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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).

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.

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).

Berque, J, Lubin D, Somerville RCJ.  2004.  Infrared radiative properties of the Antarctic plateau from AVHRR data. Part I: Effect of the snow surface. Journal of Applied Meteorology. 43:350-362.   10.1175/1520-0450(2004)043<0350:irpota>;2   AbstractWebsite

The effective scene temperature, or "brightness temperature," measured in channel 3 (3.5-3.9 m m) of the Advanced Very High Resolution Radiometer (AVHRR) is shown to be sensitive, in principle, to the effective particle size of snow grains on the Antarctic plateau, over the range of snow grain sizes reported in field studies. In conjunction with a discrete ordinate method radiative transfer model that couples the polar atmosphere with a scattering and absorbing snowpack, the thermal infrared channels of the AVHRR instrument can, therefore, be used to estimate effective grain size at the snow surface over Antarctica. This is subject to uncertainties related to the modeled top-of-atmosphere bidirectional reflectance distribution function resulting from the possible presence of sastrugi and to lack of complete knowledge of snow crystal shapes and habits as they influence the scattering phase function. However, when applied to NOAA-11 and NOAA-12 AVHRR data from 1992, the snow grain effective radii of order 50 mum are retrieved, consistent with field observations, with no apparent discontinuity between two spacecraft having different viewing geometries. Retrieved snow grain effective radii are 10-20-mum larger when the snow grains are modeled as hexagonal solid columns rather than as spheres with a Henyey-Greenstein phase function. Despite the above-mentioned uncertainties, the retrievals are consistent enough that one should be able to monitor climatically significant changes in surface snow grain size due to major precipitation events. It is also shown that a realistic representation of the surface snow grain size is critical when retrieving the optical depth and effective particle radius of clouds for the optically thin clouds most frequently encountered over the Antarctic plateau.

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   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.

Lubin, D, Cutchin D, Conant W, Grassl H, Schmid U, Biselli W.  1995.  Spectral Longwave Emission in the Tropics - Ftir Measurement at the Sea-Surface and Comparison with Fast Radiation Codes. Journal of Climate. 8:286-295.   10.1175/1520-0442(1995)008<0286:sleitt>;2   AbstractWebsite

Longwave emission by the tropical western Pacific atmosphere has been measured at the ocean surface by a Fourier Transform Infrared (FTIR) spectroradiometer deployed aboard the research vessel John Vickers as part of the Central Equatorial Pacific Experiment. The instrument operated throughout a Pacific Ocean crossing, beginning on 7 March 1993 in Honiara, Solomon Islands, and ending on 29 March 1993 in Los Angeles, and recorded longwave emission spectra under atmospheres associated with sea surface temperatures ranging from 291.0 to 302.8 K. Precipitable water vapor abundances ranged from 1.9 to 5.5 column centimeters. Measured emission spectra (downwelling zenith radiance) covered the middle infrared (5-20 mu m) with one inverse centimeter spectral resolution. FTIR measurements made under an entirely clear field of view are compared with spectra generated by LOWTRAN 7 and MODTRAN 2, as well as downwelling flux calculated by the NCAR Community Climate Model (CCM-2) radiation code, using radiosonde profiles as input data for these calculations. In the spectral interval 800-1000 cm(-1), these comparisons show a discrepancy between FTIR data and MODTRAN 2 having an overall variability of 6-7 mW m(-2) sr(-1) cm and a concave shape that may be related to the representation of water vapor continuum emission in MODTRAN 2. Another discrepancy appears in the spectral interval 1200-1300 cm(-1), where MODTRAN 2 appears to overestimate zenith radiance by 5 mW m(-2) sr(-1) cm. These discrepancies appear consistently; however, they become only slightly larger at the highest water vapor abundances. Because these radiance discrepancies correspond to broadband (500-2000 cm(-1)) flux uncertainties of around 3 W m(-2), there appear to be no serious inadequacies with the performance of MODTRAN 2 or LOWTRAN 7 at high atmospheric temperatures and water vapor abundances. On average, CCM-2 flux calculations agree to within 1 W m(-2) with downwelling flux estimates from the FTIR data over all sea surface temperatures, although this result has a scatter of +/-12 W m(-2) at high sea surface temperatures.