Effects of threshold retrievals on estimates of the aerosol indirect radiative forcing

Empirical estimates of the aerosol indirect radiative forcing often rely on threshold cloud retrievals applied to multispectral satellite imagery data. In such retrievals, pixels having radiances that surpass prescribed thresholds are assumed to be overcast even if they are only partially cloud covered. This assumption leads to cloud visible optical depths that are underestimated and droplet radii that are overestimated. As regional cloud cover increases, overcast pixels become more common and the biases in cloud properties decrease. Because aerosol optical depths derived from cloud-free pixels also increase with regional cloud cover (Loeb and Manalo-Smith, 2005), the biases in threshold-derived cloud properties can be mistakenly interpreted as being evidence for the effects of aerosols on clouds. Because of the biases, threshold retrievals of cloud properties are likely to lead to overestimates of the aerosol indirect forcing. A retrieval scheme that accounts for fractional cloud cover within an imager pixel is used to estimate the enhancement in the indirect radiative forcing that arises from threshold cloud retrievals. The enhancements prove to be relatively small, approximately 20%. If cloud liquid water is held fixed to estimate the forcing, the biases in threshold-derived droplet radii and in the sensitivity of droplet radii to changes in aerosol nearly cancel so that the estimates are almost the same for threshold and partly cloudy pixel retrievals

Multiyear Advanced Very High Resolution Radiometer observations of summertime stratocumulus collocated with aerosols in the northeastern Atlantic

Advanced Very High Resolution Radiometer (AVHRR) 4-km data were collected over the northeast Atlantic for May–August 1995–1999. Aerosol optical depth at 0.55 μm was retrieved in pixels identified as being cloud-free ocean. In pixels identified as containing clouds from single-layered, low-level cloud systems over oceans, the following cloud properties were retrieved: 0.64-μm cloud optical depth, droplet effective radius, cloud layer altitude, pixel-scale fractional cloud cover, column liquid water amount and column droplet concentration. Aerosol and cloud properties were averaged in 1º x 1º latitude-longitude regions. Regions that contained clouds were limited to those in which all the clouds were part of a single-layered, low-level cloud system. Aerosol and cloud properties were compared only in 1º regions that had sufficient numbers of both cloud-free pixels that yielded aerosol retrievals and cloudy pixels that yielded retrievals of cloud properties within a single overpass. The comparisons were collected in 5º x 5º latitude-longitude regions to determine trends. Within each 5º region the cloud properties were similar from year to year, permitting the data to be composited for all 5 years. Aerosol optical depth decreased systematically with time, probably as a result of the increase in solar zenith angle due to the precession of the satellite orbit. Within the 5º regions, as aerosol optical depth increased, droplet effective radius decreased, cloud optical depth increased, and droplet column number concentration increased, qualitatively consistent with the trends expected for the aerosol indirect effect. In some regions, liquid water path decreased as aerosol optical depth increased, contrary to the trends expected for the suppression of drizzle. Within each 5º region, clouds in clean air, as indicated by their collocation with relatively small aerosol optical depths, had larger droplets and smaller cloud optical depths than clouds in polluted air, as indicated by their collocation with relatively large aerosol optical depths. On average, the aerosol indirect radiative forcing for overcast conditions was about twice as large as the direct radiative forcing for cloud-free conditions. In most of the 5º regions increases in cloud liquid water with increasing aerosol optical depth enhanced the ratios by 20–30% over those calculated from the changes in droplet effective radius and an assumption of constant cloud liquid water. In some 5º regions, however, like those just to the west of the Iberian peninsula, the column amount of cloud liquid water decreased with increasing aerosol optical depth

Aerosol and cloud property relationships for summertime stratiform clouds in the northeastern Atlantic from Advanced Very High Resolution Radiometer observations

Advanced Very High Resolution Radiometer (AVHRR) 4-km data collected over the northeastern Atlantic off the coast of the Iberian Peninsula for May to August 1995 were used to investigate the feasibility of empirically deriving estimates of the aerosol indirect radiative forcing. A retrieval scheme was used to derive cloud visible optical depth, droplet effective radius, cloud layer altitude, and pixel-scale fractional cloud cover. A two-channel aerosol retrieval scheme was used to determine aerosol optical depth in cloud-free pixels. Mean aerosol optical depths derived from the cloud-free pixels in 1º x 1º latitude-longitude regions on a given satellite overpass were associated with mean cloud properties derived from the cloudy pixels in the same region for the same satellite overpass. The analysis was restricted to 1º regions that contained only singlelayered, low-level cloud systems. Because aerosol and cloud properties are highly variable, results for the 4-month period were composited into 5º x 5º latitude-longitude regions and averaged to obtain reliable trends in the cloud properties as functions of aerosol burden. Consistent with expectations for the aerosol indirect effect, in some 5º regions, droplet effective radii decreased, and cloud visible optical depths increased as aerosol optical depths increased. The hypothesis that drizzle is suppressed in polluted clouds predicts that liquid water path should increase as aerosol burden increases. In three of the thirteen 5º regions studied, the liquid water path increased as aerosol optical depth increased, but in none of the regions was the increase in cloud liquid water statistically significant. In the remaining regions, cloud liquid water remained constant or even decreased with increasing aerosol optical depth. In many of the 5º regions, the retrieved aerosol optical depth increased as the percentage of cloudy pixels increased. Consistent with expectations from adiabatic cloud parcel models, droplet effective radius, cloud optical depth, and cloud liquid water path also increased as fractional cloud cover increased. The simultaneous increase in retrieved aerosol and cloud optical depths with increasing fractional cloud cover might have been due to the aerosol indirect effect, but it might also have resulted from processes that affect both the cloud and aerosol properties as cloud cover changes. The dependence on fractional cloud cover suggests that some of the trends between aerosol optical depth and the cloud properties cannot be solely attributed to the effects of the aerosols. For comparison with previous studies, the simultaneous changes in aerosol and cloud properties were used to estimate the daily average aerosol indirect forcing for overcast conditions in the summertime northeastern Atlantic. The magnitude of the indirect forcing relative to that of the direct forcing reported here is smaller than estimates reported by others

GENE SILENCING. Epigenetic silencing by the HUSH complex mediates position-effect variegation in human cells.

Forward genetic screens in Drosophila melanogaster for modifiers of position-effect variegation have revealed the basis of much of our understanding of heterochromatin. We took an analogous approach to identify genes required for epigenetic repression in human cells. A nonlethal forward genetic screen in near-haploid KBM7 cells identified the HUSH (human silencing hub) complex, comprising three poorly characterized proteins, TASOR, MPP8, and periphilin; this complex is absent from Drosophila but is conserved from fish to humans. Loss of HUSH components resulted in decreased H3K9me3 both at endogenous genomic loci and at retroviruses integrated into heterochromatin. Our results suggest that the HUSH complex is recruited to genomic loci rich in H3K9me3, where subsequent recruitment of the methyltransferase SETDB1 is required for further H3K9me3 deposition to maintain transcriptional silencing.This work was supported by a Wellcome Trust Principal Research Fellowship to P.J.L. (084957/Z/08/Z) and studentship to I.A.T., an MRC Centenary Award to R.T.T., and the Cambridge Biomedical Research Centre (UK). The CIMR is in receipt of a Wellcome Trust Strategic Award.This is the author accepted manuscript. The final version is available from AAAS via http://dx.doi.org/10.1126/science.aaa722

Imaging and Demography of the Host Galaxies of High-Redshift Type Ia Supernovae

We present the results of a study of the host galaxies of high redshift Type Ia supernovae (SNe Ia). We provide a catalog of 18 hosts of SNe Ia observed with the Hubble Space Telescope (HST) by the High-z Supernova Search Team (HZT), including images, scale-lengths, measurements of integrated (Hubble equivalent) BVRIZ photometry in bands where the galaxies are brighter than m ~ 25 mag, and galactocentric distances of the supernovae. We compare the residuals of SN Ia distance measurements from cosmological fits to measurable properties of the supernova host galaxies that might be expected to correlate with variable properties of the progenitor population, such as host galaxy color and position of the supernova. We find mostly null results; the current data are generally consistent with no correlations of the distance residuals with host galaxy properties in the redshift range 0.42 < z < 1.06. Although a subsample of SN hosts shows a formally significant (3-sigma) correlation between apparent V-R host color and distance residuals, the correlation is not consistent with the null results from other host colors probed by our largest samples. There is also evidence for the same correlations between SN Ia properties and host type at low redshift and high redshift. These similarities support the current practice of extrapolating properties of the nearby population to high redshifts pending more robust detections of any correlations between distance residuals from cosmological fits and host properties.Comment: 35 pages, 12 figures, 4 tables, accepted for publication in A

Type Ia Supernova Rate Measurements To Redshift 2.5 From CANDELS: Searching For Prompt Explosions In The Early Universe

dThe Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) was a multi-cycle treasury program on the Hubble Space Telescope (HST) that surveyed a total area of -0.25 deg2 with -900 HST orbits spread across five fields over three years. Within these survey images we discovered 65 supernovae (SNe) of all types, out to z 2.5. We classify -24 of these as Type Ia SNe (SNe Ia) based on host galaxy redshifts and SN photometry (supplemented by grism spectroscopy of six SNe). Here we present a measurement of the volumetric SN Ia rate as a function of redshift, reaching for the first time beyond z =- 2 and putting new constraints on SN Ia progenitor models. Our highest redshift bin includes detections of SNe that exploded when the universe was only -3 Gyr old and near the peak of the cosmic star formation history. This gives the CANDELS high redshift sample unique leverage for evaluating the fraction of SNe Ia that explode promptly after formation ( 40 Myr. However, mild tension is apparent between ground-based low-z surveys and space-based high-z surveys. In both CANDELS and the sister HST program CLASH (Cluster Lensing And Supernova Survey with Hubble), we find a low rate of SNe Ia at z > 1. This could be a hint that prompt progenitors are in fact relatively rare, accounting for only 20% of all SN Ia explosions-though further analysis and larger samples will be needed to examine that suggestion. Key words: infrared: general - supernovae:Astronom

The fast declining Type Ia supernova 2003gs, and evidence for a significant dispersion in near-infrared absolute magnitudes of fast decliners at maximum light

We obtained optical photometry of SN 2003gs on 49 nights, from 2 to 494 days after T(B_max). We also obtained near-IR photometry on 21 nights. SN 2003gs was the first fast declining Type Ia SN that has been well observed since SN 1999by. While it was subluminous in optical bands compared to more slowly declining Type Ia SNe, it was not subluminous at maximum light in the near-IR bands. There appears to be a bimodal distribution in the near-IR absolute magnitudes of Type Ia SNe at maximum light. Those that peak in the near-IR after T(B_max) are subluminous in the all bands. Those that peak in the near-IR prior to T(B_max), such as SN 2003gs, have effectively the same near-IR absolute magnitudes at maximum light regardless of the decline rate Delta m_15(B). Near-IR spectral evidence suggests that opacities in the outer layers of SN 2003gs are reduced much earlier than for normal Type Ia SNe. That may allow gamma rays that power the luminosity to escape more rapidly and accelerate the decline rate. This conclusion is consistent with the photometric behavior of SN 2003gs in the IR, which indicates a faster than normal decline from approximately normal peak brightness.Comment: 41 pages, 13 figures, to be published in the December, 2009, issue of the Astronomical Journa

Hubble Space Telescope and Ground-Based Observations of Type Ia Supernovae at Redshift 0.5: Cosmological Implications

We present observations of the Type Ia supernovae (SNe) 1999M, 1999N, 1999Q, 1999S, and 1999U, at redshift z~0.5. They were discovered in early 1999 with the 4.0~m Blanco telescope at Cerro Tololo Inter-American Observatory by the High-z Supernova Search Team (HZT) and subsequently followed with many ground-based telescopes. SNe 1999Q and 1999U were also observed with the Hubble Space Telescope. We computed luminosity distances to the new SNe using two methods, and added them to the high-z Hubble diagram that the HZT has been constructing since 1995. The new distance moduli confirm the results of previous work. At z~0.5, luminosity distances are larger than those expected for an empty universe, implying that a ``Cosmological Constant,'' or another form of ``dark energy,'' has been increasing the expansion rate of the Universe during the last few billion years.Comment: 68 pages, 22 figures. Scheduled for the 01 February 2006 issue of Ap.J. (v637

The Expanding Photosphere Method Applied to SN 1992 AM At cz=14 600km/s

We present photometry and spectroscopy of SN 1992am for five months following its discovery by the Calan/CTIO SN search. These data show SN 1992am to be type II-P, displaying hydrogen in its spectrum and the typical shoulder in its light curve. The photometric data and the distance from our own analysis are used to construct the supernova's bolometric light curve. Using the bolometric light curve, we estimate SN 1992am ejected approximately 0.30 M_sun_ of ^56^Ni, an amount four times larger than that of other well studied SNe II. SN 1992am's; host galaxy lies at a redshift of cz = 14600 km s ~, making it one of the most distant SNe II discovered, and an important application of the Expanding Photosphere Method. Since z = 0.05 is large enough for redshift-dependent effects to matter, we develop the technique to derive luminosity distances with the Expanding Photosphere Method at any redshift, and apply this method to SN 1992am, The derived distance, D = 180_-25_^+30^ Mpc, is independent of all other rungs in the extragalactic distance ladder. The redshift of SN 1992am's host galaxy is sufficiently large that uncertainties due to perturbations in the smooth Hubble flow should be smaller than 10%. The Hubble ratio derived from the distance and redshift of this single object is H_0_ = 81_-15_^+17^ km s^-1^ Mpc^-1^. In the future with more of these distant objects, we hope to establish an independent and statistically robust estimate of H_0_ based solely on type II supernovae.Supernova research at Harvard University is supported by NSF Grant No. AST 92-18475, and NASA Grants Nos. NAG 5-841 and NGT-51002. M.H. and J.M. gratefully acknowledge financial support from Grant No. 92/0312 from Fondo Nacional de Ciencias y Tecnologia (Fondecyt-Chile). The Berkeley group was financially supported by NSF Grants Nos. AST-8957063 and AST-9115174 to A. V. F, by an NSF Graduate Fellowship to T.M., and by the NSF Cooperative Agreement No. AST-8809616. N.D.T. acknowledges support from NSF Grant No. AST-931469

PTF11eon/SN2011dh: Discovery of a Type IIb Supernova From a Compact Progenitor in the Nearby Galaxy M51

On May 31, 2011 UT a supernova (SN) exploded in the nearby galaxy M51 (the Whirlpool Galaxy). We discovered this event using small telescopes equipped with CCD cameras, as well as by the Palomar Transient Factory (PTF) survey, and rapidly confirmed it to be a Type II supernova. Our early light curve and spectroscopy indicates that PTF11eon resulted from the explosion of a relatively compact progenitor star as evidenced by the rapid shock-breakout cooling seen in the light curve, the relatively low temperature in early-time spectra and the prompt appearance of low-ionization spectral features. The spectra of PTF11eon are dominated by H lines out to day 10 after explosion, but initial signs of He appear to be present. Assuming that He lines continue to develop in the near future, this SN is likely a member of the cIIb (compact IIb; Chevalier and Soderberg 2010) class, with progenitor radius larger than that of SN 2008ax and smaller than the eIIb (extended IIb) SN 1993J progenitor. Our data imply that the object identified in pre-explosion Hubble Space Telescope images at the SN location is possibly a companion to the progenitor or a blended source, and not the progenitor star itself, as its radius (~10^13 cm) would be highly inconsistent with constraints from our post-explosion photometric and spectroscopic data