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Nicolas, JP, Vogelmann AM, Scott RC, Wilson AB, Cadeddu MP, Bromwich DH, Verlinde J, Lubin D, Russell LM, Jenkinson C, Powers HH, Ryczek M, Stone G, Wille JD.  2017.  January 2016 extensive summer melt in West Antarctica favoured by strong El Niño. 8:15799.   10.1038/ncomms15799   Abstract

Over the past two decades the primary driver of mass loss from the West Antarctic Ice Sheet (WAIS) has been warm ocean water underneath coastal ice shelves, not a warmer atmosphere. Yet, surface melt occurs sporadically over low-lying areas of the WAIS and is not fully understood. Here we report on an episode of extensive and prolonged surface melting observed in the Ross Sea sector of the WAIS in January 2016. A comprehensive cloud and radiation experiment at the WAIS ice divide, downwind of the melt region, provided detailed insight into the physical processes at play during the event. The unusual extent and duration of the melting are linked to strong and sustained advection of warm marine air toward the area, likely favoured by the concurrent strong El Niño event. The increase in the number of extreme El Niño events projected for the twenty-first century could expose the WAIS to more frequent major melt events.

Markus, T, Neumann T, Martino A, Abdalati W, Brunt K, Csatho B, Farrell S, Fricker H, Gardner A, Harding D, Jasinski M, Kwok R, Magruder L, Lubin D, Luthcke S, Morison J, Nelson R, Neuenschwander A, Palm S, Popescu S, Shum CK, Schutz BE, Smith B, Yang YK, Zwally J.  2017.  The Ice, Cloud, and land Elevation Satellite-2 (ICESat-2): Science requirements, concept, and implementation. Remote Sensing of Environment. 190:260-273.   10.1016/j.rse.2016.12.029   AbstractWebsite

The Ice, Cloud, and land Elevation Satellite (ICESat) mission used laser altimetry measurements to determine changes in elevations of glaciers and ice sheets, as well as sea ice thickness distribution. These measurements have provided important information on the response of the cryopshere (Earth's frozen surfaces) to changes in atmosphere and ocean condition. ICESat operated from 2003 to 2009 and provided repeat altimetry measurements not only to the cryosphere scientific community but also to the ocean, terrestrial and atmospheric scientific communities. The conclusive assessment of significant ongoing rapid changes in the Earth's ice cover, in part supported by ICESat observations, has strengthened the need for sustained, high accuracy, repeat observations similar to what was provided by the ICESat mission. Following recommendations from the National Research Council for an ICESat follow-on mission, the ICESat-2 mission is now under development for planned launch in 2018. The primary scientific aims of the ICESat-2 mission are to continue measurements of sea ice freeboard and ice sheet elevation to determine their changes at scales from outlet glaciers to the entire ice sheet, and from 105 of meters to the entire polar oceans for sea ice freeboard. ICESat carried a single beam profiling laser altimeter that produced similar to 70 m diameter footprints on the surface of the Earth at similar to 150 m along-track intervals. In contrast, ICESat-2 will operate with three pairs of beams, each pair separated by about 3 km cross-track with a pair spacing of 90 m. Each of the beams will have a nominal 17 m diameter footprint with an along -track sampling interval of 0.7 m. The differences in the ICESat-2 measurement concept are a result of overcoming some limitations associated with the approach used in the ICESat mission. The beam pair configuration of ICESat-2 allows for the determination of local cross -track slope, a significant factor in measuring elevation change for the outlet glaciers surrounding the Greenland and Antarctica coasts. The multiple beam pairs also provide improved spatial coverage. The dense spatial sampling eliminates along -track measurement gaps, and the small footprint diameter is especially useful for sea surface height measurements in the often narrow leads needed for sea ice freeboard and ice thickness retrievals. The ICESat-2 instrumentation concept uses a low energy 532 nm (green) laser in conjunction with single-photon sensitive detectors to measure range. Combining ICESat-2 data with altimetry data collected since the start of the ICESat mission in 2003, such as Operation IceBridge and ESA's CryoSat-2, will yield a 15+ year record of changes in ice sheet elevation and sea ice thickness. ICESat-2 will also provide information of mountain glacier and ice cap elevations changes, land and vegetation heights, inland water elevations, sea surface heights, and cloud layering and optical thickness. Published by Elsevier Inc. This is an open access article under the CC BY license

Scott, RC, Lubin D.  2016.  Unique manifestations of mixed-phase cloud microphysics over Ross Island and the Ross Ice Shelf, Antarctica. Geophysical Research Letters. 43:2936-2945.   10.1002/2015gl067246   AbstractWebsite

Spaceborne radar and lidar observations from the CloudSat and CALIPSO satellites are used to compare seasonal variations in the microphysical and radiative properties of clouds over Ross Island, Antarctica, with two contrasting Arctic atmospheric observatories located in Barrow, Alaska, and Summit, Greenland. At Ross Island, downstream from recurrent intrusions of marine air over the West Antarctic Ice Sheet and eastern Ross Ice Shelf, clouds exhibit a tendency toward the greatest geometrical thickness and coldest temperatures in summer, the largest average ice water content, IWC, at low altitude during summer and autumn, the most abundant IWC at cold mixed-phase temperatures (-40 degrees C

Lubin, D, Kahn BH, Lazzara MA, Rowe P, Walden V.  2015.  Variability in AIRS-retrieved cloud amount and thermodynamic phase over west versus east Antarctica influenced by the SAM. Geophysical Research Letters. 42:1259-1267.   10.1002/2014gl062285   AbstractWebsite

In a sample of summertime cloud retrievals from the NASA Atmospheric Infrared Sounder (AIRS), a positive Southern Annular Mode (SAM) index polarity is associated with greater cloud frequency and larger effective cloud fraction over West Antarctica compared with a negative SAM index polarity. The opposite result appears over the high East Antarctic Plateau. Comparing AIRS-retrieved cloud fraction with Antarctic Automatic Weather Station 2 m air temperature data, a positive and significant correlation is found over most of West Antarctica, signifying a longwave heating effect of clouds. Over East Antarctica correlations between Sun elevation and 2 m air temperature are strongest, consistent with lower cloud amount.

Kahn, BH, Irion FW, Dang VT, Manning EM, Nasiri SL, Naud CM, Blaisdell JM, Schreier MM, Yue Q, Bowman KW, Fetzer EJ, Hulley GC, Liou KN, Lubin D, Ou SC, Susskind J, Takano Y, Tian B, Worden JR.  2014.  The Atmospheric Infrared Sounder version 6 cloud products. Atmospheric Chemistry and Physics. 14:399-426.   10.5194/acp-14-399-2014   AbstractWebsite

The version 6 cloud products of the Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Sounding Unit (AMSU) instrument suite are described. The cloud top temperature, pressure, and height and effective cloud fraction are now reported at the AIRS field-of-view (FOV) resolution. Significant improvements in cloud height assignment over version 5 are shown with FOV-scale comparisons to cloud vertical structure observed by the CloudSat 94 GHz radar and the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP). Cloud thermodynamic phase (ice, liquid, and unknown phase), ice cloud effective diameter (D-e), and ice cloud optical thickness (tau) are derived using an optimal estimation methodology for AIRS FOVs, and global distributions for 2007 are presented. The largest values of tau are found in the storm tracks and near convection in the tropics, while D-e is largest on the equatorial side of the midlatitude storm tracks in both hemispheres, and lowest in tropical thin cirrus and the winter polar atmosphere. Over the Maritime Continent the diurnal variability of tau is significantly larger than for the total cloud fraction, ice cloud frequency, and D-e, and is anchored to the island archipelago morphology. Important differences are described between northern and southern hemispheric midlatitude cyclones using storm center composites. The infrared-based cloud retrievals of AIRS provide unique, decadal-scale and global observations of clouds over portions of the diurnal and annual cycles, and capture variability within the mesoscale and synoptic scales at all latitudes.

Bromwich, DH, Nicolas JP, Hines KM, Kay JE, Key EL, Lazzara MA, Lubin D, McFarquhar GM, Gorodetskaya IV, Grosvenor DP, Lachlan-Cope T, van Lipzig NPM.  2012.  Tropospheric clouds in Antarctica. Reviews of Geophysics. 50   10.1029/2011rg000363   AbstractWebsite

Compared to other regions, little is known about clouds in Antarctica. This arises in part from the challenging deployment of instrumentation in this remote and harsh environment and from the limitations of traditional satellite passive remote sensing over the polar regions. Yet clouds have a critical influence on the ice sheet's radiation budget and its surface mass balance. The extremely low temperatures, absolute humidity levels, and aerosol concentrations found in Antarctica create unique conditions for cloud formation that greatly differ from those encountered in other regions, including the Arctic. During the first decade of the 21st century, new results from field studies, the advent of cloud observations from spaceborne active sensors, and improvements in cloud parameterizations in numerical models have contributed to significant advances in our understanding of Antarctic clouds. This review covers four main topics: (1) observational methods and instruments, (2) the seasonal and interannual variability of cloud amounts, (3) the microphysical properties of clouds and aerosols, and (4) cloud representation in global and regional numerical models. Aside from a synthesis of the existing literature, novel insights are also presented. A new climatology of clouds over Antarctica and the Southern Ocean is derived from combined measurements of the CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellites. This climatology is used to assess the forecast cloud amounts in 20th century global climate model simulations. While cloud monitoring over Antarctica from space has proved essential to the recent advances, the review concludes by emphasizing the need for additional in situ measurements.

Mulmenstadt, J, Lubin D, Russell LM, Vogelmann AM.  2012.  Cloud properties over the North Slope of Alaska: Identifying the prevailing meteorological regimes. Journal of Climate. 25:8238-8258.   10.1175/jcli-d-11-00636.1   AbstractWebsite

Long time series of Arctic atmospheric measurements are assembled into meteorological categories that can serve as test cases for climate model evaluation. The meteorological categories are established by applying an objective k-means clustering algorithm to 11 years of standard surface-meteorological observations collected from 1 January 2000 to 31 December 2010 at the North Slope of Alaska (NSA) site of the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM). Four meteorological categories emerge. These meteorological categories constitute the first classification by meteorological regime of a long time series of Arctic meteorological conditions. The synoptic-scale patterns associated with each category, which include well-known synoptic features such as the Aleutian low and Beaufort Sea high, are used to explain the conditions at the NSA site. Cloud properties, which are not used as inputs to the k-means clustering, are found to differ significantly between the regimes and are also well explained by the synoptic-scale influences in each regime. Since the data available at the ARM NSA site include a wealth of cloud observations, this classification is well suited for model observation comparison studies. Each category comprises an ensemble of test cases covering a representative range in variables describing atmospheric structure, moisture content, and cloud properties. This classification is offered as a complement to standard case-study evaluation of climate model parameterizations, in which models are compared against limited realizations of the Earth atmosphere system (e.g., from detailed aircraft measurements).

Lubin, D, Tytler D, Kirkman D.  2012.  Frequency of Maunder minimum events in solar-type stars inferred from activity and metallicity observations.. The Astrophysical Journal Letters. (747 L32)   10.1088/2041-8205/747/2/L32  
Lubin, D, Vogelmann AM.  2011.  The influence of mixed-phase clouds on surface shortwave irradiance during the Arctic spring. Journal of Geophysical Research-Atmospheres. 116   10.1029/2011jd015761   AbstractWebsite

The influence of mixed-phase stratiform clouds on the surface shortwave irradiance is examined using unique spectral shortwave irradiance measurements made during the Indirect and Semi-Direct Aerosol Campaign (ISDAC), supported by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program. An Analytical Spectral Devices (ASD, Inc.) spectroradiometer measured downwelling spectral irradiance from 350 to 2200 nm in one-minute averages throughout April-May 2008 from the ARM Climate Research Facility's North Slope of Alaska (NSA) site at Barrow. This study examines spectral irradiance measurements made under single-layer, overcast cloud decks having geometric thickness <3000 m. Cloud optical depth is retrieved from irradiance in the interval 1022-1033 nm. The contrasting surface radiative influences of mixed-phase clouds and liquid-water clouds are discerned using irradiances in the 1.6-mu m window. Compared with liquid-water clouds, mixed-phase clouds during the Arctic spring cause a greater reduction of shortwave irradiance at the surface. At fixed conservative-scattering optical depth (constant optical depth for wavelengths lambda < 1100 nm), the presence of ice water in cloud reduces the near-IR surface irradiance by an additional several watts-per-meter-squared. This additional reduction, or supplemental ice absorption, is typically similar to 5 W m(-2) near solar noon over Barrow, and decreases with increasing solar zenith angle. However, for some cloud decks this additional absorption can be as large as 8-10 W m(-2).

McFarquhar, GM, Ghan S, Verlinde J, Korolev A, Strapp JW, Schmid B, Tomlinson JM, Wolde M, Brooks SD, Cziczo D, Dubey MK, Fan JW, Flynn C, Gultepe I, Hubbe J, Gilles MK, Laskin A, Lawson P, Leaitch WR, Liu P, Liu XH, Lubin D, Mazzoleni C, Macdonald AM, Moffet RC, Morrison H, Ovchinnikov M, Shupe MD, Turner DD, Xie SC, Zelenyuk A, Bae K, Freer M, Glen A.  2011.  Indirect and semi-direct aerosol campaign: The impact of Arctic aerosols on clouds. Bulletin of the American Meteorological Society. 92:183-+.   10.1175/2010bams2935.1   AbstractWebsite

INDIRECT AND SEMI-DIRECT AEROSOL CAMPAIGN (ISDAC): THE IMPACT OF ARCTIC AEROSOLS ON CLOUDS A comprehensive dataset of microphysical and radiative properties of aerosols and clouds in the boundary layer in the vicinity of Barrow, Alaska, was collected in April 2008 during the Indirect and Semi-Direct Aerosol Campaign (ISDAC). ISDAC's primary aim was to examine the effects of aerosols, including those generated by Asian wildfires, on clouds that contain both liquid and ice. ISDAC utilized the Atmospheric Radiation Measurement Pro-gram's permanent observational facilities at Barrow and specially deployed instruments measuring aerosol, ice fog, precipitation, and radiation. The National Research Council of Canada Convair-580 flew 27 sorties and collected data using an unprecedented 41 state-of-the-art cloud and aerosol instruments for more than 100 h on 12 different days. Aerosol compositions, including fresh and processed sea salt, biomass-burning particles, organics, and sulfates mixed with organics, varied between flights. Observations in a dense arctic haze on 19 April and above, within, and below the single-layer stratocumulus on 8 and 26 April are enabling a process-oriented understanding of how aerosols affect arctic clouds. Inhomogeneities in reflectivity, a close coupling of upward and downward Doppler motion, and a nearly constant ice profile in the single-layer stratocumulus suggests that vertical mixing is responsible for its longevity. observed during ISDAC. Data acquired in cirrus on flights between Barrow and Fairbanks, Alaska, are improving the understanding of the performance of cloud probes in ice. Ultimately, ISDAC data will improve the representation of cloud and aerosol processes in models covering a variety of spatial and temporal scales, and determine the extent to which surface measurements can provide retrievals of aerosols, clouds, precipitation, and radiative heating.

Berque, J, Lubin D, Somerville RCJ.  2011.  Transect method for Antarctic cloud property retrieval using AVHRR data. International Journal of Remote Sensing. 32:2887-2903.   10.1080/01431161003745624   AbstractWebsite

For studies of Antarctic climate change, the Advanced Very High Resolution Radiometer (AVHRR) offers a time series spanning more than two decades, with numerous overpasses per day from converging polar orbits, and with radiometrically calibrated thermal infrared channels. However, over the Antarctic Plateau, standard multispectral application of AVHRR data for cloud optical property retrieval with individual pixels is problematic due to poor scene contrasts and measurement uncertainties. We present a method that takes advantage of rapid changes in radiances at well-defined cloud boundaries. We examine a transect of AVHRR-measured radiances in the three thermal infrared channels across a boundary between cloudy and cloud-free parts of the image. Using scatter diagrams, made from the data along this transect, of the brightness temperature differences between channels 3 and 4, and channels 4 and 5, it is possible to fit families of radiative transfer solutions to the data to estimate cloud effective temperature, thermodynamic phase, and effective particle radius. The major approximation with this method is that along such a transect, cloud water path has considerable spatial variability, while effective radius, phase, and cloud temperature have much less variability. To illustrate this method, two AVHRR images centred about the South Pole are analysed. The two images are chosen based on their differing contrasts in brightness temperature between clear and cloud-filled pixels, to demonstrate that our method can work with varying cloud top heights. In one image the data are consistent with radiative transfer simulations using ice cloud. In the other, the data are inconsistent with ice cloud and are well simulated with supercooled liquid water cloud at 241.5 K. This method therefore has potential for climatological investigation of the radiatively important phase transition in the extremely cold and pristine Antarctic environment.

Lubin, D, Vogelmann AM.  2010.  Observational quantification of a total aerosol indirect effect in the Arctic. Tellus Series B-Chemical and Physical Meteorology. 62:181-189.   10.1111/j.1600-0889.2010.00460.x   AbstractWebsite

We use 6 yr of multisensor radiometric data (1998-2003) from the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program to provide an observational quantification of the short-wave aerosol first indirect effect in the Arctic. Combined with the previously determined long-wave indirect effect, the total (short-wave and long-wave) first indirect effect in the high Arctic is found to yield a transition from surface warming of +3 W m(-2) during March to a cooling of -11 W m(-2) during May, therefore altering the seasonal cycle of energy input to the Arctic Earth atmosphere system. These data also reveal evidence of a first indirect effect that affects optically thinner clouds during summer. which may represent an additional negative climate feedback that responds to a warming Arctic Ocean with retreating sea ice.

Lubin, D, Ayres G, Hart S.  2009.  REMOTE SENSING OF POLAR REGIONS Lessons and Resources for the International Polar Year. Bulletin of the American Meteorological Society. 90:825-+.   10.1175/2008bams2596.1   AbstractWebsite

Polar researchers have historically been innovative and adaptive users of satellite remote sensing data, and their experiences can suggest ways to enhance the use of remote sensing throughout the climate sciences. We performed a semistructured survey of the polar research community on the use of remote sensing at the beginning of the NASA Earth Observing System (EOS) era. For the most part, remote sensing plays a supporting but critical role in the research as described by the respondents. Data acquisition and analysis is mostly at the home institution, with field telemetry appearing in a small minority of responses. Most polar researchers have not had formal training in remote sensing, but they have adapted and trained themselves very thoroughly. Although a significant number of polar researchers are content with visual inspection of satellite images, a roughly equal number develop their own algorithms for derivation of geophysical products, and more have become adept at using high-level graphical programming languages to work with data. Given the self-sufficiency in remote sensing training that characterizes polar researchers, nontraditional satellite data users (e.g., life scientists) tend to view the "learning curve" as steep, as compared with physical scientists. Although up to a third of respondents report no significant obstacles in accessing satellite data, obstacles such as a) difficulty locating data centers for their needs, b) the cost of acquiring data, and c) insider or restricted access to data were each reported by about one-quarter of the respondents. The major ongoing challenges with remote sensing in polar research can be met with aspects of modern cyberinfrastructure involving data interoperability.

Lubin, D, Wittenmyer RA, Bromwich DH, Marshall GJ.  2008.  Antarctic Peninsula mesoscale cyclone variability and climatic impacts influenced by the SAM. Geophysical Research Letters. 35   10.1029/2007gl032170   AbstractWebsite

The frequency of mesoscale cyclones in the Western Antarctic Peninsula (WAP) region during 1991-94 is correlated with the Southern Hemisphere Annular Mode (SAM) index, most strongly during winter and spring. Also, during periods of positive SAM index polarity there is a shift in the storm tracks to favor more east-bound trajectories, consistent with strengthening of circumpolar westerlies. The presence of mesoscale cyclones is associated with positive near-surface-air temperature anomalies in the WAP region year-round, largest during winter.

Lubin, D, Vogelmann AM.  2007.  Expected magnitude of the aerosol shortwave indirect effect in springtime Arctic liquid water clouds. Geophysical Research Letters. 34   10.1029/2006gl028750   AbstractWebsite

Radiative transfer simulations are used to assess the expected magnitude of the diurnally-averaged shortwave aerosol first indirect effect in Arctic liquid water clouds, in the context of recently discovered longwave surface heating of order 3 to 8 W m(-2) by this same aerosol effect detected at the Barrow, Alaska, ARM Site. We find that during March and April, shortwave surface cooling by the first indirect effect is comparable in magnitude to the longwave surface heating. During May and June, the shortwave surface cooling exceeds the longwave heating. Due to multiple reflection of photons between the snow or sea ice surface and cloud base, the shortwave first indirect effect may be easier to detect in surface radiation measurements than from space.

Lubin, D, Massom R.  2007.  Remote sensing of Earth's polar regions - Opportunities for computational science. Computing in Science & Engineering. 9:58-71.   10.1109/mcse.2007.16   AbstractWebsite

Polar remote sensing offers numerous opportunities for computer scientists, including spacecraft design and data processing, the development of algorithms for geophysical product retrieval, operational assistance for aircraft and ship navigation, and database management at national archives.

Bais, AF, Lubin D, Arola A, Bernhard G, Blumthaler M, Chubarova N, Erlick C, Gies HP, Krotkov NA, Lantz K, Mayer B, Mckenzie RL, d. Piacentini R, Seckmeyer G, Slusser JR, Zerefos CZ.  2007.  Surface Ultraviolet Radiation: Past, Present, and Future. Scientific assessment of ozone depletion: 2006. ( Organization W, Ed.)., Geneva, Switzerland: World Meteorological Organization Abstract
McComiskey, A, Ricchiazzi P, Gautier C, Lubin D.  2006.  Assessment of a three dimensional model for atmospheric radiative transfer over heterogeneous land cover. Geophysical Research Letters. 33   10.1029/2005gl025356   AbstractWebsite

A three-dimensional (3D) atmospheric radiative transfer model that explicitly represents surface albedo heterogeneity is tested against a one-dimensional model and surface irradiance observations in a polar region where land cover heterogeneity is high. For observations located near high latitude coastlines, the contrast between the highly absorbing ocean and reflective snow surface creates spatial heterogeneity, or a 3D effect, around the observation site. The resulting effect on radiation at the sensor should be taken into account when using a solar radiative transfer model to interpret measurements. This assessment shows that better closure is obtained with a three-dimensional model (<= 5%) versus a plane-parallel model (<= 7%). The importance of the surface 3D effect increases with aerosol or cloud optical depth and with surface albedo contrast. The model used here can be implemented at any surface site given the surrounding land cover properties.

Lubin, D, Vogelmann AM.  2006.  A climatologically significant aerosol longwave indirect effect in the Arctic. Nature. 439:453-456.   10.1038/nature04449   AbstractWebsite

The warming of Arctic climate and decreases in sea ice thickness and extent(1,2) observed over recent decades are believed to result from increased direct greenhouse gas forcing, changes in atmospheric dynamics having anthropogenic origin(3-5), and important positive reinforcements including ice - albedo and cloud - radiation feedbacks(6). The importance of cloud - radiation interactions is being investigated through advanced instrumentation deployed in the high Arctic since 1997 (refs 7, 8). These studies have established that clouds, via the dominance of longwave radiation, exert a net warming on the Arctic climate system throughout most of the year, except briefly during the summer(9). The Arctic region also experiences significant periodic influxes of anthropogenic aerosols, which originate from the industrial regions in lower latitudes(10). Here we use multisensor radiometric data(7,8) to show that enhanced aerosol concentrations alter the microphysical properties of Arctic clouds, in a process known as the 'first indirect' effect(11,12). Under frequently occurring cloud types we find that this leads to an increase of an average 3.4 watts per square metre in the surface longwave fluxes. This is comparable to a warming effect from established greenhouse gases and implies that the observed longwave enhancement is climatologically significant.

Lubin, D, Massom R, SpringerLink.  2006.  Polar Remote Sensing Volume I: Atmosphere and Oceans. , Berlin, Heidelberg: Praxis Pub., Chichester, UK Abstract
Massom, R, Lubin D, SpringerLink.  2006.  Polar Remote Sensing Volume II: Ice Sheets. , Berlin, Heidelberg: Praxis Pub., Chichester, UK Abstract
Kirkman, D, Tytler D, Suzuki N, Melis C, Hollywood S, James K, So G, Lubin D, Jena T, Norman ML, Paschos P.  2005.  The HI opacity of the intergalactic medium at redshifts 1.6 < z < 3.2. Monthly Notices of the Royal Astronomical Society. 360:1373-1380.   10.1111/j.1365-2966.2005.09126.x   AbstractWebsite

We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Ly alpha forest absorption (DA) over the redshift range 2.2 < z < 3.2. Our measurement of DA excludes absorption from metal lines or the Ly alpha lines of Lyman-limit systems and damped Ly alpha systems. We use artificial spectra with realistic flux calibration errors to show that we are able to place continuum levels that are accurate to better than 1 per cent. When we combine our results with our previous results between 1.6 < z < 2.2, we find that the redshift evolution of DA is well described over f1.6 < z < 3.2 as A (1 +z)(gamma), where A = 0.0062 and gamma = 2.75. We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less H i absorption than expected at z = 3, implying that the ultraviolet background is similar to 40 per cent higher than expected. Our data appears to be consistent with an H i ionization rate of Gamma similar to 1.4 x 10(-12) s(-1).

Suzuki, N, Tytler D, Kirkman D, O'Meara JM, Lubin D.  2005.  Predicting QSO continua in the Ly alpha forest. Astrophysical Journal. 618:592-600.   10.1086/426062   AbstractWebsite

We present a method to make predictions with sets of correlated data values, in this case QSO flux spectra. We predict the continuum in the Lyalpha forest of a QSO, from 1020 to 1216 8, using the spectrum of that QSO from 1216 to 1600 Angstrom. We find correlations between the unabsorbed flux in these two wavelength regions in the Hubble Space Telescope (HST) spectra of 50 QSOs. We use principal component analysis to summarize the variety of these spectra, relate the weights of the principal components for 1020-1600 Angstrom to the weights for 1216-1600 Angstrom, and apply this relation to make predictions. We test the method on the HST spectra and find an average absolute flux error of 9%, with a range of 3%-30%, where individual predictions are systematically too low or too high. We mention several ways in which the predictions might be improved.