Clear-sky Albedo Research Articles

Hemispheric Albedo Asymmetries across Three Phases of CMIP

Abstract Earth’s planetary albedo shows a remarkable hemispheric symmetry. We assess to what extent CMIP models symmetrize the hemispheric clear-sky albedo asymmetry and what the role of clouds is for this. Following Voigt et al., we calculate a reference TOA reflected solar radiation considering the masking of clear-sky asymmetry by symmetric cloud contributions. We use the simple radiation model of Donohoe and Battisti to estimate this benchmark and to separate surface, aerosol, and cloud contributions to the compensation of this benchmark. In CERES, tropical clouds enhance the reference asymmetry while extratropical cloud asymmetries balance the reference asymmetry and the additional asymmetry introduced by tropical clouds. CMIP multimodel means show similar results as CERES. Clouds compensate reference asymmetries by 85% (CMIP3), 65% (CMIP5), and 78% (CMIP6) as compared with 98% for CERES. Spatial distributions of hemispheric differences indicate clear improvements across the CMIP phases. Remaining all-sky reflection asymmetries predominantly result from too-small, partly compensating cloud asymmetries: a too-weak enhancement of the reference asymmetry in the tropical Atlantic and eastern Pacific Oceans is accompanied by a too-weak compensation by extratropical clouds. Thus, tropical clouds and extratropical storm track regions are largely responsible for the compensation of hemispheric clear-sky asymmetries in CERES and CMIP, and for remaining biases in the GCMs. An unexpected result is the magnitude of model biases in the clear-sky asymmetries, which potentially condition systematic cloud biases. Experiments testing cloud-controlling factors influencing hemispheric asymmetry could help us to understand what drives hemispheric cloud differences.

Correction: Sun et al. A Method to Estimate Clear-Sky Albedo of Paddy Rice Fields. Remote Sens. 2022, 14, 5185

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The implications of maintaining Earth's hemispheric albedo symmetry for shortwave radiative feedbacks

Abstract. The Earth's albedo is observed to be symmetric between the hemispheres on the annual mean timescale, despite the clear-sky albedo being asymmetrically higher in the Northern Hemisphere due to more land area and aerosol sources; this is because the mean cloud distribution currently compensates for the clear-sky asymmetry almost exactly. We investigate the evolution of the hemispheric difference in albedo in the Coupled Model Intercomparison Project Phase 6 (CMIP6) coupled model simulations following an abrupt quadrupling of CO2 concentrations, to which all models respond with an initial decrease of albedo in the Northern Hemisphere (NH) due to loss of Arctic sea ice. Models disagree over whether the net effect of NH cloud responses is to reduce or amplify initial NH albedo reductions. After the initial response, the evolution of the hemispheric albedo difference diverges among models, with some models remaining stably at their new hemispheric albedo difference and others returning towards their pre-industrial difference primarily through a reduction in SH cloud cover. Whereas local increases in cloud cover contribute to negative shortwave cloud feedback, the cross-hemispheric communicating mechanism found to be primarily responsible for restoring hemispheric symmetry in the models studied implies positive shortwave cloud feedback.

The role of baroclinic activity in controlling Earth’s albedo in the present and future climates

Clouds are one of the most influential components of Earth's climate system. Specifically, the midlatitude clouds play a vital role in shaping Earth's albedo. This study investigates the connection between baroclinic activity, which dominates the midlatitude climate, and cloud-albedo and how it relates to Earth's existing hemispheric albedo symmetry. We show that baroclinic activity and cloud-albedo are highly correlated. By using Lagrangian tracking of cyclones and anticyclones and analyzing their individual cloud properties at different vertical levels, we explain why their cloud-albedo increases monotonically with intensity. We find that while for anticyclones, the relation between strength and cloudiness is mostly linear, for cyclones, in which clouds are more prevalent, the relation saturates with strength. Using the cloud-albedo strength relationships and the climatology of baroclinic activity, we demonstrate that the observed hemispheric difference in cloud-albedo is well explained by the difference in the population of cyclones and anticyclones, which counter-balances the difference in clear-sky albedo. Finally, we discuss the robustness of the hemispheric albedo symmetry in the future climate. Seemingly, the symmetry should break, as the northern hemisphere's storm track response differs from that of the southern hemisphere due to Arctic amplification. However, we show that the saturation of the cloud response to storm intensity implies that the increase in the skewness of the southern hemisphere storm distribution toward strong storms will decrease future cloud-albedo in the southern hemisphere. This complex response explains how albedo symmetry might persist even with the predicted asymmetric hemispheric change in baroclinicity under climate change.

Improved cloudy-sky snow albedo estimates using passive microwave and VIIRS data

Land surface albedo (LSA) is an essential component of the surface radiation budget, and has been retrieved extensively as a basic remote sensing product; however, daily LSA products suffer from extensive data gaps primarily caused by cloud cover. Accordingly, several gap-filling methods were developed (e.g., spatiotemporal interpolation and data fusion with albedo climatology), although the traditional methods are limited by cloud scale and surface heterogeneity. Further, as the largest varying surface landscape feature, seasonal snow cover substantially influences LSA and represents a major uncertainty factor of gap recovery because previous studies failed to employ actual surface signals to capture such ephemeral but intense albedo changes under cloud cover. To address this issue, a three-step framework was proposed for estimating 1 km cloudy-sky LSA using passive microwave (PMW) data, albedo climatology, and Visible Infrared Imaging Radiometer Suite (VIIRS) clear-sky albedo: (1) All-sky snow albedo was estimated from PMW brightness temperatures using a statistical model, (2) Continuous albedo dynamics were generated by combining the all-sky snow albedo with snow-free climatological albedo, and (3) The 1 km cloudy-sky LSA was predicted after filtering 1 km VIIRS clear-sky LSA by the albedo dynamic series. PMW-derived snow albedo was assessed over the Contiguous US (CONUS), and the final 1 km cloudy-sky LSA was validated across 10 sites from SURFRAD and Core AmeriFlux in 2013. Based on the comparison with high-quality MODIS pixels, the estimated snow albedo yielded an overall RMSE of 0.064 over CONUS, with a bias of −0.010 (R2 = 0.845). The recovered 1 km cloudy-sky LSA produced RMSEs of 0.074 (0.137) for all (snow) samples, a significant improvement over the Global Land Surface Satellite (GLASS) gap-free albedo products especially on snow cases (p-value = 0.027). Corresponding RMSE in calculating surface net radiation was also decreased by 38.91 W·m−2; and anomalous snow samples were corrected as well. The temporal analysis and all-sky LSA mapping suggest that the recovered LSA has satisfactory spatiotemporal continuity, and successfully captured details of spatiotemporal variability, especially for ephemeral snow events. This study provides an innovative solution to recover gaps in LSA data, and considerably improves the LSA accuracy under cloud cover, which can inform snow melting modeling, hazard forecasting, and irrigation management.

Persistence and Variability of Earth’s Interhemispheric Albedo Symmetry in 19 Years of CERES EBAF Observations

AbstractDespite the unequal partitioning of land and aerosol sources between the hemispheres, Earth’s albedo is observed to be persistently symmetric about the equator. This symmetry is determined by the compensation of clouds to the clear-sky albedo. Here, the variability of this interhemispheric albedo symmetry is explored by decomposing observed radiative fluxes in the CERES EBAF satellite data record into components reflected by the atmosphere, clouds, and the surface. We find that the degree of interhemispheric albedo symmetry has not changed significantly throughout the observational record. The variability of the interhemispheric difference in reflected solar radiation (asymmetry) is strongly determined by tropical and subtropical cloud cover, particularly those related to nonneutral phases of El Niño–Southern Oscillation (ENSO). As ENSO is the most significant source of interannual variability in reflected radiation on a global scale, this underscores the interhemispheric albedo symmetry as a robust feature of Earth’s current annual mean climate. Comparing this feature in observations with simulations from coupled models reveals that the degree of modeled albedo symmetry is mostly dependent on biases in reflected radiation in the midlatitudes, and that models that overestimate its variability the most have larger biases in reflected radiation in the tropics. The degree of model albedo symmetry is improved when driven with historical sea surface temperatures, indicating that the degree of symmetry in Earth’s albedo is dependent on the representation of cloud responses to coupled ocean–atmosphere processes.

SALBEC – A Python Library and GUI Application to Calculate the Diurnal Variation of the Soil Albedo

AbstractThis study presents the SALBEC – Soil ALBEdo Calculator – a Python library and Graphical User Interface designed to predict the diurnal variation of the clear-sky albedo based on the soil surface properties. Such predictions are becoming more and more necessary with the increasing role of remote measurements. The software uses the following input parameters: the soil spectrum, soil roughness, day of the year (DOY) and sample location. It returns the diurnal albedo variation and, as a unique feature, optimal observation time in the form of tables and graphs as outputs. Models created with the SALBEC were compared with the data acquired under near clear-sky conditions. The comparison shows that the differences between the models and measured data do not exceed the variation of input parameters. The software is directed towards scientists and professionals who require precise estimations of the albedo of soils for different field observation times. Our software is issued as free and open source software (FOSS) and is publicly available athttps://github.com/jarekj71/salbec.

The effect of snow at forest floor on boreal forest albedo diurnal and seasonal variation during the melting season

Seasonal snow cover affects markedly the albedo values of the boreal forest zone. It increases the forest albedo values and changes the diurnal variation of albedo compared to snow-free situations. Snow is also a high scatterer in visible and near-infrared wavelengths, and thus the probability of multiple scattering between forest floor and canopy is high. A photon recollision probability based model called PARAS, where multiple scattering components are included, was used to simulate albedo values of needleleaved evergreen forest with and without snow cover on the forest floor. We focused on two example dates representing typical premelt and melting season snow conditions in clear-sky cases, and multiple overcast days over melting season, and compared the diurnal and seasonal variations to estimate the effect of snow cover and multiple scattering. We found that changing the forest floor surface from snow-free to snow-covered changes the diurnal variation characteristics of the clear-sky albedo from U-shaped to W-shaped. In addition, ignoring the effect of the forest floor or multiple scattering leads to relative underestimation of forest albedo values and reflected solar energy, of up to 80% (up to 1.0 kWh/m2) and 30% (up to 0.25 kWh/m2), respectively. The underestimations depend on the forest floor cover type and the season.

Evaluation of Hemispheric Asymmetries in Marine Cloud Radiative Properties

Abstract The hemispheric symmetry of albedo and its contributing factors in satellite observations and global climate models is evaluated. The analysis is performed on the annual mean time scale, on which a bimodality in the joint distribution of albedo and cloud fraction is evident, resulting from tropical and subtropical clouds and midlatitude clouds, respectively. Hemispheric albedo symmetry is not found in individual ocean-only latitude bands; comparing the Northern and Southern Hemisphere (NH and SH), regional mean albedo is higher in the NH tropics and lower in the NH subtropics and midlatitudes than in the SH counterparts. This follows the hemispheric asymmetry of cloud fraction. In midlatitudes and tropics the hemispheric asymmetry in cloud albedo also contributes to the asymmetry in total albedo, whereas in the subtropics the cloud albedo is more hemispherically symmetric. According to the observations, cloud contributions to compensation for higher clear-sky albedo in the NH come primarily from cloud albedo in midlatitudes and cloud amount in the subtropics. Current-generation climate models diverge in their representation of these relationships, but common features of the model–data comparison include weaker-than-observed asymmetry in cloud fraction and cloud albedo in the tropics, weaker or reversed cloud fraction asymmetry in the subtropics, and agreement with observed cloud albedo asymmetry in the midlatitudes. Models on average reproduce the NH–SH asymmetry in total albedo over the 60°S–60°N ocean but show higher occurrence of brighter clouds in the SH compared to observations. The albedo bias in both hemispheres is reinforced by overestimated clear-sky albedo in the models.

Geographically coherent patterns of albedo enhancement and suppression associated with aerosol sources and sinks

Earth's albedo is the primary determinant of the amount of energy absorbed by the Earth–atmosphere system. It is a function of the fractional cloud cover and the cloudy- and clear-sky albedos, and thereby of the aerosol loading of the atmosphere. Here, we introduce a method by which we can examine the spatial distribution of the albedo variability that is independent of variations in the two dominant factors of albedo: cloud fraction and liquid water path (LWP). The analysis is based on data simultaneously retrieved from the CERES and MODIS instruments carried on board the Aqua satellite. We analysed the daily overpass data between July 2002 and June 2014 and showed that perturbations in albedo, accounting for variations induced by cloud fraction and LWP, display a coherent geographical pattern. Positive deviations occur in proximity to known anthropogenic aerosol sources, and negative deviations coincide with areas of intense precipitation, acting as aerosol sinks. A simple multiplication of the observed positive perturbations in albedo with the solar flux of 340 W m−2 yields a magnitude of that effect of several watts per square meter locally. While the location and scale of the geographical pattern might suggest an anthropogenic contribution to the positive albedo perturbations, it is imperative to first carefully examine all other possible causal factors behind the perturbations. Finally, although we have not attempted a full calculation of detection limits, the analysis is capable of sensing very small changes in average albedo of the order of 0.003 out of a total albedo of the order of 0.3. Hence, the applied method might find utilisation in a variety of situations where there is a need to quantify small perturbations of a dependent variable in noisy global data sets.

Compensation of Hemispheric Albedo Asymmetries by Shifts of the ITCZ and Tropical Clouds

Abstract Despite a substantial hemispheric asymmetry in clear-sky albedo, observations of Earth’s radiation budget reveal that the two hemispheres have the same all-sky albedo. Here, aquaplanet simulations with the atmosphere general circulation model ECHAM6 coupled to a slab ocean are performed to study to what extent and by which mechanisms clouds compensate hemispheric asymmetries in clear-sky albedo. Clouds adapt to compensate the imposed asymmetries because the intertropical convergence zone (ITCZ) shifts into the dark surface hemisphere. The strength of this tropical compensation mechanism is linked to the magnitude of the ITCZ shift. In some cases the ITCZ shift is so strong as to overcompensate the hemispheric asymmetry in clear-sky albedo, yielding a range of climates for which the hemisphere with lower clear-sky albedo has a higher all-sky albedo. The ITCZ shift is sensitive to the convection scheme and the depth of the slab ocean. Cloud–radiative feedbacks explain part of the sensitivity to the convection scheme as they amplify the ITCZ shift in the Tiedtke (TTT) scheme but have a neutral effect in the Nordeng (TNT) scheme. A shallower slab ocean depth, and thereby reduced thermal inertia of the underlying surface and increased seasonal cycle, stabilizes the ITCZ against annual-mean shifts. The results lend support to the idea that the climate system adjusts so as to minimize hemispheric albedo asymmetries, although there is no indication that the hemispheres must have exactly the same albedo.

Evaluating the dominant components of warming in Pliocene climate simulations

Abstract. The Pliocene Model Intercomparison Project (PlioMIP) is the first coordinated climate model comparison for a warmer palaeoclimate with atmospheric CO2 significantly higher than pre-industrial concentrations. The simulations of the mid-Pliocene warm period show global warming of between 1.8 and 3.6 °C above pre-industrial surface air temperatures, with significant polar amplification. Here we perform energy balance calculations on all eight of the coupled ocean–atmosphere simulations within PlioMIP Experiment 2 to evaluate the causes of the increased temperatures and differences between the models. In the tropics simulated warming is dominated by greenhouse gas increases, with the cloud component of planetary albedo enhancing the warming in most of the models, but by widely varying amounts. The responses to mid-Pliocene climate forcing in the Northern Hemisphere midlatitudes are substantially different between the climate models, with the only consistent response being a warming due to increased greenhouse gases. In the high latitudes all the energy balance components become important, but the dominant warming influence comes from the clear sky albedo, only partially offset by the increases in the cooling impact of cloud albedo. This demonstrates the importance of specified ice sheet and high latitude vegetation boundary conditions and simulated sea ice and snow albedo feedbacks. The largest components in the overall uncertainty are associated with clouds in the tropics and polar clear sky albedo, particularly in sea ice regions. These simulations show that albedo feedbacks, particularly those of sea ice and ice sheets, provide the most significant enhancements to high latitude warming in the Pliocene.

Planetary albedo in strongly forced climate, as simulated by the CMIP3 models

In an ensemble of general circulation models, the global mean albedo significantly decreases in response to strong CO2 forcing. In some of the models, the magnitude of this positive feedback is as large as the CO2 forcing itself. The models agree well on the surface contribution to the trend, due to retreating snow and ice cover, but display large differences when it comes to the contribution from shortwave radiative effects of clouds. The “cloud contribution” defined as the difference between clear-sky and all-sky albedo anomalies and denoted as ΔCC is correlated with equilibrium climate sensitivity in the models (correlation coefficient 0.76), indicating that in high sensitivity models the clouds to a greater extent act to enhance the negative clear-sky albedo trend, whereas in low sensitivity models the clouds rather counteract this trend. As a consequence, the total albedo trend is more negative in more sensitive models (correlation coefficient 0.73). This illustrates in a new way the importance of cloud response to global warming in determining climate sensitivity in models. The cloud contribution to the albedo trend can primarily be ascribed to changes in total cloud fraction, but changes in cloud albedo may also be of importance.

Comparison of radiative fluxes at the top of the atmosphere from INSAT and ERBE

An accurate knowledge of radiative fluxes at the Top of the Atmosphere (TOA) is necessary to study the variability of climate. Many geostationary satellites have been measuring radiative fluxes in a narrow spectral band in the infrared and visible regions during the past 30 years. This data will be useful for climate studies if it can be converted to total radiative fluxes. In this paper we demonstrate that the errors in monthly mean Outgoing Longwave Radiation (OLR) at the TOA obtained from the Indian geostationary satellite INSAT-1B is less than 15 W m-2 in most of the regions when compared to the data from the Earth Radiation Budget Experiment (ERBE). This indicates that the conversion of INSAT narrowband flux to broadband flux does not result in large errors. This could be on account of high water vapour content in the Indian region. The error in INSAT OLR has been shown to be dependent on number of images available per month. INSAT albedo has a negative bias over ocean when compared to ERBE on account of the isotropic reflectance assumption. No such bias was noticed over land. This low albedo bias in INSAT was removed by adding a constant term equal to 2% over ocean. It has been shown that the effect of Sun Glint in clear sky albedo can be removed if two images per day are used for the calculation of albedo. The difference in net radiation at the TOA between INSAT and ERBE has been shown to be within 10 W m-2. In some regions, such as Saudi Arabia (from December 1988 to March 1989) the difference between ERBE and INSAT estimated net radiation (~10 W m-2) was found to be less than the difference between two ERBE satellite observations (~20 W m-2). This indicates clearly that diurnal variation of net radiation can cause large errors in the estimates of monthly mean net radiation.

Clear-Sky Albedo Measured at Seven Rangeland Sites in Southwest Idaho

As a component of the energy budget, albedo is used in the calculation of evapotranspiration in many natural resource and hydrologic models. There are few measurements of albedo over sagebrush-dominated rangelands, so albedo was measured at seven sites on the Reynolds Creek Experimental Watershed in southwest Idaho during 1989–1993. For all sites, the average albedo measured during midday under clear skies was 0.14 during the growing season. Albedo varied from a low of 0.11 during June at the Mountain big sagebrush site to a high of 0.17 at the low-elevation Wyoming big sagebrush site. Albedo varied little between about 2 h after sunrise and 2 h before sunset.

The Impact of the Diurnal Variation of Albedo on the Remote Sensing of the Daily Mean Albedo of Grassland

Abstract The correction of a land surface albedo estimate made at one solar zenith angle (SZA) from a polar-orbiting satellite to a standard SZA or to a daily mean albedo requires knowledge of the dependence of the albedo on SZA. This paper uses ground-based measurements of the clear-sky albedo at a uniform grassland site at Uardry (34.39°S, 145.30°E) in southeastern Australia to investigate the accuracy to which the daily mean albedo can be inferred from the albedo at 1030 LST, given knowledge of the SZA dependence of albedo to various levels of detail. During nine months in which the daily mean albedo varied from 0.20 to 0.27, the albedo always had the expected minimum near noon but the strength of the albedo’s SZA dependence varied greatly. For a few months, albedos were up to 0.04 higher in the afternoon than in the morning, and variations on finer timescales of up to 0.02 also appeared in the diurnal albedo cycle for days or weeks. These features of the diurnal variation were all seen at two or three...

The potential for collocated AGLP and ERBE data for fire, smoke, and radiation budget studies

One month of the Advanced Very High Resolution Radiometer (AVHRR) Global Area Coverage (GAC) Land Pathfinder (AGLP) data from September 1985 are used to examine the spatial and temporal distribution of fires over four major ecosystems in South America. The Earth Radiation Budget Experiment (ERBE) scanner data are used to examine the top of atmosphere (TOA) shortwave and longwave fluxes over smoke generated from biomass burning. The relationship between the AGLP-derived Normalized Difference Vegetation Index (NDVI) and the ERBE-estimated clear sky albedos are also examined as a function of the four ecosystems. This study shows that the grassland areas in South America have the highest number of fires for September 1985, and their corresponding NDVI values are smaller than the tropical rainforest region where the number of fires were comparatively small. Clear sky statistics accumulated during the days when smoke was not present show that clear sky albedos derived from ERBE are higher for grassland areas when compared to the tropical rainforest. The results show that the AGLP can be used to determine the spatial and temporal distribution of fires along with vegetation characteristics, while ERBE data can provide necessary information on broadband albedos and regional top of atmosphere radiative impacts of biomass burning aerosols. Since the AGLP data are available from 1981 to the present day, several climate-related issues can be addressed.

Contribution of sea salt aerosol to the planetary clear-sky albedo

Contribution of sea salt aerosol to the planetary clear-sky albedo

Contribution of sea salt aerosol to the planetary clear-sky albedo

In order to understand the response of the earth-atmosphere-aerosol system to climate forcing by greenhouse gases, it is essential to know the relative importance of the different factors within it. The purpose of the present work is to determine how much of the planetary clear sky albedo is due to the layer of sea salt aerosol that exists over the oceans. We calculate 94F (the change in solar flux entering at the top of the atmosphere) due to the presence of the sea salt aerosol layer, then show that this 9F is opposite in sign and comparable in magnitude to the predicted radiative forcing by increased levels of CO2. Since sea salt is a stable component of the atmosphere, its radiative effects are already included in any measurement or calculation of the current planetary albedo. But when studying climates with stronger winds (leading to a greater sea salt aerosol loading), it is important to factor in the greater cooling due to sea salt, as an omission of this effect would lead to an underestimate of the planetary albedo.

WINTER RADIATION EXTINCTION AND REFLECTION IN A BOREAL PINE CANOPY: MEASUREMENTS AND MODELLING

Predicting the rate of snowmelt and intercepted snow sublimation in boreal forests requires an understanding of the effects of snow-covered conifers on the exchange of radiant energy. This study examined the amount of intercepted snow on a jack pine canopy in the boreal forest of central Saskatchewan and the shortwave and net radiation exchange with this canopy, to determine the effect of intercepted snow and canopy structure on shortwave radiation reflection and extinction and net radiation attenuation in a boreal forest. The study focused on clear sky conditions, which are common during winter in the continental boreal forest. Intercepted snow was found to have no influence on the clear-sky albedo of the canopy, the extinction of short wave radiation by the canopy or ratio of net radiation at the canopy top to that at the surface snow cover. Because of the low albedo of the snow-covered canopy, net radiation at the canopy top remains positive and a large potential source of energy for sublimation. The canopy albedo declines somewhat as the extinction efficiency of the underlying canopy increases. The extinction efficiency of short wave radiation in the canopy depends on solar angle because of the approximately horizontal orientation of pine branches. For low solar angles above the horizon, the extinction efficiency is quite low and short wave transmissivity through the canopy is relatively high. As the solar angle increases, extinction increases up to angles of about 50°, and then declines. Extinction of short wave radiation in the canopy strongly influences the attenuation of net radiation by the canopy. Short wave radiation that is extinguished by branches is radiated as long wave, partly downwards to the snow cover. The ratio of net radiation at the canopy top to that at the snow cover surface increases with the extinction of short wave radiation and is negative for low extinction efficiencies. For the pine canopy examined, the daily mean net radiation at the snow cover surface became positive when daily mean solar angles exceeded 22° in late March. Hence, canopy structure and solar angle control the net radiation at the snow cover surface during clear sky conditions and will govern the timing and rate of snowmelt. Models of intercepted snow sublimation and forest snowmelt could beneficially incorporate the canopy radiation balance, which can be extrapolated to stands of various canopy densities, coverage and heights in a physically based manner. Such models could hence avoid 'empirical' temperature index measures that cannot be extrapolated with confidence.