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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.

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

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

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

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

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

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

Lubin, D.  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.

Lubin, D, Lynch S, Clarke R, Morrow E, Hart S.  2003.  Increasing reflectivity of the Antarctic ocean-atmosphere system: Analysis of Total Ozone Mapping Spectrometer (TOMS) and passive microwave data for 1979-1994. Journal of Geophysical Research-Atmospheres. 108   10.1029/2002jd002702   AbstractWebsite

Measurements of Lambert equivalent reflectance at 380 nm from the Total Ozone Mapping Spectrometer (TOMS) instrument have shown increases in reflectivity between 1979 and 1994 over much of the Southern Ocean, encompassing 280degrees in longitude. These trends represent a possible change in the state of the Antarctic ocean-atmosphere system related to recent climate warming. To determine if these reflectivity trends are due to changes in cloud cover or sea ice, or both, the TOMS data were collocated with a contemporaneous passive microwave satellite data set from the scanning multichannel microwave radiometer and the Special Sensor Microwave Imager. The passive microwave data sets specify total sea ice concentration, retrieved by a uniform method for all years using the NASA Team algorithm. To first order the locations of TOMS reflectivity increases coincide with regions where sea ice concentration has increased over the past 2 decades, signifying that the TOMS trends are the result of trends in underlying sea ice and not cloud cover. However, when the TOMS reflectivity measurements are sorted into fixed sea ice concentration bins of 0.1 width, the TOMS data also show increasing reflectivity trends in regions where sea ice extent has been decreasing (Amundsen and Bellingshausen Seas and the Western Antarctic Peninsula). Over open water, TOMS reflectivity trends are less convincing and may be artifacts related to uncertainties in passive microwave sea ice identification. These results suggest that a significant component of the Southern Ocean TOMS reflectivity trends may be a gradual increase in the albedo of the underlying sea ice. This could be caused by a gradual lengthening of the sea ice season, with a concomitant increase in the persistence of dry snow on the sea ice cover.

Xiong, XZ, Li W, Lubin D, Stamnes K.  2002.  Evaluating the principles of cloud remote sensing with AVHRR and MAS imagery over SHEBA. Journal of Geophysical Research-Oceans. 107   10.1029/2000jc000424   AbstractWebsite

[1] A rigorous discrete ordinates radiative transfer formulation has been applied to two Advanced Very High Resolution Radiometer (AVHRR) images extracted from telemetry collected by the CCGS Des Groseilliers satellite tracking system during SHEBA to estimate cloud optical depth and effective radius of the cloud droplet size distribution. The two cases, from 2 and 3 June 1998, were chosen for analysis because (1) the images contained mostly liquid water clouds and (2) contemporaneous MODIS Airborne Simulator (MAS) overflight imagery was available for these AVHRR overpasses. The objective is to apply the same detailed radiative transfer formulation to both the MAS and AVHRR data so that the quality of the retrievals from the latter can be evaluated. Retrievals of cloud optical properties from MAS are assumed to be more reliable, because (1) all MAS channels have direct radiometric calibration, (2) the higher spatial resolution of MAS (50 m nadir versus 1.1 km nadir with AVHRR) should yield smaller uncertainties related to partially cloudy pixels in a given study area, and (3) effective droplet radius can be retrieved directly from the MAS 1.62-mum m channel without additional uncertainties involved with subtracting a thermal radiance component. Examination of the retrievals from both sensors in these two cases reveals considerable spatial variability (more than a factor of 2) in cloud optical depth, on a variety of scales ranging from tens of meters to tens of kilometers, even for relatively uniform liquid water clouds. Retrievals of cloud effective droplet radius from AVHRR are generally consistent with those from MAS, suggesting that AVHRR can be reliably used to estimate this quantity. However, AVHRR-based retrievals of cloud optical depth are subject to large errors that result from small uncertainties in the absolute radiometric calibration of AVHRR channel 2. Using recalibration coefficients from one of the more robust AVHRR postlaunch calibration efforts, the cloud optical depths that we retrieved from NOAA 14 AVHRR channel 2 are consistently 30-50% larger than those obtained from MAS. The intercomparison of MAS and AVHRR retrievals of cloud optical depth also revealed errors with AVHRR due to partial cloud cover, and these errors are not immediately apparent when examining the AVHRR retrievals alone. If the AVHRR retrievals are averaged to spatial resolutions of order 10-30 km, they appear to become more stable for use in applications such as atmospheric energy budget calculations.

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, 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.

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, 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, 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.  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.