Export 10 results:
Sort by: Author Title Type [ Year  (Desc)]
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.

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.

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   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.

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.

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, 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   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.

Lubin, D, Jensen EH.  1995.  Effects of Clouds and Stratospheric Ozone Depletion on Ultraviolet-Radiation Trends. Nature. 377:710-713.   10.1038/377710a0   AbstractWebsite

ANTHROPOGENIC depletion of ozone in the lower stratosphere has been of global environmental concern for two decades, but the environmentally relevant quantity-the flux of solar ultraviolet radiation (UVR) reading the Earth's surface-remains poorly quantified on a global basis. The three most important parameters governing surface UVR fluxes and trends are solar elevation, total vertically integrated ozone abundance and cloud opacity. Here we use global satellite measurements of total ozone abundance and cloud reflectance to examine how the trends in UVR resulting from established trends in total ozone abundance(1,2) compare with the potentially large natural variability in UVR that results from variations in cloud opacity. We find that throughout many temperate regions-including large parts of continental Europe, North and South America, New Zealand, Australia and southern Africa-interannual variability in cloud opacity is sufficiently small that by the end of this century, trends in summer average local-noon UVR dose rates relevant to mammalian skin cancer or plant damage should be significant with respect to cloud variability.

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.

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.