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2001
O'Meara, JM, Tytler D, Kirkman D, Suzuki N, Prochaska JX, Lubin D, Wolfe AM.  2001.  The deuterium to hydrogen abundance ratio toward a fourth QSO: HS 0105+1619. Astrophysical Journal. 552:718-730.   10.1086/320579   AbstractWebsite

We report the measurement of the primordial D/H abundance ratio toward QSO HS 0105+1619. The column density of the neutral hydrogen in the z similar or equal to 2.536 Lyman limit system is high, log N-HI = 19.422 +/- 0.009 cm(-2), allowing for the deuterium to be seen in five Lyman series transitions. The measured value of the D/H ratio toward QSO HS 0105 + 1619 is found to be D/H = 2.54 +/- 0.23 x 10(-5). The metallicity of the system showing D/H is found to be similar or equal to 0.01 solar, indicating that the measured D/H is the primordial D/H within the measurement errors. The gas that shows D/H is neutral, unlike previous D/H systems that were more highly ionized. Thus, the determination of the D/H ratio becomes more secure since we are measuring it in different astrophysical environments, but the error is larger because we now see more dispersion between measurements. Combined with prior measurements of D/H, the best D/H ratio is now D/H = 3.0 +/- 0.4 x 10(-5), which is 10% lower than the previous value. The new values for the baryon-to-photon ratio and baryonic matter density derived from D/H are eta = 5.6 +/- 0.5 x 10(-10) and Omega (b), h(2) = 0.0205 +/- 0.0018, respectively.

Morrow, E, Scheeres DJ, Lubin D.  2001.  Solar sail orbit operations at asteroids. Journal of Spacecraft and Rockets. 38:279-286.   10.2514/2.3682   AbstractWebsite

The inherent capabilities of solar sails and that they need no onboard supplies of fuel for propulsion make them well suited for use in long-term, multiple-objective missions. They are especially well suited for the exploration of asteroids, where one spacecraft could rendezvous with a number of asteroids in succession. The orbital mechanics of solar sail operations about an asteroid, however, have not yet been studied in detail. Building an previous studies, we find both hovering points and orbiting trajectories about various sized asteroids using equations of motion for a solar sail spacecraft. The orbiting trajectories are stable and offer good coverage of the asteroid surface, although restrictions on sail acceleration are needed for smaller asteroids.

Lubin, D, Li W, Dustan P, Mazel CH, Stamnes K.  2001.  Spectral signatures of coral reefs: Features from space. Remote Sensing of Environment. 75:127-137.   10.1016/s0034-4257(00)00161-9   AbstractWebsite

The special signatures of coral reefs and related scenes, as they would be measured above the Earth's atmosphere, are calculated using a coupled atmosphere-ocean discrete ordinates radiative transfer model. Actual measured reflectance spectra from field work are used as input data. Four coral species are considered, to survey the natural range of coral reflectance: Montastrea cavernosa, Acropora palmata, Dichocoenia stokesii, and Siderastrea siderea. Four noncoral objects associated with reefs are also considered: sand, coralline algae, green macroalgae, and algal turf. The reflectance spectra as would be measured at the top of the atmosphere are substantially different from the in situ spectra, due to differential attenuation by the water column and, most importantly, by atmospheric Rayleigh scattering. The result is that many of the spectral features that can be used to distinguish coral species from their surroundings or from one another, which have been used successfully with surface or aircraft data, would be obscured in spectral measurements from a spacecraft. However, above the atmosphere, the radiance contrasts between most coral species and most brighter noncoral objects remain noticeable for water column depths up to 20 m. Over many spectral intervals, the reflectance from dark coral under shallow water is smaller than that of deep water. The maximum top-of-atmosphere radiances, and maximum contrasts between scene types, occur between 400 nm and 600 nm. This study supports the conclusions of recent satellite reef mapping exercises, suggesting that coral reef identification should be feasible using satellite remote sensing, but that detailed reef mapping (e.g., species identification) may be more difficult. (C) Elsevier Science Inc., 2001.

Lubin, D, Morrow E.  2001.  Ultraviolet radiation environment of Antarctica 1. Effect of sea ice on top-of-atmosphere albedo and on satellite retrievals. Journal of Geophysical Research-Atmospheres. 106:33453-33461.   10.1029/2001jd000687   AbstractWebsite

The backscattered ultraviolet radiance measured by the Total Ozone Mapping Spectrometer (TOMS) over the Southern Ocean is influenced by both cloud cover and sea ice concentration. In TOMS data alone, these influences cannot be separated. To assess the relative importance of cloud opacity and sea ice concentration, TOMS level 2 data are colocated with AVHRR and SSM/I data. AVHRR provides independent cloud identification at a spatial resolution sufficient to estimate cloud fraction within a TOMS level 2 footprint, while the SSM/I provides useful estimates of sea ice concentration over clear and cloudy scenes. The sea ice cover is shown to have a stronger influence than cloud cover on the backscattered ultraviolet radiance at the top of the atmosphere. Over overcast scenes the mean TOMS reflectivity increases from 45 to 84% as the underlying sea ice concentration increases from 0 to 1. Over scenes containing sea ice concentrations greater than 0.5, the increase in TOMS-measured radiance with increasing cloud amount (0-1) is generally less than 30% and is negligible over high sea ice concentrations. Over clear-sky scenes the characteristic UV-A albedos of the sea ice components of the scenes are retrieved from the TOMS data. These albedos range from 0.19 +/- 0.14 for sea ice concentration 0.1, increasing rapidly to 0.53 +/- 0.15 for sea ice concentration 0.3, and then approximately linearly to 0.80 +/- 0.11 for sea ice concentration 1.0. There is the potential to develop a climatology of surface ultraviolet and photosynthetically active radiation for southern high latitudes, which utilizes a combination of TOMS and SSM/I data. Such a climatology could cover the entire Southern Ocean throughout the duration of the modern springtime ozone depletion phenomenon. Analysis of uncertainties related to sea ice concentration retrieval from SSM/I, and related uncertainties in surface albedo identification and their influence on the estimated surface radiative flux, shows that such a climatology would have the most quantitative value for sea ice concentrations less than 0.5.

2000
Tytler, D, O'Meara JM, Suzuki N, Lubin D.  2000.  Big bang nucleosynthesis. Nuclear Physics B-Proceedings Supplements. 87:464-473.   10.1016/s0920-5632(00)00721-0   AbstractWebsite

Big Bang Nucleosynthesis (BBN) is the synthesis of the light nuclei, Deuterium (D or H-2), He-3, He-4 and Li-7 during the first few minutes of the universe. This review concentrates on recent improvements in the measurement of the primordial (after BBN, and prior to modification) abundances of these nuclei.

Collins, WD, Bucholtz A, Flatau P, Lubin D, Valero FPJ, Weaver CP, Pilewski P.  2000.  Determination of surface heating by convective cloud systems in the central equatorial Pacific from surface and satellite measurements. Journal of Geophysical Research-Atmospheres. 105:14807-14821.   10.1029/2000jd900109   AbstractWebsite

The heating of the ocean surface by longwave radiation from convective clouds has been estimated using measurements from the Central Equatorial Pacific Experiment (CEPEX). The ratio of the surface longwave cloud forcing to the cloud radiative forcing on the total atmospheric column is parameterized by the f factor. The f factor is a measure of the partitioning of the cloud radiative effect between the surface and the troposphere. Estimates of the f factor have been obtained by combining simultaneous observations from ship, aircraft, and satellite instruments. The cloud forcing near the ocean surface is determined from radiometers on board the National Oceanic and Atmospheric Administration P-3 aircraft and the R/V John Vickers. The longwave cloud forcing at the top of the atmosphere has been estimated from data obtained from the Japanese Geostationary Meteorological Satellite GMS 4. A new method for estimating longwave fluxes from satellite narrowband radiances is described. The method is based upon calibrating the satellite radiances against narrowband and broadband infrared measurements from the high-altitude NASA ER-2 aircraft. The average value of f derived from the surface and satellite observations of convective clouds is 0.15 +/- 0.02. The area-mean top-of-atmosphere longwave forcing by convective clouds in the region 10 degrees S-10 degrees N, 160 degrees E-160 degrees W is 40 W/m(2) during CEPEX. Those results indicate that the surface longwave forcing by convective clouds was approximately 5 W/m(2) in the central equatorial Pacific and that this forcing is the smallest radiative component of the surface energy budget.

Kirkman, D, Tytler D, Burles S, Lubin D, O'Meara JM.  2000.  QSO 0130-4021: A third QSO showing a low deuterium-to-hydrogen abundance ratio. Astrophysical Journal. 529:655-660.   10.1086/308317   AbstractWebsite

We have discovered a third quasar absorption system which is consistent with a low deuterium-to-hydrogen abundance ratio, D/H = 3.4 x 10(-5). The z(abs) similar to 2.8 partial Lyman limit system toward Q0130-4021 provides the strongest evidence to date against large D/H ratios because the H I absorption, which consists of a single high column density component with unsaturated high-order Lyman series lines, is readily modeled-a task which is more complex in other D/H systems. We have obtained 22 hr of spectra from the High-Resolution Echelle Spectrograph on the W. RI. Keek Telescope, which allow a detailed description of the hydrogen. We see excess absorption on the blue wing of the H I Ly alpha line, near the expected position of deuterium. However, we find that deuterium cannot explain all of the excess absorption, and hence there must be contamination by additional absorption, probably H I. This extra H I can account for most or all of the absorption at the D position, and hence D/H = 0 is allowed. We find an upper limit of D/H less than or equal to 6.7 x 10(-5) in this system, consistent with the value of D/H similar or equal to 3.4 x 10(-5) deduced toward QSO 1009 + 2956 and QSO 1937 - 1009 by Buries and Tytler. This absorption system shows only weak metal-line absorption, and we estimate [Si/H] less than or equal to -2.6, indicating that the D/H ratio of the system is likely primordial. All four of the known high-redshift absorption-line systems simple enough to provide useful limits on D are consistent with D/H = 3.4 +/- 0.25 x 10(-5). Conversely, this QSO provides the third case which is inconsistent with much larger values.

Tytler, D, O'Meara JM, Suzuki N, Lubin D.  2000.  Deuterium and the baryonic density of the universe. Physics Reports-Review Section of Physics Letters. 333:409-432.   10.1016/s0370-1573(00)00032-6   AbstractWebsite

Big bang nucleosynthesis (BBN) is the creation of the light nuclei, deuterium, He-3, He-4 and Li-7 during the first few minutes of the universe. Here we discuss recent measurements of the D to H abundance ratio, D/H, in our galaxy and towards quasars. We have achieved an order of magnitude improvement in the precision of the measurement of primordial D/H, using the HIRES spectrograph on the W. M. Keck telescope to measure D in gas with very nearly primordial abundances towards quasars. From 1994 to 1996, it appeared that there could be a factor of 10 range in primordial D/H, but today four examples of low D are secure. High D/H should be much easier to detect, and since there are no convincing examples, it must be extremely rare or non-existent. All data are consistent with a single low value for D/H, and the examples which are consistent with high D/H are readily interpreted as H contamination near the position of D. The new D/H measurements give the most accurate value for the baryon-to-photon ratio, eta, and hence the cosmological baryon density. A similar density is required to explain the amount of Ly alpha absorption from neutral hydrogen in the intergalactic medium (IGM) at redshift z similar or equal to 3, and to explain the fraction of baryons in local clusters of galaxies. The D/H measurements lead to predictions for the abundances of the other light nuclei, which generally agree with measurements. The remaining differences with some measurements can be explained by a combination of measurement and analysis errors or changes in the abundances after BBN. The measurements do not require physics beyond the standard BBN model. Instead, the agreement between the abundances is used to limit the non-standard physics. (C) 2000 Elsevier Science B.V. All rights reserved.

Tytler, D, O'Meara JM, Suzuki N, Lubin D, Burles S, Kirkman D.  2000.  Measurements of the primordial D/H abundance towards quasars. Light Elements and Their Evolution. ( DaSilva L, Spite M, DeMedeiros JR, Eds.).:125-134., San Francisco: Astronomical Soc Pacific Abstract

Big Bang Nucleosynthesis (BBN) is the synthesis of the light nuclei, Deuterium (D or H-2), He-3, He-4 and Li-7 during the first few minutes of the universe. In this review we concentrate on recent data which give the primordial deuterium (D) abundance. We have measured the primordial D/H in gas with very nearly primordial abundances. We use the Lyman series absorption lines seen in the spectra of quasars. We have measured D/H towards three QSOs, while a fourth gives a consistent upper limit. All QSO spectra are consistent with a single value for D/H: 3.325(-0.25)(+0.22) x 10(-5). From about 1994-1996, there was much discussion of the possibility that some QSOs show much higher D/H, but the best such example was shown to be contaminated by H, and no other no convincing examples have been found. Since high D/H should be much easier to detect, and hence it must be extremely rare or non-existent. The new D/H measurements give the most accurate value for the baryon to photon ratio, eta, and hence the cosmological baryon density: Omega(b) = 0.0190 +/- 0.0009 (1sigma) A similar density is required to explain the amount of Lyalpha absorption from neutral Hydrogen in the intergalactic medium (IGM) at redshift z similar or equal to 3, and to explain the fraction of baryons in local clusters of galaxies. The D/H measurements lead to predictions for the abundances of the other light nuclei, which generally agree with measurements. The remaining differences with some measurements can be explained by a combination of measurement and analysis errors or changes in the abundances after BBN. The measurements do not require physics beyond the standard BBN model. Instead, the agreement between the abundances is used to limit the non-standard physics.

Tytler, D, O'Meara JM, Suzuki N, Lubin D.  2000.  Review of big bang nucleosynthesis and primordial abundances. Physica Scripta. T85:12-31.   10.1238/Physica.Topical.085a00012   AbstractWebsite

Big Bang Nucleosynthesis (BBN) is the synthesis of the light nuclei, Deuterium (D or H-2), He-3, He-4 and Li-7 during the first few minutes of the universe. This review concentrates on recent improvements in the measurement of the primordial (after BBN, and prior to modification) abundances of these nuclei. We mention improvement in the standard theory, and the non-standard extensions which are limited by the data. We have achieved an order of magnitude improvement in the precision of the measurement of primordial D/H, using the HIRES spectrograph on the W M. Keck telescope to measure D in gas with very nearly primordial abundances towards quasars. From 1994 - 1996, it appeared that there could be a factor of ten range in primordial D/H, but today four examples of low D are secure. High D/H should be much easier to detect, and since there are no convincing examples, it must be extremely rare or non-existent. All data are consistent with a single low value for D/H, and the examples which an consistent with high D/H are readily interpreted as H contamination near the position of D. The new D/H measurements give the most accurate value for the baryon to photon ratio II, and hence the cosmological baryon density. A similar density is required to explain the amount of Ly alpha absorption from neutral Hydrogen in the intergalactic medium (IGM) at redshift z similar or equal to 3, and to explain the fraction of baryons in local clusters of galaxies. The D/H measurements lead to predictions for the abundances of the other light nuclei, which generally agree with measurements. The remaining differences with some measurements can be explained by a combination of measurement and analysis errors or changes in the abundances after BBN. The measurements do not require physics beyond the standard BBN model. instead, the agreement between the abundances is used to limit the non-standard physics. New measurements are giving improved understanding of the difficulties in estimating the abundances of all the light nuclei, but unfortunately in most cases we are not yet seeing much improvement in the accuracy of the primordial abundances. Since we are now interested in the highest accuracy and reliability for all nuclei, the few objects with the most extensive observations give by far the most convincing results. Earlier measurements of He-4 may have obtained too low a value because the He emission line strengths were reduced by undetected stellar absorption lines. The systematic errors associated with the He-4 abundance have frequently been underestimated in the past, and this problem persists. When two groups use the same data and different ways to estimate the electron density and He-4 abundance, the results differ by more than the quoted systematic errors. While the methods used by Izotov and Thuan [1] seem to be an advance on those used before, the other method is reasonable, and hence the systematic error should encompass the range in results. The abundance of Li-7 is measured to high accuracy, but we do not know how much was produced prior to the formation of the stars, and how much was destroyed (depleted) in the stars. Li-6 helps limit the amount of depletion of Li-7, but by an uncertain amount since it too has been depleted. BBN is successful because it uses known physics and measured cross-sections for the nuclear reactions. It gives accurate predictions for the abundances of five light nuclei as a function of the one free parameter eta. The other initial conditions seem natural: the universe began homogeneous and hotter than T > 10(11) K (30 Mev). The predicted abundances agree with most observations, and the required eta is consistent with other, less accurate, measurements of the baryon density. Abundance measurements of the baryon density, from the CMB, clusters of galaxies and the Ly alpha forest, will give II. Although the accuracy might not exceed that obtained from D/H, this is an important advance because BBN then gives abundance predictions with no adjustable parameters. New measurement in the coming years will give improved accuracy. Measurement of D/H in many more quasar spectra would improve the accuracy of D/H by a factor of a few, to a few percent, but even with improved methods of selecting the target quasars, this would need much more time on the largest telescopes. More reliable He-4 abundances might be obtained from spectra which have higher spectral and spatial resolution, to help correct for stellar absorption, higher signal to noise to show weaker emission lines, and more galaxies with low metal abundances, to minimize the extrapolation to primordial abundances. Measurements of Li-6, Be and Boron in the same stars and observations of a variety of stars should give improved models for the depletion of Li-7 in halo stars, and hence tighter constraints on the primordial abundance. However, in general, it is hard to think of any new methods which could give any primordial abundances with an order of magnitude higher accuracy than those used today. This is a major unexploited opportunity because it means that we can not yet test BBN to the accuracy of the predictions.

Lubin, D, Vogelmann A, Lehr PJ, Kressin A, Ehramjian J, Ramanathan V.  2000.  Validation of visible/near-IR atmospheric absorption and solar emission spectroscopic models at 1 cm-1 resolution. Journal of Geophysical Research. 105:22445–22454.   10.1029/2000JD900317   Abstract

A Fourier transform infrared (FTIR) spectrometer, operating at 1 cm-1 resolution between 9000 and 24,669 cm-1 (0.405-1.111 μm) has been used to check the spectral composition of databases that form the basis for most atmospheric absorption parameterizations used in climate models, remote sensing, and other radiative transfer simulations. The spectrometer, operating near sea level under clear skies, obtained relative atmospheric transmission measurements of the direct solar beam by means of a heliostat. The spectroscopic data were compared with a line-by-line radiative transfer model (LBLRTM) calculation of direct solar beam flux, which used a input data a monochromatic model extraterrestrial solar flux spectrum currently in common use. This intercomparison revealed that the extraterrestrial solar flux spectrum contains 266 solar absorption features that do not appear in the data, resulting in an excess of approximately 1.92 W m-2 in the model's solar constant. The intercomparison also revealed 97 absorption features in the data that do not appear in the HITRAN-96 database as used by LBLRTM, resulting in a model underestimate of shortwave absorption of ˜0.23 W m-2 for a solar zenith angle of 42°. These small discrepancies revealed by the intercomparison indicate that current extraterrestrial solar irradiance models and spectroscopic databases used by shortwave atmospheric radiative transfer models are nearly entirely complete for purposes of atmospheric energy budget calculation. Thus clear or cloudy sky `excess absorption' is unlikely to be related to an incomplete identification of atmospheric absorbing gases and their spectroscopic features, at 1 cm-1 resolution, for a clean troposphere of normal composition.

1999
Han, W, Stamnes K, Lubin D.  1999.  Remote sensing of surface and cloud properties in the Arctic from AVHRR measurements. Journal of Applied Meteorology. 38:989-1012.   10.1175/1520-0450(1999)038<0989:rsosac>2.0.co;2   AbstractWebsite

Algorithms to retrieve cloud optical depth and effective radius in the Arctic using Advanced Very High Resolution Radiometer (AVHRR) data are developed, using a comprehensive radiative transfer model in which the atmosphere is coupled to the snowpack. For dark surfaces AVHRR channel 1 is used to derive visible cloud optical depth, while for bright surfaces AVHRR channel 2 is used. Independent inference of cloud effective radius from AVHRR channel 3 (3.75 mu m) allows for derivation cloud liquid water path (proportional to the product of optical depth and effective radius). which is a fundamental parameter of the climate system. The algorithms are based on the recognition that the reflection function of clouds at a nonabsorbing wavelength (such as AVHRR channel 1) in the solar spectrum is primarily a function of cloud optical thickness, whereas the reflection function at a liquid water absorbing wavelength (such as AVHRR channel 3) is primarily a function of cloud particle size. For water clouds over highly reflecting surfaces (snow and ice), the reflectance in AVHRR channel 1 is insensitive to cloud optical depth due to the multiple reflections between cloud base and the underlying surface; channel 2 (0.85 mu m) must be used instead for optical depth retrieval. Water clouds over tundra or ocean are more straightforward cases similar to those found at lower latitudes, and in these cases a comprehensive atmospheric radiative transfer model with a Lambertian surface under cloud is used. Thus, for water cloud over tundra and ocean, channel 1 is used for cloud optical depth retrieval. In all cases, channel 3 is used for independent retrieval of cloud droplet effective radius. The thermal component of channel 3 is estimated by making use of channel 4 (11 mu m) and is subtracted from the total channel 3 radiance. Over clear-sky scenes, the bidirectional reflectance properties of snow are calculated directly by the coupled snowpack-atmosphere model. This results in greater overall accuracy in retrieved surface properties as compared with the simplified approach that uses a Lambertian approximation for the surface albedo. To test the physical soundness of the algorithms the authors have applied them to AVHRR data over Barrow, Alaska, from April to August 1992. Downwelling irradiances at the surface calculated using the retrieved cloud optical depth and effective radius are compared with field irradiance measurements, and encouraging agreement is found. The algorithms are also applied to three areas of about 100-km dimension around Barrow, each having a different underlying surface (ocean, tundra, snow).

1998
Lubin, D, Jensen EH, Gies HP.  1998.  Global surface ultraviolet radiation climatology from TOMS and ERBE data. Journal of Geophysical Research-Atmospheres. 103:26061-26091.   10.1029/98jd02308   AbstractWebsite

A global climatology of biologically active solar ultraviolet radiation (UVR) at the Earth's surface is derived using NASA total ozone mapping spectrometer (TOMS) measurements of column ozone abundance and NASA Earth Radiation Budget Experiment (ERBE) measurements of solar reflectance from the Earth-atmosphere system. These two sources of satellite data are used as input to a delta-Eddington radiative transfer model to estimate climatological cloud opacity and thereby demonstrate how surface UVR varies with geography and season. The surface UVR fluxes are spectrally resolved to enable weighted integration with any biological action spectrum. Solar elevation is shown to be more important than total column ozone abundance in governing the variability of surface UVR over large geographic areas, although some regions with pronounced local minima in ozone (30 Dobson units or more) will cause noticeable enhancements of integrated UV-B (280-315 nm) flux relative to UV-A (315-400 nm). The greatest variability in surface UVR within a given climate zone is induced by cloud cover. During summer, regions that show lower surface UVR fluxes relative to their surrounding regions include the eastern United States (versus the western United States), India, China (in the vicinity of the Yangtze River), and Japan (relative to the surrounding oceans). Cloud cover over tropical rainforest areas reduces the surface UVR flux relative to ocean areas at the same latitudes. The UVR cloud transmission derived from the TOMS and ERBE data correlates with an independent climatology of global cloud coverage. The UVR mapping method, based on the TOMS and ERBE data, allows a direct investigation of diurnal variability and a rigorous calculation of the biologically relevant integrated daily dose of UVR. However, it is shown that a UVR mapping method based on TOMS data alone, which is limited to only local noon satellite measurements, can make defensible estimates of the integrated daily UVR dose and the instantaneous local noon UVR surface flux.

Lubin, D, Chen B, Bromwich DH, Somerville RCJ, Lee WH, Hines KM.  1998.  The impact of Antarctic cloud radiative properties on a GCM climate simulation. Journal of Climate. 11:447-462.   10.1175/1520-0442(1998)011<0447:tioacr>2.0.co;2   AbstractWebsite

A sensitivity study to evaluate the impact upon regional and hemispheric climate caused by changing the optical properties of clouds over the Antarctic continent is conducted with the NCAR Community Model version 2 (CCM2). Sensitivity runs are performed in which radiation interacts with ice clouds with particle sizes of 10 and 40 mu m rather than with the standard 10-mu m water clouds. The experiments are carried out for perpetual January conditions with the diurnal cycle considered. The effects of these cloud changes on the Antarctic radiation budget are examined by considering cloud forcing at the top of the atmosphere and net radiation at the surface. Changes of the cloud radiative properties to those of 10-mu m ice clouds over Antarctica have significant Impacts on regional climate: temperature increases throughout the Antarctic troposphere by 1 degrees-2 degrees C and total cloud fraction over Antarctica is smaller than that of the control at low levels but is larger than that of the control in the mid- to upper troposphere. As a result of Antarctic warming and changes in the north-south temperature gradient, the drainage flows at the surface as well as the meridional mass circulation are weakened. Similarly, the circumpolar trough weakens significantly by 4-8 hPa and moves northward by about 4 degrees-5 degrees latitude. This regional mass field adjustment halves the strength of the simulated surface westerly winds. As a result of indirect thermodynamic and dynamic effects, significant changes are observed in the zonal mean circulation and eddies in the middle latitudes. In fact, the simulated impacts of the Antarctic cloud radiative alteration are not confined to the Southern Hemisphere. The meridional mean mass flux, zonal wind, and latent heat release exhibit statistically significant changes in the Tropics and even extratropics of the Northern Hemisphere. The simulation with radiative properties of 40-mu m ice clouds produces colder surface temperatures over Antarctica by up to 3 degrees C compared to the control. Otherwise, the results of the 40-mu m ice cloud simulation are similar to those of the 10-mu m ice cloud simulation.

Jayaraman, A, Lubin D, Ramachandran S, Ramanathan V, Woodbridge E, Collins WD, Zalpuri KS.  1998.  Direct observations of aerosol radiative forcing over the tropical Indian Ocean during the January-February 1996 pre-INDOEX cruise. Journal of Geophysical Research-Atmospheres. 103:13827-13836.   10.1029/98jd00559   AbstractWebsite

Simultaneous measurements of aerosol optical depth, size distribution, and incoming solar radiation flux were conducted with spectral and broadband radiometers over the coastal Indian region, Arabian Sea, and Indian Ocean in January-February 1996. Columnar aerosol optical depth, delta a, at visible wavelengths was found to be 0.2-0.5 over the Arabian Sea and <0.1 over the equatorial Indian Ocean. Aerosol mass concentration decreased from about 80 mu g/m(3) near the coast to just a few mu g/m(3) over the interior ocean. The sub-micron-size particles showed more than an order of magnitude increase in number concentration near the coast versus the interior ocean. This large gradient in particle concentration was consistent with a corresponding large increase in the Sun-photometer-derived Angstrom exponent, which increased from 0.2 over the Indian Ocean to about 1.4 near the coast. The change in surface-reaching solar flux with delta a was obtained for both the direct and the global solar flux in the visible spectral region. The solar-zenith-angle-normalized global and diffuse fluxes vary almost linearly with normalized delta a. The direct visible (<780 nm) solar flux decreases by about 42 +/- 4 Wm(-2) and the diffuse sky radiation increases by about 30 +/- 3 Wm(-2) with every 0.1 increase in delta a, for solar zenith angles smaller than 60 degrees. For the same extinction optical depth the radiative forcing of the coastal aerosols is larger than the open ocean aerosol forcing by a factor of 2 or larger.

Podgorny, I, Lubin D.  1998.  Biologically active insolation over Antarctic waters: Effect of a highly reflecting coastline. Journal of Geophysical Research-Oceans. 103:2919-2928.   10.1029/97jc02763   AbstractWebsite

Near an Antarctic coastline or sea ice edge, multiple reflection of photons between the high-albedo surface and a cloud will increase the downwelling surface insolation not only over the high-albedo surface itself but also out over the adjacent open water. This insolation enhancement is examined with a Monte Carlo radiative transfer model. The insolation enhancement extends to a typical distance of 4 km out to sea, with the most important effects being within 2 km of the coastline. The strength of the multiple reflection effect depends primarily on cloud base height and cloud optical depth and only slightly on cloud geometrical thickness. The insolation enhancement is also a function of wavelength, being larger for ultraviolet wavelengths than for the visible. This is due to a slightly greater contribution from Rayleigh scattering at the shorter wavelengths, although at ultraviolet wavelengths where ozone absorption is strong, tropospheric ozone absorption can offset the Rayleigh scattering contribution at larger cloud optical depths. On the basis of the limited range of the multiple reflection effect (2-4 km out to sea) the insolation enhancement due to the high-albedo coastline is unlikely to be a major influence on the primary productivity of all Antarctic waters; however, it may influence phytoplankton blooms near the coast and photobiological experiments carried out at coastal research stations. Also, the insolation enhancement may have significance in sea ice leads and polynyas.

Lubin, D, Morrow E.  1998.  Evaluation of an AVHRR cloud detection and classification method over the Central Arctic Ocean. Journal of Applied Meteorology. 37:166-183.   10.1175/1520-0450(1998)037<0166:eoaacd>2.0.co;2   AbstractWebsite

A cloud classification method that uses both multispectral and textural features with a maximum likelihood discriminator is applied to full-resolution AVHRR (Advanced Very High Resolution Radiometer) data from 100 NOAA polar-orbiter overpasses tracked from an icebreaker during the 1994 Arctic Ocean Section. The cloud classification method is applied to the 32 x 32 pixel cell centered about the ship's position during each overpass. These overpasses have matching surface weather observations in the form of all-sky photographs or, during a period of heavy weather, an objective record that the sky was overcast with low water clouds. The cloud classifications from the maximum likelihood method are compared with the surface weather observations to determine if the automated satellite cloud classifier actually produces realistic descriptions of the scene. These comparisons are favorable in most cases, with the exception of a frequent error in which the classifier confuses Ci/Cc/Ac with extensive low water clouds over sea ice. This overall evaluation does not change appreciably if global area coverage resolution is used instead of full resolution or if the authors attempt to recalibrate the data to the NOAA-7 data for which the algorithm was originally developed. The authors find that the Ci/Cc/Ac cloud error can usually be avoided by 1) modifying the textural feature values for some cloud-over-ice categories and 2) applying a threshold value of 30% to the AVHRR channel 2 albedo averaged over the cell (and normalized by the cosine of the solar zenith angle). For a cell that the classifier identifies as containing Ci/Cc/Ac over sea ice, a cell-average channel 2 albedo greater than 30% usually indicates that the cell instead contains extensive low water clouds. When compared to the surface weather observations, the skill score of the satellite cloud classifier thus modified is 81%, which is very close to that claimed by its original author, This study suggests that satellite cloud detection and classification schemes based on both spectral signatures and texture recognition may indeed yield realistic results.

1997
Lubin, D, Garrity C, Ramseier RO, Whritner RH.  1997.  Total sea ice concentration retrieval from the SSM/I 85.5 GHz channels during the Arctic summer. Remote Sensing of Environment. 62:63-76.   10.1016/s0034-4257(97)00081-3   AbstractWebsite

During the 1994 Arctic Ocean Section, a joint voyage across the Arctic Ocean, by the U.S. Coast Guard Cutter Polar Sea and the Canadian Coast Guard Ship Louis S. St.-Laurent, telemetry from the Defense Meteorological Satellite Program (DMSP) polar orbiters was tracked by a shipboard antenna. Special Sensor Microwave Imager (SSM/I) data was used to generate maps of total sea. ice concentration, using the NASA Team algorithm with the 19 GHz and 37 GHz channels, and using a polarization-based algorithm with the 85.5 GHz channels. When compared with shipboard ice observations, the total sea ice concentration estimated from the 85.5 GHz algorithm are at least as accurate as those from the algorithm that uses only the lower SSM/I frequencies, despite the potential for greater difficulty in dealing with cloud liquid water contamination in the 85.5 GHz signal during the Arctic summer. Near the edge of the ice pack, the 85.5 GHz algorithm often provided more accurate estimates of total ice concentration when compared with surface observations, most likely because of the finer grid spacing at 85.5 GHz (12.5 km vs. 25 km for 37 GHz). However, when using the 85.5 GHz algorithm over regions of lower ice concentration, the reference polarizations in a given image must be chosen with care because over lower sea ice concentration the polarization-based algorithm is more sensitive to cloud opacity and can easily and substantially underestimate the ice concentration. The 85.5 GHz total sea ice retrievals are compared with in situ snow wetness measurements. This comparison suggests that, despite the higher atmospheric opacity at 85.5 GHz, information about sea ice surface properties that affect emissivity can be obtained from these SSM/I channels. (C) Elsevier Science Inc., 1997.

Lubin, D, Simpson AS.  1997.  Measurement of surface radiation fluxes and cloud optical properties during the 1994 Arctic Ocean Section. Journal of Geophysical Research-Atmospheres. 102:4275-4286.   10.1029/96jd03215   AbstractWebsite

During a voyage to the north pole from Alaska by the icebreakers USCGC Polar Sea and Canadian CGC Louis S. St.-Laurent (the 1994 Arctic Ocean Section, July 24 to September 3) an atmospheric radiation and remote sensing experiment measured downwelling shortwave and longwave radiation reaching the sea ice surface. The experiment included a Fourier transform infrared (FTIR) spectroradiometer which measured zenith radiance at 1 cm(-1) resolution in the middle infrared wavelength range 5-20 mu m, an Eppley pyranometer measuring most of the downwelling shortwave flux (0.28-2.80 mu m), an Eppley pyranometer measuring the downwelling near-infrared flux (0.78-2.80 mu m), and an Eppley pyrgeometer measuring the downwelling longwave flux. In conjunction with a discrete-ordinates radiative transfer model, the FTIR emission spectra are used to estimate 8-12 mu m cloud emissivity and effective radius of the cloud droplet size distribution. The broadband shortwave flux measurements are used to estimate shortwave cloud scattering optical depth. Most of the FTIR emission spectra recorded under overcast skies are consistent with cloud effective radius in the range 10-12 mu m, but 27% of the spectra are more consistent with the range 4-6 mu m, suggesting an occasional continental aerosol influence to Arctic cloud microphysics. The average daily shortwave cloud-scattering optical depth ranged from 2 to 46, which is similar to a range inferred from radiometer data recorded at Barrow, Alaska, during the same season. The downwelling shortwave flux measurements and estimates of net surface flux are generally consistent with a four-decade Russian climatology but also suggest that the frequency of cloud cover sampled during the 1994 Arctic Ocean Section was somewhat larger than the climatological average. These radiation measurement data from the 1994 Arctic Ocean Section should be useful for examining the treatment of atmospheric radiation and surface energy input in Arctic climate model simulations.

Lubin, D, Jensen EH.  1997.  Satellite mapping of solar ultraviolet radiation at the earth's surface. Solar ultraviolet radiation : modelling, measurements, and effects. ( Zerefos CS, Bais AF, Eds.)., Berlin; New York: Springer Abstract
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1996
Collins, WD, Valero FPJ, Flatau PJ, Lubin D, Grassl H, Pilewskie P.  1996.  Radiative effects of convection in the tropical Pacific. Journal of Geophysical Research-Atmospheres. 101:14999-15012.   10.1029/95jd02534   AbstractWebsite

The radiative effects of tropical clouds at the tropopause and the ocean surface have been estimated by using in situ measurements from the Central Equatorial Pacific Experiment (CEPEX). The effect of clouds is distinguished from the radiative effects of the surrounding atmosphere by calculating the shortwave and longwave cloud forcing. These terms give the reduction in insolation and the increase in absorption of terrestrial thermal emission associated with clouds. At the tropopause the shortwave and longwave cloud forcing are nearly equal and opposite, even on daily timescales. Therefore the net effect of an ensemble of convective clouds is small compared to other radiative terms in the surface-tropospheric heat budget. This confirms the statistical cancellation of cloud forcing observed in Earth radiation budget measurements from satellites. At the surface the net effect of clouds is to reduce the radiant energy absorbed by the ocean. Under deep convective clouds the diurnally averaged reduction exceeds 150 W m(-2). The divergence of flux in the cloudy atmosphere can be estimated from the difference in cloud forcing at the surface and tropopause. The CEPEX observations show that the atmospheric cloud forcing is nearly equal and opposite to the surface forcing. Based upon the frequency of convection, the atmospheric forcing approaches 100 W m(-2) when the surface temperature is 303 K. The cloud forcing is closely related to the frequency of convective cloud systems. This relation is used in conjunction with cloud population statistics derived from satellite to calculate the change in surface cloud forcing with sea surface temperature. The net radiative cooling of the surface by clouds increases at a rate of 20 W m(-2)K(-1)during the CEPEX observing period.

Lubin, D, Chen JP, Pilewskie P, Ramanathan V, Valero FPJ.  1996.  Microphysical examination of excess cloud absorption in the tropical atmosphere. Journal of Geophysical Research-Atmospheres. 101:16961-16972.   10.1029/96jd01154   AbstractWebsite

To investigate the excess shortwave absorption by clouds, a numerical cloud generation model has been coupled to a plane-parallel discrete ordinates radiative transfer model. The former was used in a time-dependent fashion to generate a cumulonimbus turret and three types of cirrus anvil (precipitating, extended, detached) representing three stages of cloud evolution outward from the turret. The cloud particle size distributions, as a function of altitude, were used as input to the radiative transfer model using indices of refraction for pure water and pure ice and equivalent sphere Mie theory. The radiative transfer model was used to calculate the ratio of cloud forcing at the surface to cloud forcing at the top of the atmosphere, both for the broadband shortwave and as a function of wavelength. Recent empirical studies have placed this cloud forcing ratio at around 1.5, and our coupled model results approach this value for small solar zenith angles, when the cloud contains large (>100 mu m) ice particles that absorb significantly in the near infrared (primarily the 1.6-mu m window). However, the empirical studies are based on diurnal averages, and our plane-parallel radiative transfer model yields an area and diurnally averaged cloud forcing ratio of only 1.18 for a tropical cumulonimbus and cirrus anvil system, primarily because of the rapid decrease of the ratio with solar zenith angle. The ratio decreases because of the increase in albedo with solar zenith angle, which is a characteristic feature of plane-parallel clouds. Adding dust or aerosol to the cloud layers, to make them absorb at visible wavelengths, makes the instantaneous cloud forcing ratio larger for an overhead Sun but also makes the solar zenith angle dependence in the cloud forcing ratio more pronounced. These two effects cancel, eliminating interstitial aerosol as a possible explanation for the excess cloud absorption in plane-parallel radiative transfer modeling. The strong dependence of the surface/top of the atmosphere cloud forcing ratio on solar zenith angle may be a fundamental defect with the plane-parallel approach to solar radiative transfer in a cloudy atmosphere.

Lubin, D, Harper DA.  1996.  Cloud radiative properties over the South Pole from AVHRR infrared data. Journal of Climate. 9:3405-3418.   10.1175/1520-0442(1996)009<3405:crpots>2.0.co;2   AbstractWebsite

Over the Antarctic plateau, the radiances measured by the AVHRR middle infrared (11 and 12 mu m) channels are shown to depend on effective cloud temperature, emissivity, ice water path, and effective radius of the particle size distribution. The usefulness of these dependencies is limited by radiometric uncertainties of up to 2 K in brightness temperature and by the fact that the radiative transfer solutions are not single valued over all possible ranges of temperature, effective radius, and ice water path. Despite these limitations, AVHRR imagery can be used to characterize cloud optical properties over the Antarctic continent if surface weather observations and/or radiosonde data can be collocated with the satellite overpasses. From AVHRR imagery covering the South Pole during 1992, the mean cloud emissivity is estimated at 0.43 during summer and 0.37 during winter, while the mean summer and winter effective radii are estimated at 12.3 and 5.6 mu m, respectively. When a radiative transfer model is used to evaluate these results in comparison with surface pyrgeometer measurements, the comparison suggests that the AVHRR retrieval method captures the overall seasonal behavior in cloud properties. During months when the polar vortex persists, AVHRR infrared radiances may be noticeably influenced by polar stratospheric clouds.

Aagaard, K, Barrie L, Carmack E, Garrity C, Jones EP, Lubin D, Macdonald RW, Swift JH, Tucker W, Wheeler PA, Whritner R.  1996.  U.S., Canadian researchers explore Arctic Ocean. EOS, Transactions American Geophysical Union. 77:209,213.   10.1029/96EO00141   Abstract

During July–September 1994, two Canadian and U.S. ice breakers crossed the Arctic Ocean (Figure 1) to investigate the biological, chemical, and physical systems that define the role of the Arctic in global change. The results are changing our perceptions of the Arctic Ocean as a static environment with low biological productivity to a dynamic and productive system. The experiment was called the Arctic Ocean Section (AOS) and the ships were the Canadian Coast Guard ship Louis S. St.-Laurent and the U.S. Coast Guard cutter Polar Sea.

1995
Ricchiazzi, P, Gautier C, Lubin D.  1995.  Cloud Scattering Optical Depth and Local Surface Albedo in the Antarctic - Simultaneous Retrieval Using Ground-Based Radiometry. Journal of Geophysical Research-Atmospheres. 100:21091-21104.   10.1029/95jd01461   AbstractWebsite

We have used solar irradiance measurements from a ground-based multichannel radiometer system deployed at Palmer Station, Antarctica (64 degrees 46'S, 64 degrees 04'W), during spring 1991 to simultaneously estimate cloud scattering optical depth and surface albedo. Irradiance measurements at 410 and 630 nm, in conjunction with a discrete ordinate radiative transfer (RT) model, enable this simultaneous retrieval by exploiting the wavelength dependence in Rayleigh scattering strength. The RT model is used in an inverse mode to find the values of surface albedo and cloud optical depth that match calculated and measured irradiances at both wavelengths. Under the homogeneous stratiform cloud cover for which the technique applies, surface albedo at 630 nm was consistently retrieved at above 0.9. For most homogeneous, overcast conditions, cloud optical depth (at 630 nm) is found to be in the range 20-50, with a most probable value of 25. This measurement and retrieval technique should be useful for compiling high-latitude cloud opacity and surface albedo climatologies of interest for global change and photobiology research.