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Wilson, A, Scott RC, Cadeddu MP, Ghate V, Lubin D.  2018.  Cloud optical properties over West Antarctica from shortwave spectroradiometer measurements during AWARE. Journal of Geophysical Research-Atmospheres. 123:9559-9570.   10.1029/2018jd028347   AbstractWebsite

A shortwave spectroradiometer was deployed on the West Antarctic Ice Sheet (WAIS) as part of the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program ARM West Antarctic Radiation Experiment (AWARE). This instrument recorded 1-min averages of downwelling hemispheric spectral irradiance covering the wavelength range 350-2,200nm with spectral resolution 3 and 10nm for wavelengths shorter and longer than 1,000nm, respectively. Using simultaneous micropulse lidar data to identify the thermodynamic phase of stratiform clouds, a radiative transfer algorithm is used to retrieve optical depth and effective droplet (or particle) size for single-phase liquid water and ice water clouds. The AWARE campaign on the WAIS first sampled typical climatological conditions between 7 December 2015 and 9 January 2016 and then a much warmer air mass with more moisture associated with a surface melt event between 10 and 17 January 2016. Before the melt event most liquid cloud effective droplet radii were consistent with pristine polar maritime clouds (mode radius 13.5m) but showed a second local maximum in the distribution (at 8m) consistent with colder, moisture-limited conditions. Most ice clouds sampled occurred before the melt event (mode optical depth 4 and effective particle size 19m). During the melt event liquid water cloud optical depth nearly doubled (mode value increasing from 8 to 14). AWARE therefore sampled on the WAIS two cases relevant to climate model simulations: typical current climatological conditions, followed by warmer meteorology possibly consistent with future increasing surface melt scenarios.

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).

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.  2004.  Thermodynamic phase of maritime Antarctic clouds from FTIR and supplementary radiometric data. Journal of Geophysical Research-Atmospheres. 109   10.1029/2003jd003979   AbstractWebsite

A Fourier Transform Infrared (FTIR) spectroradiometer was deployed at Palmer Station, Antarctica, from 1 September to 17 November 1991. This instrument is similar to the Atmospheric Emitted Radiance Interferometer (AERI) deployed with the U. S. Department of Energy Atmospheric Radiation Measurement (ARM) program. The instrument measured downwelling zenith radiance in the spectral interval 400 2000 cm(-1), at a resolution of 1 cm(-1). The spectral radiance measurements, which can be expressed as spectral brightness temperature T-b(nu), contain information about cloud optical properties in the middle infrared window (800-1200 cm(-1) 1, 8.3-12.5 mm). In this study, this information is exploited to (1) provide additional characterization of Antarctic cloud radiative properties, and (2) demonstrate how multisensor analysis of ARM data can potentially retrieve cloud thermodynamic phase. Radiative transfer simulations demonstrate how T-b(nu) is a function of cloud optical depth tau, effective particle radius r(e), and thermodynamic phase. For typical values of tau and r(e), the effect of increasing the ice fraction of the total optical depth is to flatten the slope of T-b(nu) between 800 1000 cm(-1). For optically thin clouds (tau similar to 3) and larger ice particles (re(ice) > 50 mm) the behavior of T-b(nu) in this interval switches from a decrease with increasing wavenumber n to an increase with nu, once the ice fraction of the total optical depth exceeds similar to0.7. The FTIR spectra alone cannot be interpreted to obtain thermodynamic phase, because a relatively small slope in T-b(nu) between 800-1000 cm(-1) could represent either an optically thin cloud in the ice or mixed phase, or an optically thick cloud radiating as a blackbody. Sky observations and ancillary radiometric data are needed to sort the FTIR spectra into categories of small cloud optical depth, where the mid-IR window data can be interpreted; and larger cloud optical depth, where the FTIR data contain information only about cloud base temperature. Spectral solar ultraviolet (UV) irradiance measurements from the U. S. National Science Foundation's UV Monitor at Palmer Station are used to estimate area-averaged effective cloud optical depth tau(sw), and these estimates are used to sort the FTIR data. FTIR measurements with colocated tau(sw) < 16 are interpreted to estimate cloud thermodynamic phase. Precipitating cloud decks generally show flatter slopes in T-b(ν), consistent with the ice or mixed phase. Altostratus decks show a larger range in T-b(ν) slope than low cloud decks, including increasing slopes with ν, suggesting a more likely occurrence of the ice phase. This study illustrates how cloud thermodynamic phase can be defensibly retrieved from FTIR data if high quality shortwave radiometric data are also available to sort the FTIR measurements by cloud opacity, and both data types are available at the ARM sites.

Xiong, XZ, Storvold R, Stamnes K, Lubin D.  2004.  Derivation of a threshold function for the Advanced Very High Resolution Radiometer 3.75 mu m channel and its application in automatic cloud discrimination over snow/ice surfaces. International Journal of Remote Sensing. 25:2995-3017.   10.1080/01431160310001619553   AbstractWebsite

The distinct contrast between the reflectance of solar radiation in Advanced Very High Resolution Radiometer (AVHRR) channel 3 (3.75 mum) by clouds and by bright surfaces provides an effective means of cloud discrimination over snow/ice surfaces. A threshold function for the top-of-atmosphere (TOA) albedo in channel 3 (r(3)) is derived and used to develop an improved method for cloud discrimination over snow/ice surfaces that makes explicit use of TOA r(3) . Corrections for radiance anisotropy and temperature effects are required to derive accurate values of r(3) from satellite measurements and to utilize the threshold function. It has been used to retrieve cloud cover fractions from National Oceanic and Atmospheric Administration (NOAA)-14 AVHRR data over the Arctic Ocean and over the North Slope of Alaska (NSA) Atmospheric Radiation Measurement (ARM) site in Barrow, Alaska. The retrieved cloud fractions are in good agreement with SHEBA (Surface HEat Budget of the Arctic Ocean) surface visual observations and with NSA cloud radar and lidar observations, respectively. This method can be utilized to improve cloud discrimination over snow/ice surfaces for any satellite sensor with a channel near 3.7 mum.

Lubin, D, Satheesh SK, McFarquar G, Heymsfield AJ.  2002.  Longwave radiative forcing of Indian Ocean tropospheric aerosol. Journal of Geophysical Research-Atmospheres. 107   10.1029/2001jd001183   AbstractWebsite

A spectrally resolved discrete-ordinates radiative transfer model is used to calculate the change in downwelling surface and top-of-the-atmosphere (TOA) outgoing longwave (3.9-500 mum) radiative fluxes induced by tropospheric aerosols of the type observed over the Indian Ocean during the Indian Ocean Experiment (INDOEX). Both external and internal aerosol mixtures were considered. Throughout the longwave, the aerosol volume extinction depends more strongly on relative humidity than in most of the shortwave (0.28-3.9 mum), implying that particle growth factors and realistic relative humidity profiles must be taken into account when modeling the longwave radiative effects of aerosols. A typical boundary layer aerosol loading, with a 500-nm optical depth of 0.3, will increase the downwelling longwave flux at the surface by 7.7 W m(-2) over the clean air case while decreasing the outgoing longwave radiation by 1.3 W m(-2). A more vertically extended aerosol loading, exhibiting a high opacity plume between 2 and 3 km above the surface and having a typical 500-nm optical depth of 0.7, will increase the downwelling longwave flux at the surface by 11.2 W m(-2) over the clean air case while decreasing the outgoing longwave radiation by 2.7 W m(-2). For a vertically extended aerosol profile, approximately 30% of the TOA radiative forcing comes from sea salt and approximately 60% of the forcing comes from the combination of sea salt and dust. The remaining forcing is from anthropogenic constituents. These results are for the external mixture. For an internal mixture, TOA longwave forcings can be up to a factor of two larger. Therefore, to complete our understanding of this region's longwave aerosol radiative properties, more detailed information is needed about aerosol mixing states. These longwave radiative effects partially offset the large shortwave aerosol radiative forcing and should be included in regional and global climate modeling simulations.

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, Weber PG.  1995.  The Use of Cloud Reflectance Functions with Satellite Data for Surface Radiation Budget Estimation. Journal of Applied Meteorology. 34:1333-1347.   10.1175/1520-0450(1995)034<1333:tuocrf>;2   AbstractWebsite

The bidirectional reflectance distribution function (BRDF) of an overcast atmosphere above an ocean surface has been calculated as a function of wavelength using a discrete-ordinates radiative transfer model. This plane-parallel BRDF appears qualitatively similar to the empirically derived angular dependence models from the Earth Radiation Budget Experiment. But when these two different BRDFs are used to estimate net shortwave flux at the ocean surface, discrepancies of 20-60 W m(-2) can occur between the respective net surface nux estimations. When using either BRDF with Advanced Very High Resolution Radiometer data for surface radiation budget estimation, this uncertainty can be minimized by restricting the satellite viewing( polar) angle to between 30 degrees and 50 degrees. Accurate measurements of the planetary BRDF would help resolve these differences.

Lubin, D.  1994.  Infrared Radiative Properties of the Maritime Antarctic Atmosphere. Journal of Climate. 7:121-140.   10.1175/1520-0442(1994)007<0121:irpotm>;2   AbstractWebsite

The longwave radiation environment of the Antarctic Peninsula and Southern Ocean has been investigated using radiometric Fourier Transform Infrared (FTIR) measurements of atmospheric emission in conjunction with detailed radiative transfer theory. The California Space Institute FTIR Spectroradiometer was deployed at Palmer Station, Antarctica (64 degrees 46'S, 64 degrees 04'W), where it made zenith sky emission measurements several times daily between 25 August 1991 and 17 November 1991. Emission spectra covered the entire middle infrared (5-20 mu m) with one inverse centimeter spectral resolution. For FTIR data obtained under cloudy skies, a least-squares algorithm is used to match the emission spectra with discrete-ordinate radiative transfer calculations that are based on marine cloud microphysics. This algorithm provides a determination of cloud emissivity, and useful estimates of cloud optical depth and equivalent radius of the droplet size distribution. Temperatures in the lower troposphere between 259 K and 273 K diminish the radiative importance of water vapor and enhance the importance of clouds and CO2 relative to midlatitudes. Springtime variability in stratospheric temperature and ozone abundance has a small but noticeable impact of about 1.0 W m(-2) on surface longwave flux under clear skies. The mid-IR window emissivities of low stratiform clouds are most often between 0.90 and 0.98, with few as large as unity. Most low stratiform clouds appear to have moderate mid-IR optical depth (5-10), but relatively large equivalent radius (9-11 mu m). However, clouds with base height between 1 and 2 km have noticeably smaller emissivities and optical depths. The emissivity of maritime antarctic clouds is determined to be smaller for a given liquid water path than the parameterization used in the NCAR Community Climate Model (CCM1), and an appropriate mass absorption coefficient for antarctic clouds is 0.065 m(2) g(-1) for the mid-IR window.