evidence for second-generation star formation outside the boundary of the cluster, which may be triggered by massive stars in the cluster. The slope of the initial mass function, ,, in the mass range 2.5 < M/M,, 28 is found to be ,1.01 ± 0.11 which is shallower than the Salpeter value (,1.35), whereas in the mass range 1.5 < M/M,, 2.5 the slope is almost flat. The slope of the K -band luminosity function is estimated as 0.27 ± 0.02, which is smaller than the average value (,0.4) reported for young embedded clusters. Approximately 32 per cent of H, emission stars of Be 59 exhibit NIR excess indicating that inner discs of the T Tauri star (TTS) population have not dissipated. The Midcourse Space Experiment (MSX) and IRAS-HIRES images around the cluster region are also used to study the emission from unidentified infrared bands and to estimate the spatial distribution of optical depth of warm and cold interstellar dust. [source] Reionization bias in high-redshift quasar near-zonesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008J. Stuart B. Wyithe ABSTRACT Absorption spectra of high-redshift quasars exhibit an increasingly thick Ly, forest, suggesting that the fraction of neutral hydrogen in the intergalactic medium (IGM) is increasing towards z, 6. However, the interpretation of these spectra is complicated by the fact that the Ly, optical depth is already large for neutral hydrogen fractions in excess of 10,4, and also because quasars are expected to reside in dense regions of the IGM. We present a model for the evolution of the ionization state of the IGM which is applicable to the dense, biased regions around high-redshift quasars as well as more typical regions in the IGM. We employ a cold dark matter based model in which the ionizing photons for reionization are produced by star formation in dark matter haloes spanning a wide range of masses, combined with numerical radiative transfer simulations which model the resulting opacity distribution in quasar absorption spectra. With an appropriate choice for the parameter which controls the star formation efficiency, our model is able to simultaneously reproduce the observed Ly, forest opacity at 4 < z < 6, the ionizing photon mean-free-path at z, 4 and the rapid evolution of highly ionized near-zone sizes around high-redshift quasars at 5.8 < z < 6.4. In our model, reionization extends over a wide redshift range, starting at z, 10 and completing as H ii regions overlap at z, 6,7. We find that within 5 physical Mpc of a high-redshift quasar, the evolution of the ionization state of the IGM precedes that in more typical regions by around 0.3 redshift units. More importantly, when combined with the rapid increase in the ionizing photon mean-free-path expected shortly after overlap, this offset results in an ionizing background near the quasar which exceeds the value in the rest of the IGM by a factor of ,2,3. We further find that in the post-overlap phase of reionization the size of the observed quasar near-zones is not directly sensitive to the neutral hydrogen fraction of the IGM. Instead, these sizes probe the level of the background ionization rate and the temperature of the surrounding IGM. The observed rapid evolution of the quasar near-zone sizes at 5.8 < z < 6.4 can thus be explained by the rapid evolution of the ionizing background, which in our model is caused by the completion of overlap at the end of reionization by 6 ,z, 7. [source] Cosmic reionization constraints on the nature of cosmological perturbationsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006Pedro P. Avelino ABSTRACT We study the reionization history of the Universe in cosmological models with non-Gaussian density fluctuations, taking them to have a renormalized ,2 probability distribution function parametrized by the number of degrees of freedom, ,. We compute the ionization history using a simple semi-analytical model, considering various possibilities for the astrophysics of reionization. In all our models we require that reionization is completed prior to z= 6, as required by the measurement of the Gunn,Peterson optical depth from the spectra of high-redshift quasars. We confirm previous results demonstrating that such a non-Gaussian distribution leads to a slower reionization as compared to the Gaussian case. We further show that the recent WMAP three-year measurement of the optical depth due to electron scattering, ,= 0.09 ± 0.03, weakly constrains the allowed deviations from Gaussianity on the small scales relevant to reionization if a constant spectral index is assumed. We also confirm the need for a significant suppression of star formation in minihaloes, which increases dramatically as we decrease ,. [source] Constraints on the initial mass function of the first starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006Raffaella Schneider ABSTRACT Motivated by theoretical predictions that the first stars were predominantly very massive, we investigate the physics of the transition from an early epoch dominated by massive Pop III stars to a later epoch dominated by familiar low-mass Pop II/I stars by means of a numerically generated catalogue of dark matter haloes coupled with a self-consistent treatment of chemical and radiative feedback. Depending on the strength of the chemical feedback, Pop III stars can contribute a substantial fraction (several per cent) of the cosmic star formation activity even at moderate redshifts, z, 5. We find that the three z, 10 sources tentatively detected in Near Infrared Camera and Multi-Object Spectrometer (NICMOS) Ultra Deep Fields (UDFs) should be powered by Pop III stars, if these are massive; however, this scenario fails to reproduce the derived Wilkinson Microwave Anisotropy Probe (WMAP) electron scattering optical depth. Instead, both the UDFs and WMAP constraints can be fulfilled if stars at any time form with a more standard, slightly top-heavy, Larson initial mass function. [source] A very extended reionization epoch?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005A. Melchiorri ABSTRACT The recent observations of cross temperature,polarization power spectra of the cosmic microwave background (CMB) made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite are in better agreement with a high value of the Thomson scattering optical depth ,, 0.17. This value is close to ,= 0.3, which is taken as the upper limit in the parameter extraction analysis made by the WMAP team. However, models with ,, 0.3 provide a good fit to current CMB data and are not significantly excluded when combined with large-scale structure data. By making use of a self-consistent reionization model, we verify the astrophysical feasibility of models with ,, 0.3. It turns out that current data on various observations related to the thermal and ionization history of the intergalactic medium are not able to rule out ,, 0.3. The possibility of a very extended reionization epoch can significantly undermine the WMAP constraints on crucial cosmological parameters such as the Hubble constant, the spectral index of primordial fluctuations and the amplitude of dark matter clustering. [source] The effect of condensates on the characterization of transiting planet atmospheres with transmission spectroscopyMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005Jonathan J. Fortney ABSTRACT Through a simple physical argument we show that the slant optical depth through the atmosphere of a ,hot Jupiter' planet is ,35,90 times greater than the normal optical depth. This not unexpected result has direct consequences for the method of transmission spectroscopy for characterizing the atmospheres of transiting giant planets. The atmospheres of these planets likely contain minor condensates and hazes, which at normal viewing geometry have negligible optical depth, but at slant viewing geometry have appreciable optical depth that can obscure absorption features of gaseous atmospheric species. We identify several possible condensates. We predict that this is a general masking mechanism for all planets, not just for HD 209458b, and will lead to weaker than expected or undetected absorption features. Constraints on an atmosphere from transmission spectroscopy are not the same as constraints on an atmosphere at normal viewing geometry. [source] Background radiation from sterile neutrino decay and reionizationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005M. Mapelli ABSTRACT Sterile neutrinos are one of the most promising warm dark matter (WDM) candidates. By considering their radiative- and pion-decay channels, we derive the allowed contribution of sterile neutrinos to the X-ray, optical and near-infrared (NIR) cosmic backgrounds. The X-ray background puts a strong constraint on the mass of radiatively decaying neutrinos (m,s, 14 keV), whereas the allowed mass range for pion-decay neutrinos (for a particle lifetime > 4 × 1017 s) is 150 ,m,s/MeV , 500. Taking into account these constraints, we find that sterile neutrinos do not significantly contribute to the optical and NIR background. We further consider the impact of sterile neutrinos on reionization. We find that the Thomson optical depth due to sterile neutrinos is ,e= (0.4,3) × 10,2 in the case of radiative decays and it is ,10,3 for the pion-decay channel. We conclude that these particles must have played only a minor role in cosmic reionization history. [source] Constraining dark energy with X-ray galaxy clusters, supernovae and the cosmic microwave backgroundMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005David Rapetti ABSTRACT We present new constraints on the evolution of dark energy from an analysis of cosmic microwave background, supernova and X-ray galaxy cluster data. Our analysis employs a minimum of priors and exploits the complementary nature of these data sets. We examine a series of dark energy models with up to three free parameters: the current dark energy equation of state w0, the early-time equation of state wet, and the scalefactor at transition at. From a combined analysis of all three data sets, assuming a constant equation of state and that the Universe is flat, we measure w0=,1.05+0.10,0.12. Including wet as a free parameter and allowing the transition scalefactor to vary over the range 0.5 < at < 0.95 where the data sets have discriminating power, we measure w0=,1.27+0.33,0.39 and wet=,0.66+0.44,0.62. We find no significant evidence for evolution in the dark energy equation-of-state parameter with redshift. Marginal hints of evolution in the supernovae data become less significant when the cluster constraints are also included in the analysis. The complementary nature of the data sets leads to a tight constraint on the mean matter density ,m and alleviates a number of other parameter degeneracies, including that between the scalar spectral index ns, the physical baryon density ,bh2 and the optical depth ,. This complementary nature also allows us to examine models in which we drop the prior on the curvature. For non-flat models with a constant equation of state, we measure w0=,1.09+0.12,0.15 and obtain a tight constraint on the current dark energy density ,de= 0.70 ± 0.03. For dark energy models other than a cosmological constant, energy,momentum conservation requires the inclusion of spatial perturbations in the dark energy component. Our analysis includes such perturbations, assuming a sound speed c2s= 1 in the dark energy fluid as expected for quintessence scenarios. For our most general dark energy model, not including such perturbations would lead to spurious constraints on wet, which would be tighter than those mentioned above by approximately a factor of 2 with the current data. [source] MSX mid-infrared imaging of massive star birth environments , II.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2004Giant H ii regions ABSTRACT We conduct a Galactic census of giant H ii (GH ii) regions, based on the all-sky 6-cm data set of Kuchar & Clark, plus the kinematic distances obtained by Russeil. From an inspection of mid-infrared (MIR) Mid-course Space Experiment (MSX) and far-IR IRAS Sky Survey Atlas images, we identify a total of 56 GH ii regions in the Milky Way, of which 15 per cent (65 per cent) can be seen at optical (near-IR) wavelengths. The mid to far-IR fluxes from each GH ii region are measured, and sample the thermal emission from the ubiquitous dust present within the exciting clusters of OB stars, arising from the integrated luminosity of the hot stars heating the cluster dust, for which we obtain log L(IR) = 5.5,7.3 L,. The MIR 21-,m spatial morphology is presented for each GH ii region, and often indicates multiple emission sources, suggesting complicated cluster formation. IR colour,colour diagrams are presented, providing information concerning the temperature distribution and the optical depth of the dust. For the clusters of our study, the dust is not optically thick to all stellar radiation, thus the measured infrared luminosity is lower than Lbol. As the dust environment of a cluster begins to dissipate, the thermal emission and its optical depth ought to decrease even before the stars evolve appreciably. We see evidence of this in our empirical relationship between the integrated IR and Lyman continuum luminosities. [source] Can non-Gaussian cosmological models explain the WMAP high optical depth for reionization?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2003Xuelei Chen ABSTRACT The first-year Wilkinson Microwave Anisotropy Probe data suggest a high optical depth for Thomson scattering of 0.17 ± 0.04, implying that the Universe was reionized at an earlier epoch than previously expected. Such early reionization is likely to be caused by ultraviolet (UV) photons from first stars, but it appears that the observed high optical depth can be reconciled within the standard structure formation model only if star formation in the early Universe was extremely efficient. With normal star formation efficiencies, cosmological models with non-Gaussian density fluctuations may circumvent this conflict as high density peaks collapse at an earlier epoch than in models with Gaussian fluctuations. We study cosmic reionization in non-Gaussian models and explore to what extent, within available constraints, non-Gaussianities affect the reionization history. For mild non-Gaussian fluctuations at redshifts of 30 to 50, the increase in optical depth remains at a level of a few per cent and appears unlikely to aid significantly in explaining the measured high optical depth. On the other hand, within available observational constraints, increasing the non-Gaussian nature of density fluctuations can easily reproduce the optical depth and may remain viable in underlying models of non-Gaussianity with a scale-dependence. [source] Constraints on the ultraviolet metagalactic emissivity using the Ly, forestMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2003Avery Meiksin ABSTRACT Numerical hydrodynamical simulations have proven a successful means of reproducing many of the statistical properties of the Ly, forest as measured in high redshift quasar spectra. The source of ionization of the intergalactic medium (IGM), however, remains unknown. We investigate how the Ly, forest may be used to probe the nature of the sources. We show that the attenuation of Lyman continuum photons by the IGM depends sensitively on the emissivity of the sources, permitting a strong constraint to be set on the required emissivity to match the measured values of the mean IGM Ly, optical depth. We find that, within the observational errors, quasi-stellar object (QSO) sources alone are able to account for the required ultraviolet (UV) background at z, 4. By contrast, the emissivity of Lyman-break galaxies (LBGs) must decline sharply with redshift, compared with the estimated emissivity at z, 3, so as not to overproduce the UV background and drive the mean Ly, optical depth to values that are too low. We also investigate the effect of fluctuations in the UV background, as would arise if QSOs dominated. To this end, we derive the distribution function of the background radiation field produced by discrete sources in an infinite universe, including the effects of attenuation by an intervening absorbing medium. We show that, for z, 5, the fluctuations significantly boost the mean Ly, optical depth, and so increase the estimate for the mean ionization rate required to match the measured mean Ly, optical depths. The fluctuations will also result in large spatial correlations in the ionization level of the IGM. We show that the large mean Ly, optical depth measured at z, 6 suggests such large correlations will be present if QSOs dominate the UV background. A secondary, smaller effect of the UV background fluctuations is a distortion of the pixel flux distribution. While the effect on the distribution may be too small to detect with existing telescopes, it may be measurable with the extremely large telescopes planned for the future. We also show that if QSOs dominate the UV background at z, 6, then they will be sufficient in number to rejuvenate the ionization of a previously ionized IGM if it has not yet fully recombined. [source] Probing the dark ages with redshift distribution of gamma-ray burstsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002T. Roy Choudhury Abstract In this article, we explore the possibility of using the properties of gamma-ray bursts (GRBs) to probe the physical conditions in the epochs prior to reionization. The redshift distribution of GRBs is modelled using the Press,Schechter formalism with an assumption that they follow the cosmic star formation history. We reproduce the observed star formation rate obtained from galaxies in the redshift range 0 < z < 5, as well as the redshift distribution of the GRBs inferred from the luminosity,variability correlation of the burst light curve. We show that the fraction of GRBs at high redshifts, the afterglows of which cannot be observed in the R and I bands owing to H i Gunn,Peterson optical depth can, at the most, account for one third of the dark GRBs. The observed redshift distribution of GRBs, with much less scatter than the one available today, can put stringent constraints on the epoch of reionization and the nature of gas cooling in the epochs prior to reionization. [source] Cuspy dark matter haloes and the GalaxyMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2001J.J. Binney The microlensing optical depth to Baade's Window constrains the minimum total mass in baryonic matter within the Solar circle to be greater than ,, assuming the inner Galaxy is barred with viewing angle ,20°. From the kinematics of solar neighbourhood stars, the local surface density of dark matter is ,. We construct cuspy haloes normalized to the local dark matter density and calculate the circular-speed curve of the halo in the inner Galaxy. This is added in quadrature to the rotation curve provided by the stellar and ISM discs, together with a bar sufficiently massive so that the baryonic matter in the inner Galaxy reproduces the microlensing optical depth. Such models violate the observational constraint provided by the tangent-velocity data in the inner Galaxy (typically at radii . The high baryonic contribution required by the microlensing is consistent with implications from hydrodynamical modelling and the pattern speed of the Galactic bar. We conclude that the cuspy haloes favoured by the cold dark matter cosmology (and its variants) are inconsistent with the observational data on the Galaxy. [source] SCUBA observations of Hawaii 167MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2000Geraint F. Lewis We present the first submillimetre observations of the z=2.36 broad absorption line system Hawaii 167. Our observations confirm the hypothesis that Hawaii 167 contains a massive quantity of dust, the optical depth of which is sufficient to extinguish completely our ultraviolet view of a central, buried quasar. The submillimetre luminosity and associated dust mass of Hawaii 167 are similar to those of the ultraluminous class of infrared galaxies, supporting the existence of an evolutionary link between these and the active galaxy population. Hawaii 167 appears to be a young quasar that is emerging from its dusty cocoon. [source] Albedo, atmospheric solar absorption and heating rate measurements with stacked UAVsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 629 2007M. V. Ramana Abstract This paper reports unique measurements of albedo, atmospheric solar absorption, and heating rates in the visible (0.4 to 0.7 µm) and broadband (0.3 to 2.8 µm) spectral regions using vertically stacked multiple lightweight autonomous unmanned aerial vehicles (UAVs). The most significant finding of this study is that when absorbing aerosols and water vapour concentrations are measured accurately and accounted for in models, and when heating rates are measured directly with stacked aircraft, the simulated clear sky heating rates are consistent with the observed broadband heating rates within experimental errors (about 15%). We conclude that there is no need to invoke anomalous or excess absorption or unknown physics in clear skies. Aerosol,radiation,cloud measurements were made over the tropical Indian Ocean within the lowest 3 km of the atmosphere during the Maldives Autonomous UAV Campaign (MAC). The UAVs and ground-based remote sensing instruments determined most of the parameters required for calculating the albedo and vertical distribution of solar fluxes. The paper provides a refined analytical procedure to reduce errors and biases due to the offset errors arising from mounting of the radiometers on the aircraft and due to the aircraft attitude. Measured fluxes have been compared with those derived from a Monte-Carlo radiative transfer algorithm which can incorporate both gaseous and aerosol components. Under cloud-free conditions the calculated and measured incoming fluxes agree within 2,10 W m,2 (<1%) depending upon the altitudes. Similarly, the measured and calculated reflected fluxes agreed within 2,5 W m,2 (<5%). The analysis focuses on a cloud-free day when the air was polluted due to long-range transport from India, and the mean aerosol optical depth (AOD) was 0.31 and mean single scattering albedo was 0.92. The UAV-measured absorption AOD was 0.019 which agreed within 20% of the value of 0.024 reported by a ground-based instrument. The observed and simulated solar absorption agreed within 5% above 1.0 km and aerosol absorption accounted for 30% to 50% of the absorption depending upon the altitude and solar zenith angle. Thus there was no need to invoke spurious or anomalous absorption, provided we accounted for aerosol black carbon. The diurnal mean absorption values for altitudes between 0.5 and 3.0 km above mean sea level were observed to be 41 ± 3 W m,2 (1.5 K/day) in the broadband region and 8 ± 2 W m,2 (0.3 K/day) in the visible region. The contribution of absorbing aerosols to the heating rate was an order of magnitude larger than the contribution of CO2 and one-third that of the water vapour. In the lowest 3 km of the tropical atmosphere, aerosols accounted for more than 80% of the atmospheric absorption in the visible region. Copyright © 2007 Royal Meteorological Society [source] In situ and remote-sensing measurements of the mean microphysical and optical properties of industrial pollution aerosol during ADRIEXTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue S1 2007S. R. Osborne Abstract We present a summary of the principal physical and optical properties of aerosol particles using the FAAM BAE-146 instrumented aircraft during ADRIEX between 27 August and 6 September 2004, augmented by sunphotometer, lidar and satellite retrievals. Observations of anthropogenic aerosol, principally from industrial sources, were concentrated over the northern Adriatic Sea and over the Po Valley close to the aerosol sources. An additional flight was also carried out over the Black Sea to compare east and west European pollution. Measurements show the single-scattering albedo of dry aerosol particles to vary considerably between 0.89 and 0.97 at a wavelength of 0.55 µm, with a campaign mean within the polluted lower free troposphere of 0.92. Although aerosol concentrations varied significantly from day to day and during individual days, the shape of the aerosol size distribution was relatively consistent through the experiment, with no detectable change observed over land and over sea. There is evidence to suggest that the pollution aerosol within the marine boundary layer was younger than that in the elevated layer. Trends in the aerosol volume distribution show consistency with multiple-site AERONET radiometric observations. The aerosol optical depths derived from aircraft measurements show a consistent bias to lower values than both the AERONET and lidar ground-based radiometric observations, differences which can be explained by local variations in the aerosol column loading and by some aircraft instrumental artefacts. Retrievals of the aerosol optical depth and fine-mode (<0.5 µm radius) fraction contribution to the optical depth using MODIS data from the Terra and Aqua satellites show a reasonable level of agreement with the AERONET and aircraft measurements. © Crown Copyright 2007. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd [source] Mie simulations as an error source in mineral aerosol radiative forcing calculationsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 623 2007M. Kahnert Abstract The role of aerosols remains a major uncertainty for climate and climate change. For the direct radiative forcing by mineral aerosols, the uncertainty in the refractive index m has been regarded as the most important error source, while the impact of aerosol non-sphericity has been considered a minor issue and is neglected in climate models. Here, the errors caused by the spherical particle approximation (SPA) are evaluated by comparing radiative fluxes based on (i) Mie simulations and (ii) laboratory measurements of aerosol optical properties. Furthermore, they are contrasted with the errors related to the uncertainty in the refractive index. These two error sources are found to be of comparable magnitude, although they are strongly dependent on optical depth, surface albedo, and particle size. Thus, our results provide evidence that, contrary to common beliefs, the use of spherical model particles in radiative transfer simulations is probably among the major sources of error in quantifying the climate forcing effect of mineral aerosols. This stems from misrepresentation of the scattering phase function and the asymmetry parameter. Aerosol single-scattering computations based on non-spherical model particles are expected to reduce the shape-related errors and thus significantly improve the accuracy of radiative forcing simulations. Copyright © 2007 Royal Meteorological Society [source] Effects of CO2 and dust on present-day solar radiation and climate on MarsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 611 2005Hannu Savijärvi Abstract A comprehensive spectrum-resolving radiative transfer model (SRM) was used to simulate the average, present-day, solar radiation field on Mars. A CO2 -only 6 hPa Martian atmosphere absorbs about 1% of zenith solar radiation, producing a modest heating rate of 4,5 K day,1 in the lowest 10 km. The trace gases have an insignificant effect but airborne dust reduces the downwelling solar flux effectively, and the reflected flux somewhat less. This produces an anti-greenhouse trend (cooling at the surface, warming within the atmosphere, reflection at the top), which increases strongly with the dust load. For instance, with dust visible optical depth of unity and sun in zenith, the surface solar irradiation is attenuated by 26% and the solar heating rate increases to about 70 K day,1 in the lowest 25 km. The numbers are however strongly dependent on the optical properties of the dust, which are not known very well. Several fast two-stream methods for dust were compared with the SRM results. Their common systematic errors were reduced by a simple, physically-based correction. The global albedo of Mars was then studied as a function of dust load, dust optics and surface albedo. The crossover from added airborne dust tending to make the whole planet look whiter or darker occurred at surface albedo of about 35%, nearly independently of the dust load. We demonstrate, however, that this value is sensitive to the optical properties of the assumed dust. Copyright © 2005 Royal Meteorological Society [source] Sensitivity of one-dimensional radiative biases to vertical cloud-structure assumptions: Validation with aircraft dataTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 608 2005F. Di Giuseppe Abstract Three representations of an observed stratocumulus system are generated by combining aircraft observations with a simple statistical model. The realizations differ in their representation of the vertical cloud structure while the horizontal variability is identical. In the control case (A) both the adiabatic liquid-water profile and the effect of wind-shear induced vertical decorrelation are represented. The second simulation (B) removes the wind-shear effect by assuming maximum overlap between adjacent layers. The third case (C) instead removes vertical variability by averaging the in-cloud liquid water for each column. For each of these scenes Monte Carlo simulated solar fluxes are compared against observed flux measurements. Cases A and B agree with observed (horizontal) flux variations within statistical uncertainty, while case C, which neglects vertical variability, is not able to reproduce the observed fluxes. The comparison between the radiative fields produced by these three representations of the stratocumulus system, calculated using a three-dimensional radiative-transfer solution, an independent pixel approximation (IPA) and a plane-parallel (PP) approach, shows substantial differences. Not accounting for the adiabatic liquid-water profile generates a systematic increase in the optical depth, , when the effective radius is computed from mean liquid-water content and droplet-number concentration, that can be responsible for a 5% increase in the reflection for shallow boundary-layer cloud systems (,,1). A much stronger effect in the radiative properties is produced by varying the cloud-overlap rule applied. While changing from maximum to random overlap does not introduce any variation in the optical depth of the cloud scene, it does introduce an increase in the reflection that is proportional to the relative change in total cloud fraction. The magnitude of these latter biases is comparable to that produced by unresolved horizontal variability. Moreover, it is shown that, when the vertical cloud structure is properly resolved, the effect of horizontal fluctuations is also reduced. Copyright © 2005 Royal Meteorological Society [source] The effect of overlying absorbing aerosol layers on remote sensing retrievals of cloud effective radius and cloud optical depthTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 598 2004Jim M. Haywood Abstract Two types of partially absorbing aerosol are included in calculations that are based on intensive aircraft observations: biomass burning aerosol characterized during the Southern AFricAn Regional science Initiative (SAFARI 2000) and mineral dust aerosol characterized during the SaHAran Dust Experiment (SHADE). Measurements during SAFARI 2000 reveal that the biomass burning aerosol layer is advected over the South Atlantic ocean at elevated altitudes above the marine boundary layer which is capped by semi-permanent stratocumulus cloud sheets. Similarly, the mineral dust is measured at elevated altitudes during SHADE resulting in transport above cloud for distances of several thousands of kilometres. We perform theoretical calculations of the effect of these partially absorbing aerosol layers on satellite retrievals of cloud effective radius and cloud optical depth, and show that, in these cases, retrievals of cloud optical depth or liquid water path are likely to be subject to systematic low biases. The theoretical calculations suggest that the cloud effective radius may be subject to a significant low bias for Moderate resolution Imaging Spectrometer (MODIS) retrievals that rely on the 0.86 and 1.63 µm radiance pair for an overlying aerosol layer of either biomass burning aerosol or mineral dust. Conversely, the cloud effective radius may be subject to a significant high bias for Advanced Very High Resolution Radiometer or MODIS retrievals that rely on the 0.63 and 3.7 µm radiance pair for an overlying aerosol layer of mineral dust. Analysis of 1 km resolution MODIS data for the SAFARI 2000 period suggests that the effective radius derived from the 0.86 and 1.63 µm radiance pair is, indeed, subject to a low bias in the presence of overlying biomass burning aerosol. These results show the difficulties associated with remote sensing retrievals, which must be kept in mind when attempting to assess any potential indirect effect. © Crown copyright 2004. [source] Quasi-biennial oscillations in spectral aerosol optical depthATMOSPHERIC SCIENCE LETTERS, Issue 4 2009S. Naseema Beegum Abstract Analysis of long-term time series of monthly mean aerosol optical depths (AOD) at four tropical stations over Asia and Africa revealed the presence of significant annual oscillations (AO) and quasi-biennial oscillations (QBO). While the AOs were the dominant features, the QBOs in AOD (QBOAOD) were also quite strong and were well associated with the QBO in stratospheric zonal wind (QBOU). At the equatorial stations, QBOAOD were out of phase with QBOU, while they were in phase at the off-equatorial stations. However, QBO in both outgoing longwave radiation (OLR) and rainfall showed an out-of-phase relationship with QBOAOD at all stations. Copyright © 2009 Royal Meteorological Society [source] A new IR technique for monitoring low cloud properties using geostationary satellite dataATMOSPHERIC SCIENCE LETTERS, Issue 2 2009Qingyuan Han Abstract A new technique of using satellite infrared radiance data for retrieving cloud properties is developed and applied to SEVIRI data, which is based on direct radiative transfer calculations, not on the emissivity approximation as used by other satellite IR only techniques. Instantaneous atmospheric profiles are used in the new technique for improving the accuracy of retrievals. Comparison of the retrieved results with coincident observations of CloudSat and CALIPSO shows excellent agreement for low clouds. This study shows that, using only IR radiances, the single layer assumption would significantly underestimate cloud optical depth when multilayered cloud system is presented. Copyright © 2009 Royal Meteorological Society [source] Estimating the top-of-atmosphere longwave radiative forcing due to Saharan dust from satellite observations over a west African surface siteATMOSPHERIC SCIENCE LETTERS, Issue 3 2007H. E. Brindley Abstract This paper presents a methodology for estimating the longwave top-of-atmosphere direct radiative forcing due to Saharan dust aerosol from satellite observations made by the Spinning Enhanced Visible and Infrared Imager (SEVIRI) and Geostationary Earth Radiation Budget (GERB) instruments. Narrow-band observations from SEVIRI are used to detect dust presence and quantify its loading, while GERB provides an estimate of the dust impact on the total outgoing longwave radiation. Applying the technique to observations made over the Banizoumbou surface station in Niger through March,June 2006 indicates a midday longwave forcing efficiency of 17 ± 5 W m,2 per unit aerosol optical depth. Copyright © 2007 Royal Meteorological Society [source] Multiyear ground-based and satellite observations of aerosol properties over a tropical urban area in IndiaATMOSPHERIC SCIENCE LETTERS, Issue 1 2007K. V. S. Badarinath Abstract Aerosol particle size distributions along with their spatial and temporal variability are important for describing both direct and indirect radiative forcing. In this study, the variation of black carbon (BC) aerosols, total aerosol mass loading and aerosol optical depth (AOD) over an urban region of Hyderabad, south India, was analyzed for 3 consecutive years from 2003 to 2005. The AOD was measured using a handheld multichannel sun-photometer at six wavelengths centered on 380, 440, 500, 675, 870 and 1020 nm and aerosol mass,size distribution was made using a quartz crystal microbalance (QCM) cascade impactor. In addition, satellite remote-sensing data from nighttime DMSP-OLS images were analyzed for inferring ancillary sources of aerosols. Results from temporal analysis (2004,2006) suggest that aerosol mass loading and BC mass concentration increased considerably over the 3-year time-period mainly due to increasing vehicular traffic from urban population growth. DMSP-OLS nighttime images for different years suggested higher forest fire occurrences in the year 2004 compared to other years. The annual mean AOD at 550 nm from moderate resolution imaging spectroradiometer (MODIS) showed relatively high values during 2004. Copyright © 2007 Royal Meteorological Society [source] Microlensing by cosmic stringsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Konrad Kuijken ABSTRACT We consider the signature and detectability of gravitational microlensing of distant quasars by cosmic strings. Because of the simple image configuration such events will have a characteristic lightcurve, in which a source would appear to brighten by exactly a factor of 2, before reverting to its original apparent brightness. We calculate the optical depth and event rate, and conclude that current predictions and limits on the total length of strings on the sky imply optical depths of , 10,8 and event rates of fewer than one event per 109 sources per year. Disregarding those predictions but replacing them with limits on the density of cosmic strings from the cosmic microwave background fluctuation spectrum, leaves only a small region of parameter space (in which the sky contains about 3 × 105 strings with deficit angle of the order of 0.3 milli-seconds) for which a microlensing survey of exposure 107 source years, spanning a 20,40-year period, might reveal the presence of cosmic strings. [source] | ||||||||||