Variations In Optical Thickness Research Articles

A multilayer mechanism of omnidirectional reflection in silvery pelagic fish

Event Abstract Back to Event A multilayer mechanism of omnidirectional reflection in silvery pelagic fish Tom Jordan1, 2*, Nicholas Roberts1 and Julian Partridge1 1 University of Bristol, School of Biological Sciences, United Kingdom 2 Bristol Centre for Complexity Sciences, United Kingdom Silvery reflections from pelagic fish are produced by a multilayer “stack” of birefringent guanine crystals with cytoplasm gaps. These reflecting structures provide camouflage in the near-axially symmetric underwater pelagic light field, with reflections from the sides of the fishes matching the rays from behind [1]. For optimal concealment, the reflecting structures must be able to produce spectrally broadband and omnidirectional (non-polarizing) reflectivity. Mechanisms capable of producing spectrally broadband reflectivity are well understood [2,3]. However the low polarization, near-omnidirectional reflections reported in previous experimental studies lack an optical explanation [4]. Here we report a novel multilayer mechanism of omnidirectional reflection in silvery pelagic fish. Using reflectance spectrophotometry, we detail the reflectance spectra from two species of fish, Clupea harengus and Sardina pilchardus, and we characterize the polarization dependence of those spectra as a function of angle of incidence. We further measure the birefringent properties of individual guanine crystals from both species and discover that in the reflecting argenteum there are two distinct populations of crystal morphology. From these experimental results we develop an optical model based upon anisotropic reflectivity coefficients [5], that correctly incorporates the birefringence of the guanine crystals in the multilayer structure. We establish that the neutralization of the polarization over the whole visible range and at all angles of incidence is due to both a variation in optical thickness of the layers and a mixture of orientations of the optic axis of the birefringent guanine layers. This polarization behavior uncovered in our study is very different to existing isotropic optical models of guanine-cytoplasm multilayers, which predict full polarization at Brewster’s angle. Our study demonstrates that polarization properties can be controlled by the orientation of the crystals in the multilayer, with both a fully polarizing Brewster’s angle and an entirely polarization-neutral omnidirectional reflection possible from different orientation arrangements. These findings have important consequences for our understanding of sensory adaptations related to how animals control the reflection of polarized light, and there is wide potential for the ideas to influence current debates in vision ecology upon polarization camouflage and polarization signaling. Acknowledgements The authors acknowledge support from the BBSRC (grant no. BB/G022917/1), the EPSRC 334 (grant no. EP/E501214/1) and the Air Force Office of Scientific Research (grant no. FA- 335 9550-09-1-0149).

Remote sensing of desert dust over Kuwait: long–term variation

Aerosol optical measurements acquired with satellite–borne TERRA/Moderate Resolution Imaging Spectro–radiometer (MODIS) sensor were analyzed over Kuwait. The study focuses on the seasonal variation of aerosol optical thickness (AOT) and their relationship with meteorological parameters during the 7–year period from 2000 through 2007. The mean values of AOT, wind speed (V), air temperatures (T), diurnal temperature range (AT) and water vapor content (WVC) in the near-infrared (NIR) region were high during spring (April–May), while those of the Angstrom wavelength exponent α were low. These low values of α indicate the presence of large aerosol particles (dust). Spectral analysis shows a significant annual variation in the AOT and meteorological parameters. The second (semi–annual), third (4–month) and fourth (3–month) harmonics of the annual variation are also evident. The fourth harmonic is well defined in WVC (NIR) spectra. The power spectral densities of the exponent α, ΔT and RH have a similar spectral power ~1.9. The cross–correlation functions between the time series of the wavelength exponent and each of the meteorological parameters exhibit annual and biennial cycles. The atmosphere over Kuwait is relatively dust free 3 months after WVC (NIR) peaks.

Impact of horizontal spreading on light propagation in melt pond covered seasonal sea ice in the Canadian Arctic

[1] Melt pond covered sea ice is a ubiquitous feature of the summertime Arctic Ocean when meltwater collects in lower-lying areas of ice surfaces. Horizontal transects were conducted during June 2008 above and below landfast sea ice with melt ponds to characterize surface and bottom topography together with variations in transmitted spectral irradiance. We captured a rapid progression from a highly flooded sea ice surface with lateral drainage toward flaws and seal breathing holes to the formation of distinct melt ponds with steep edges. As the mass of the ice cover decreased due to meltwater drainage and rose upward with respect to the seawater level, the high-scattering properties of ice above the water level (i.e., white ice) were continuously regenerated, while pond waters remained transparent compared to underlying ice. The relatively stable albedos observed throughout the study, even as ice thickness decreased, were directly related to these surface processes. Transmission through the ice cover of incident irradiance in the 400–700 nm wave band ranged from 38% to 67% and from 5% to 16% beneath ponded and white ice, respectively. Our results show that this transmission varied not only as a function of surface type (melt ponds or white ice) areal coverage but also in relation to ice thickness and proximity to other surface types through the influence of horizontal spreading of light. Thus, in contrast to albedo, this implies that regional transmittance estimates need to consider melt pond size and shape distributions and variations in optical properties and thickness of the ice cover.

Variation of aerosol optical thickness with atmospheric water vapour : A case study over a continental station Mysore, India

Atmospheric measurements in a continental, low latitude station Mysore (12.3° N) has been carried out, for the period December 2003 to June 2006. Measurements were made using a sunphotometer with five bands in the visible and near-infrared range of the solar spectrum. To bring out the wavelength dependence of Aerosol Optical Thickness (AOT) on atmospheric water vapour, typically two wavelength channels are being used, one at 500 nm and the other at 1020 nm. A linear dependence between AOT and water vapour on meteorologically calm days is the important observation made. Growth rate of AOT is found to be larger at shorter wavelength (500 nm) than that of the longer wavelength (1020 nm). A mass-plot representation is followed on monthly basis, which is nothing but the graphical plot of spectral AOT versus water vapour of the scans for all the clear sky days of a particular month. Further investigations reveal that some months exhibit a single trend of growth of AOT with water vapour whereas double trend is the scenario for other months. These results provide insight into the changes in the atmospheric aerosol characteristics with precipitable water vapour, which is the subject matter of this paper.

Precipitation and Temperature Variations on the Interannual Time Scale: Assessing the Impact of ENSO and Volcanic Eruptions

Abstract The effects of ENSO and two large tropical volcanic eruptions (El Chichón, March 1982; Mt. Pinatubo, June 1991) are examined for the period of 1979–2008 using various satellite- and station-based observations of precipitation, temperature (surface and atmospheric), and tropospheric water vapor content. By focusing on the responses in the time series of tropical and global means over land, ocean, and land and ocean combined, the authors intend to provide an observational comparison of how these two phenomena, represented by Niño-3.4 and the tropical mean stratospheric aerosol optical thickness (τ), respectively, influence precipitation, temperature, and water vapor variations. As discovered in past studies, strong same-sign ENSO signals appear in tropical and global mean temperature (surface and tropospheric) over both land and ocean. However, ENSO only has very weak impact on tropical and global mean (land + ocean) precipitation, though intense anomalies are readily seen in the time series of precipitation averaged over either land or ocean. In contrast, the two volcanoes decreased not only tropical and global mean surface and tropospheric temperature but also tropical and global mean (land + ocean) precipitation. The differences between the responses to ENSO and volcanic eruptions are thus further examined by means of lag-correlation analyses. The ENSO-related peak responses in oceanic precipitation and sea surface temperature (SST) have the same time lags with Niño-3.4, 2 (4) months for the tropical (global) means. Tropical and global mean tropospheric water vapor over ocean (and land) generally follows surface temperature. However, land precipitation responds to ENSO much faster than temperature, suggesting a certain time needed for surface energy adjustment there following ENSO-related circulation and precipitation anomalies. Weak ENSO signals in the tropical and global mean mid- to lower-tropospheric atmospheric (dry) static instability are further discovered, which tend to be consistent with weak ENSO responses in the tropical and global mean (land + ocean) precipitation. For volcanic eruptions, tropical and global mean precipitation over either ocean or land responds faster than temperature (surface and atmospheric) and tropospheric water vapor averaged over the same areas, suggesting that precipitation tends to be more sensitive to volcanic-related solar forcing. The volcanic-related precipitation variations are further shown to be related to the changes in the mid- to lower-tropospheric atmospheric (dry) instability.

Decadal changes in aerosol optical thickness and single scattering albedo estimated from ground-based broadband radiometers: A case study in Japan

[1] A method to estimate aerosol optical thickness and single scattering albedo from broadband direct and diffuse irradiances was developed. Using irradiances simulated with and without errors, the accuracies of estimated optical thickness from 0.7 to 0.8 μm and single scattering albedo in the visible wavelength region were determined to be about 0.02 and 0.05, respectively. Resulting time variations in optical thickness and single scattering albedo by broadband radiometers agreed well with sky radiometer retrievals. Long-term variations in optical thickness and single scattering albedo from 1975 to 2008 at Tsukuba, Japan, were estimated by the method described. Optical thickness increased until the mid-1980s, then decreased until the late 1990s, and was almost constant in the 2000s. The single scattering albedo was about 0.8 until the late 1980s, gradually increased, and has remained at approximately 0.9 since the mid-1990s. The surface global irradiance under clear sky conditions calculated from estimated aerosol optical properties showed an apparent transition from dimming to brightening around the mid-1980s. The magnitude of the brightening was about 12.7 W m−2; of this, 8.3 W m−2 was due to a decrease in optical thickness, and the remaining 4.4 W m−2 was due to an increase of single scattering albedo. On the other hand, the surface global irradiance measured under cloudy conditions increased by 2.6 W m−2. The dimming and brightening by aerosols were weakened by the changes in clouds. The method described could be useful in evaluating aerosol influences on long-term changes in the surface solar radiation at many sites around the world.

Seasonal Characteristics of Spherical Aerosol Distribution in Eastern Asia: Integrated Analysis Using Ground/Space-Based Lidars and a Chemical Transport Model

Seasonal characteristics of spherical aerosol distributions in eastern Asia were investigated between July 2006 and December 2008 using data from ground-based and space-borne lidar observations and the Community Multi-scale Air Quality Modeling System (CMAQ) chemical transport model simulation. The seasonal variation of spherical aerosol optical thickness (AOT) observed at four ground lidar stations surrounding the East China Sea (Beijing, Guangzhou, Seoul, and Hedo/Okinawa) was generally consistent with CMAQ simulation results. Detailed analyses confirmed clear regional differences in aerosol compositions. Analyses of aerosol vertical profiles revealed that the spherical aerosol variation is greatest in Beijing, with scale height varying between 720 m and 2100 m. The analyses also revealed that the seasonal variation patterns of spherical AOT are classifiable into ‘summer peak’ and ‘summer trough’ types. Northern sites (Beijing and Seoul) are of the summer peak type; southern sites (Guangzhou and Okinawa) show the summer trough pattern. Our analyses demonstrated that the Asian summer-winter monsoon system plays a major role in regulating such seasonal variation. The CMAQ simulated variation of spherical aerosols is well correlated to the synoptic scale monsoon variation.

Electrochromism and Swelling of Polypyrrole Membranes: An Electrochemical and Ellipsometric Study

The growth of polypyrrole (Ppy) layers on gold electrodes in nearly neutral pH solutions is analysed using “in situ” voltametric and ellipsometric techniques. Different film structures are obtained depending on the potentiodynamic programme and the composition of the electrolyte. More compact dodecylsulphate-(DS) doped Ppy layers were grown at 1.2 V versus RHE than those obtained by applying a higher potential. The more compact layers correspond to the growth of an oxidised Ppy/DS layer that shows low pseudo capacity behaviour. After dipping, the doped Ppy/DS film in KCl solution-significant variations in optical indices and thickness are detected as a function of the applied potential. Higher electrochromism as well as decrease in film thickness after cathodisation is achieved. The optical indices and the thickness of the Ppy layer formed under different applied potential/time programmes are estimated.

Disturbances of aerosol optical thickness of the atmosphere caused by forest fires in Yakutia

he aerosol optical thickness (AOT) and pyrogenic event variations in central Yakutia are studied using the remote data (NOAA, Terra and Aqua) for May–September 2000–2009. It is shown that the average summer (June–August) AOT for years with a high pyrogenic activity (2001–2003) is by about 125% greater in comparison with the years with a low pyrogenic activity (2000 and 2004–2009); the daily AOT in central Yakutia on some days with the highest pyrogenic activity significantly exceeded the background (undisturbed) values reaching 1.5–1.6.

Temporal variability of the chemical composition of surface aerosol in the Moscow region in 1999–2005 from the results of infrared spectroscopy of aerosol samples

The temporal variability of the chemical composition of surface aerosol with particle diameters of 0.7–2 μm is analyzed. This analysis is based on the results of measurements of infrared transmission spectra of aerosol samples collected with the use of a cascade impactor at the Zvenigorod Scientific Station of the Institute of Atmospheric Physics (IAP) in 1999–2005. Seasonal features of the aerosol chemical composition and its dependence on the particle size are revealed. The interdiurnal variability of the aerosol composition depends on the season, and it manifests itself more strongly in winter and spring. Air-mass changes lead to changes in the relation of sulfates and nitrates in the micron fraction of aerosol. The enrichment of samples in nitrates is especially characteristic of the winter and spring seasons. Compounds containing the NO2 group are often met in the samples of aerosol with particle sizes of 0.7–1.3 μm during the cold time of the year. The estimates of the optical thickness of micron aerosol in the sulfate absorption band are obtained, and optical-thickness variations of some scales are detected. The quantitative characteristics of statistical relations between different chemical components of aerosol inside individual fractions and between chemical components of the micron and submicron fractions are obtained and analyzed.

On the origin of seasonal variation of aerosol optical thickness in UV range over Belsk, Poland

Aerosol optical thickness (AOT) and seasonal variation of AOT over Belsk, Poland, in the UV wavelength range (310–380 nm) have been analysed using results of measurements by Brewer spectrophotometer No. 064 and Cimel sunphotometer data for the 2002–2007 period. The comparison of AOT derived from direct Sun measurements by Brewer spectrophotometer in the 310–320 nm range and retrieved from Cimel measurements at longer wavelengths shows good correlation (R = 0.96), with overestimation of retrieved values compared to the measured ones by about 6%. Basing on aerosol microphysical properties taken from almucantar retrievals and Mie theory, optical properties of aerosol in the UV range has been calculated. Analysis of seasonal variation of AOT at Belsk reveals two maxima: in April and July–August. Analysis of back-trajectories in conjunction with analysis of fire maps from Fire Information For Resource Management System shows that these seasonal maxima are connected with seasonal biomass burning in Eastern and Southern Europe.

Controlled variation of the refractive index in ion-damaged diamond

A fine control of the variation of the refractive index as a function of structural damage is essential in the fabrication of diamond-based optical and photonic devices. We report here about the variation of the real part of the refractive index at λ = 632.8 nm in high-quality single-crystal diamond damaged with 2 and 3 MeV protons at low-medium fluences (10 13–10 17 ions cm − 2 ). After implanting the samples in 125 × 125 μm 2 areas with a raster scanning ion microbeam, the variation of optical thickness of the implanted regions was measured with laser interferometric microscopy. The results were analyzed with a model based on the specific damage profile. The technique allows the direct fabrication of optical structures in bulk diamond based on the localized variation of the refractive index, which will be explored in future works.

Natural versus anthropogenic aerosols in the eastern Mediterranean basin derived from multiyear TOMS and MODIS satellite data

In the present study we investigate the spatial and temporal variation of aerosol optical thickness (AOT) in the eastern Mediterranean basin, and more specifically in the area extending from 28.5°N to 42.5°N and from 18.5°E to 35.5°E, which includes large urban areas and megacities such as Cairo, Istanbul, Athens, Izmir, Ankara, and Thessaloniki. For this purpose we use long‐term AOT data from the Total Ozone Mapping Spectrometer (TOMS) version2 converted to 500 nm (AOT500) for the period 1980–2001 and Collection 005 AOT data atλ= 550 nm (AOT550) from Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra and Aqua satellites for the periods 2000–2005 and 2002–2005, respectively. The spatial and temporal variation of AOT shows a good agreement between TOMS and MODIS, in terms of geographical patterns, which maximizes the usefulness of TOMS AOT product given its long temporal coverage (climatological). According to MODIS‐Terra, the annual mean AOT550over the studied region equals 0.22 ± 0.05 (monthly values ranging from 0.14 to 0.32) and shows strong spatial inhomogeneities. The smallest values (down to 0.1) occur over the western, and especially in the northwestern, part of Greece, over the northern part of Anatolian peninsula and also over the sea of east Mediterranean. The largest AOT values (up to about 1.0) occur over northern Africa, Middle East, and the adjacent coasts, and over the Anatolian plateau. These high aerosol loadings are of natural origin, mainly desert dust. However, very large values (up to 0.8) are also found over large urban areas surrounding megacities, associated with anthropogenic, apart from natural desert, aerosols. By using TOMS AOT data as proxy for the dust source, and the difference of MODIS‐TOMS AOT as an indicator of the anthropogenic aerosol component, the relative contribution of natural versus anthropogenic sources of aerosols has been derived for the eastern Mediterranean area. Limitations of this approach are discussed and the associated uncertainties are also evaluated and discussed.

Exploring the differences in cloud properties observed by the Terra and Aqua MODIS Sensors

Abstract. The aerosol-cloud interaction in different parts of the globe is examined here using multi-year statistics of remotely sensed data from two MODIS sensors aboard NASA's Terra (morning) and Aqua (afternoon) satellites. Simultaneous retrievals of aerosol loadings and cloud properties by the MODIS sensor allowed us to explore morning-to-afternoon variation of liquid cloud fraction (CF) and optical thickness (COT) for clean, moderately polluted and heavily polluted clouds in different seasons. Data analysis for seven-years of MODIS retrievals revealed strong temporal and spatial patterns in morning-to-afternoon variation of cloud fraction and optical thickness over different parts of the global oceans and the land. For the vast areas of stratocumulus cloud regions, the data shows that the days with elevated aerosol abundance were also associated with enhanced afternoon reduction of CF and COT pointing to the possible reduction of the indirect climate forcing. A positive correlation between aerosol optical depth and morning-to-afternoon variation of trade wind cumulus cloud cover was also found over the northern Indian Ocean, though no clear relationship between the concentration of Indo-Asian haze and morning-to-afternoon variation of COT was established. Over the Amazon region during wet conditions, aerosols are associated with an enhanced convective process in which morning shallow warm clouds are organized into afternoon deep convection with greater ice cloud coverage. Analysis presented here demonstrates that the new technique for exploring morning-to-afternoon variability in cloud properties by using the differences in data products from the two daily MODIS overpasses is capable of capturing some of the major features of diurnal variations in cloud properties and can be used for better understanding of aerosol radiative effects.

A Technique for investigating the effects of planetary waves on aerosol optical thickness variations

We have developed a technique for investigating the planetary waves based on aerosol optical thickness (AOT) observations at spatially separated stations. The technique takes into account the data nonuniformity and makes it possible to investigate the spatial and temporal structure of the AOT and Angstrom parameter variations, which depends on intraseasonal-scale atmospheric wave processes. Based on this technique, we can distinguish periodic synoptic-scale variations and investigate their phase fronts, periods, and wave numbers.

Analysis of clear‐sky Antarctic snow albedo using observations and radiative transfer modeling

A radiative transfer model for studying spectral and broadband snow surface albedo has been applied to radiation data (1998–2001) from different climate regimes in Antarctica. The model makes use of the doubling‐adding method for radiative transfer, combined with the correlated k‐distribution technique to account for atmospheric gas absorption. Snow layers are described by scattering phase functions for irregular hexagonal plate‐shaped ice crystals. Multiple scattering is included, as well as the option to include soot in the snowpack, as well as clouds. Sensitivity experiments show that the model is capable of calculating spectral and broadband albedos as a function of solar zenith angle and effective snow grain radius re. The novel approach of applying the model to multiple‐year field data of clear‐sky albedo from five locations in Dronning Maud Land, Antarctica, reveals that seasonal clear‐sky albedo variations (0.77–0.88) are dominantly caused by strong spatial and temporal variations in re. Summer season averages of re range from 22 μm on the Antarctic plateau to 64 μm on the ice shelf. Maximum monthly values of re are 40–150% higher. Other factors influencing clear‐sky broadband albedo are the seasonal cycle in solar zenith angle (at most 0.02 difference in summer and spring/autumn albedo) and the spatial variation in optical thickness of the cloudless atmosphere (0.01 difference between ice shelves and plateau). The seasonal cycle in optical thickness of the atmosphere was found to be of minor importance (<0.005 between summer and spring/autumn).

Microgap Structured Optical Sensor for Fast Label-Free DNA Detection

This paper presents a new label-free DNA optical sensor based on the microgap structured Fabry-Perot interferometer. An optical cavity is formed by a microgap sealed between a singlemode fiber and a well-cleaved fiber end. The capture DNA is immobilized on the cavity end by layer-by-layer electrostatic self-assembly. This sensor detects the hybridization of target DNA with complementary capture DNA by observing the optical cavity thickness variation. Experimental results demonstrate that this sensor has a sensitivity of 6 nmol/ml and can identify a 2-bp mismatch.

The Hydrological Cycle over a Wide Range of Climates Simulated with an Idealized GCM

Abstract A wide range of hydrological cycles and general circulations was simulated with an idealized general circulation model (GCM) by varying the optical thickness of the longwave absorber. While the idealized GCM does not capture the full complexity of the hydrological cycle, the wide range of climates simulated allows the systematic development and testing of theories of how precipitation and moisture transport change as the climate changes. The simulations show that the character of the response of the hydrological cycle to variations in longwave optical thickness differs in different climate regimes. The global-mean precipitation increases linearly with surface temperature for colder climates, but it asymptotically approaches a maximum at higher surface temperatures. The basic features of the precipitation–temperature relation, including the rate of increase in the linear regime, are reproduced in radiative–convective equilibrium simulations. Energy constraints partially account for the precipitation–temperature relation but are not quantitatively accurate. Large-scale condensation is most important in the midlatitude storm tracks, and its behavior is accounted for using a stochastic model of moisture advection and condensation. The precipitation associated with large-scale condensation does not scale with mean specific humidity, partly because the condensation region moves upward and meridionally as the climate warms, and partly because the mean condensation rate depends on isentropic specific humidity gradients, which do not scale with the specific humidity itself. The local water vapor budget relates local precipitation to evaporation and meridional moisture fluxes, whose scaling in the subtropics and extratropics is examined. A delicate balance between opposing changes in evaporation and moisture flux divergence holds in the subtropical dry zones. The extratropical precipitation maximum follows the storm track in warm climates but lies equatorward of the storm track in cold climates.

Biogeography of Tropical Montane Cloud Forests. Part I: Remote Sensing of Cloud-Base Heights

AbstractCloud-base heights over tropical montane cloud forests are determined using Moderate Resolution Imaging Spectroradiometer (MODIS) cloud products and National Centers for Environmental Prediction global tropospheric final analysis (FNL) fields. Cloud-base heights are computed by subtracting cloud thickness estimates from cloud-top height estimates. Cloud-top pressures determined from the current MODIS retrieval algorithm often have serious cloud-top pressure retrieval errors at pressures > 700 hPa. The problem can be easily remedied by matching cloud-top temperature derived from the 11-μm channel to the dewpoint temperature profile (instead of the temperature profile) obtained from the FNL dataset. The FNL dataset at 1° spatial resolution produced results that were nearly equivalent to those derived from radiosonde measurements. The following three different approaches for estimating cloud thickness are examined: 1) the constant liquid water method, 2) the empirical method, and 3) the adiabatic model method. The retrieval technique is applied first for stratus clouds over U.S. airports for 12 cases, with cloud-base heights compared with ceilometer measurements. Mean square errors on the order of 200 m result. Then, the approach is applied to orographic clouds over Monteverde, Costa Rica, with estimated cloud-base heights compared with those derived from photographs. Mean square errors on the order of 100 m result. Both the empirical and adiabatic model approaches produce superior results when compared with the constant liquid water (CLW) approach. This is due to the fact that CLW is more sensitive to natural variations in cloud optical thickness.

The Plane-Parallel Albedo Bias of Liquid Clouds from MODIS Observations

Abstract The authors present the global plane-parallel shortwave albedo bias of liquid clouds for two months, July 2003 and January 2004. The cloud optical properties necessary to perform the bias calculations come from the operational Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and MODIS Aqua level-3 datasets. These data, along with ancillary surface albedo and atmospheric information consistent with the MODIS retrievals, are inserted into a broadband shortwave radiative transfer model to calculate the fluxes at the atmospheric column boundaries. The plane-parallel homogeneous (PPH) calculations are based on the mean cloud properties, while independent column approximation (ICA) calculations are based either on 1D histograms of optical thickness or joint 2D histograms of optical thickness and effective radius. The (positive) PPH albedo bias is simply the difference between PPH and ICA albedo calculations. Two types of biases are therefore examined: 1) the bias due to the horizontal inhomogeneity of optical thickness alone (the effective radius is set to the grid mean value) and 2) the bias due to simultaneous variations of optical thickness and effective radius as derived from their joint histograms. The authors find that the global bias of albedo (liquid cloud portion of the grid boxes only) is ∼+0.03, which corresponds to roughly 8% of the global liquid cloud albedo and is only modestly sensitive to the inclusion of horizontal effective radius variability and time of day, but depends strongly on season and latitude. This albedo bias translates to ∼3–3.5 W m−2 of bias (stronger negative values) in the diurnally averaged global shortwave cloud radiative forcing, assuming homogeneous conditions for the fraction of the grid box not covered by liquid clouds; zonal values can be as high as 8 W m−2. Finally, the (positive) broadband atmospheric absorptance bias is about an order of magnitude smaller than the albedo bias. The substantial magnitude of the PPH bias underlines the importance of predicting subgrid variability in GCMs and accounting for its effects on cloud–radiation interactions.