Albedo Asymmetry Research Articles

Titan’s Atmospheric Albedo Asymmetry and Seasonal Variability Observed through the Cassini Imaging Science Subsystem

Using images from Cassini, we analyzed the north–south albedo asymmetry that has been observed in the atmosphere of Saturn’s moon, Titan. Suitable images from the Cassini Imaging Science Subsystem taken at 889 nm spanned from 2004 to 2017—around half of a Titan year—and revealed seasonal changes in the characteristics and orientation of the north–south asymmetry boundary. Such circumglobal features provide insight into the dynamics and circulation of the atmosphere more broadly. The albedo asymmetry has been observed to reverse for part of the Titan year, inverting the brighter and darker hemispheres; we also observed this inversion, along with the formation of additional banding briefly during the transition (around 2014–2016). A tilt in the rotation axis of Titan’s atmosphere with respect to the solid body rotation has previously been noted. Using robust edge-detection techniques, we likewise identified a tilt offset of a few degrees in the albedo transition boundaries. The azimuth of this tilt axis remained roughly fixed in inertial space, with some smaller possible seasonal fluctuations around the fixed direction noted.

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.

Connecting Hemispheric Asymmetries of Planetary Albedo and Surface Temperature

AbstractSatellite measurements show that the Northern and Southern hemispheres reflect equal amounts of shortwave radiation (“albedo symmetry”), but no theory exists on if, how, and why the symmetry is established and maintained. Ambiguously, climate models are strongly biased in albedo symmetry but agree in the sign of the response to CO2. We find that mean‐state biases in albedo symmetry and hemispheric surface temperature asymmetry correlate negatively. Similarly, the response of albedo asymmetry to CO2 forcing correlates negatively with the magnitude of the asymmetry in surface warming. This is true across many and within single climate model simulations: a too warm or stronger warming hemisphere is darker or darkens more than its counterpart. In the 21 years of observations we find the same tendency and hypothesize (a) albedo symmetry is a function of the current climate state and (b) we will observe an evolution toward albedo asymmetry in coming decades.

A Cloud-Controlling Factor Perspective on the Hemispheric Asymmetry of Extratropical Cloud Albedo

Abstract The Northern and Southern Hemispheres reflect on average almost equal amounts of sunlight due to compensating hemispheric asymmetries in clear-sky and cloud albedo. Recent work indicates that the cloud albedo asymmetry is largely due to clouds in extratropical oceanic regions. Here, we investigate the proximate causes of this extratropical cloud albedo asymmetry using a cloud-controlling factor (CCF) approach. We develop a simple index that measures the skill of CCFs, either individually or in combination, in predicting the asymmetry. The index captures the contribution to the asymmetry due to interhemispheric differences in the probability distribution function of daily CCF values. Cloud albedo is quantified using daily MODIS satellite retrievals, and is related to range of CCFs derived from the ERA5 product. We find that sea surface temperature is the CCF that individually explains the largest fraction of the asymmetry, followed by surface wind. The asymmetry is predominantly due to low clouds, and our results are consistent with prior local-scale modeling work showing that marine boundary layer clouds become thicker and more extensive as surface wind increases and surface temperature cools. The asymmetry is consistent with large-scale control of storm-track intensity and surface winds by meridional temperature gradients: persistently cold and windy conditions in the Southern Hemisphere keep cloud albedo high year-round. Our results have important implications for global-scale cloud feedbacks and contribute to efforts to develop a theory for planetary albedo and its symmetry.

Anthropogenic aerosol and cryosphere changes drive Earth’s strong but transient clear-sky hemispheric albedo asymmetry

A striking feature of the Earth system is that the Northern and Southern Hemispheres reflect identical amounts of sunlight. This hemispheric albedo symmetry comprises two asymmetries: The Northern Hemisphere is more reflective in clear skies, whereas the Southern Hemisphere is cloudier. Here we show that the hemispheric reflection contrast from differences in continental coverage is offset by greater reflection from the Antarctic than the Arctic, allowing the net clear-sky asymmetry to be dominated by aerosol. Climate model simulations suggest that historical anthropogenic aerosol emissions drove a large increase in the clear-sky asymmetry that would reverse in future low-emission scenarios. High-emission scenarios also show decreasing asymmetry, instead driven by declines in Northern Hemisphere ice and snow cover. Strong clear-sky hemispheric albedo asymmetry is therefore a transient feature of Earth’s climate. If all-sky symmetry is maintained, compensating cloud changes would have uncertain but important implications for Earth’s energy balance and hydrological cycle.

Earth’s Albedo and Its Symmetry

AbstractThe properties of Earth's albedo and its symmetries are analyzed using twenty years of space‐based Energy Balanced And Filled product of Clouds and the Earth's Radiant Energy System measurements. Despite surface asymmetries, top of the atmosphere temporally & hemispherically averaged reflected solar irradiance appears symmetric over Northern/Southern hemispheres. This is confirmed with the use of surrogate time‐series, which provides margins of for possible hemispheric differences supported by Clouds and Earth's Radiant System data. time‐series are further analyzed by decomposition into a seasonal (yearly and half yearly) cycle and residuals. Variability in the reflected solar irradiance is almost entirely (99%) due to the seasonal variations, mostly due to seasonal variations in insolation. The residuals of hemispherically averaged are not only small, but also indistinguishable from noise, and thus not correlated across hemispheres. This makes yearly and sub‐yearly timescales unlikely as the basis for a symmetry‐establishing mechanism. The residuals however contain a global trend that is large, as compared to expected albedo feedbacks. It is also hemispherically symmetric, and thus indicates the possibility of a symmetry enforcing mechanism at longer timescales. To pinpoint precisely which parts of the Earth system establish the hemispheric symmetry, we create an energetically consistent cloud‐albedo field from the data. We show that the surface albedo asymmetry is compensated by asymmetries between clouds over extra‐tropical oceans, with southern hemispheric storm‐tracks being 11% cloudier than their northern hemisphere counterparts. This again indicates that, assuming the albedo symmetry is not a result of chance, its mechanism likely operates on large temporal and spatial scales.

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.

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.

Diurnal and seasonal variations of surface albedo in a spring wheat field of arid lands of Northwestern China

Surface albedo greatly affects the radiation energy balance of croplands and is a significant factor in crop growth monitoring and yield estimation. Precise determination of surface albedo is thus important. This study aimed to examine the influence of growth stages (tillering, jointing, heading, filling and maturity) on albedo and its diurnal asymmetry by measuring diurnal albedo variations. Results indicated that the daily mean surface albedo generally exhibited an increased tendency during tillering to heading but decreased after heading. Surface albedos were much higher in the morning than the corresponding values of the same solar elevation angles in the afternoon when the solar elevation angle was less than 40°, indicating a diurnal asymmetry in surface albedo. However, less difference was found in surface albedos between forenoon and afternoon when the solar elevation angle was greater than 40°. Dew droplets on the leaf surface in the morning were assumed to be the main factor resulting in the diurnal asymmetry in albedo of spring wheat.

Determining a tilt in Titan’s north–south albedo asymmetry from Cassini images

Abstract Analysis of Titan’s hemispheric brightness asymmetry from mapped Cassini images reveals an axis of symmetry that is tilted with respect to the rotational axis of the solid body. Twenty images taken from 2004 through 2007 show a mean axial offset of 3.8 ± 0.9° relative to the solid body’s pole, directed 79 ± 24° to the west of the sub-solar longitude. These values are consistent with recent measurements of an implied atmospheric spin axis determined from isothermal mapping by [Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A., 2008. Icarus 197, 549–555].

Saturnian icy satellites: Disk-integrated observations of the brightness opposition surge at low phase angles

Photopolarimetric observations of Saturn's satellites Enceladus, Tethys, Dione, and Rhea were obtained with the 2 m RCC telescope at the National Astronomical Observatory, Rozhen, in two different phase-angle regions (from 0.07° to 0.01° and from 5.6° to 5.2°). In this paper we present the first results of disk-integrated satellite photometry including mutually consistent solar and rotational phase curves in the red and near infrared bands ( λ=694 and 889 nm). Enceladus, Tethys, Dione, and Rhea demonstrate a sharp brightness increase in the narrow backscattering domain of the phase angles (between 0.7° and 0.01°) revealing presence of a narrow component of the brightness opposition effect. The slopes of the low phase-angle portions of the moons’ phase curves are about 0.2 mag/deg, that is at the level of or even exceeds the slope of Europa's phase curve at opposition. The amplitudes of the moons’ opposition effects, measured by the total increase in the brightness as the phase angle decreased from 5° to 0°, are largely consistent with contribution of the coherent backscattering, which originated from the fraction of scatterers with sizes comparable to the wavelength of incident radiation. At the same time, the dependence of the relative strength of the moons’ brightening on their albedos, which become noticeable at the near infrared, points out that a shadow hiding component can also contribute to the opposition effects of these icy moons. The satellites’ rotation curves confirm that the leading/training albedo asymmetry known from previous visual observations extends into red and near infrared. The amplitudes of hemispherical asymmetries in the red and near infrared are quite similar and amount to 4%, 8%, 38%, and 18% for Enceladus, Tethys, Dione and Rhea, respectively.

Radar detection of Iapetus.

Saturn's satellite Iapetus has the largest albedo asymmetry of any natural satellite, with the optical albedo of its trailing hemisphere as much as 10 times that of its leading hemisphere ([ 1 ][1]). The sharp contrast on this synchronously rotating satellite suggests that exogenic materials, such

Latitudinal transport by barotropic waves in Titan's stratosphere.: II. Results from a coupled dynamics–microphysics–photochemistry GCM

We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude–latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and C 2H 2, as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North–South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions.

Probing Titan's Atmosphere with the 1995 August Stellar Occultation

On 1995 August 21, Saturn's moon Titan occulted the 13th magnitude star GSC5254-00997. The predawn event was observed at 2.3 μm with the NASA 3.0-m Infrared Telescope Facility. We measured the average atmospheric scale height between latitudes 50°S and 67°S by fitting isothermal models to the immersion portion of the occultation lightcurve. Within the altitude range of 300 to 500 km, we find an isothermal scale height of Titan's sunset atmosphere of 55±9 km. Our derived temperature of 180±30 K is consistent with modeled temperature profiles derived from Voyager 1 IRIS observations (A. Coustenis and B. Bézard 1995, Icarus 115, 126–140) and the 28 Sagittarii occultation event (W. B. Hubbard et al. 1993, Astron. Astrophys. 269, 541–563). The atmospheric albedo asymmetry due to low level haze (Caldwell et al. 1992, Icarus 96, 1–9) is inferred from the difference in Titan's center-of-light (determined from precise astrometric measurements of images of Titan and the star before and after the occultation) and the actual center of Titan (determined from the occultation analysis). We find the southern hemisphere to be brighter than the northern hemisphere, a result consistent with Combes et al. (1997, Icarus 129, 482–497), Gibbard et al. (1999, Icarus 139, 189–201), and Smith et al. (1996 Icarus, 119, 336–349).

Leading/Trailing Albedo Asymmetries of Thebe, Amalthea, and Metis

Using Galileo clear-filter images (effective wavelength ≈0.64 μm), we have created the first albedo maps of the small inner jovian satellites Thebe, Amalthea, and Metis. These maps clearly show that the leading sides of all three satellites are significantly brighter than their corresponding trailing sides, confirming and extending a result first reported by P. C. Thomas et al. (1998, Icarus 135, 360–371). In particular, on all three moons the leading side is brighter than the trailing side by 25 to 30%. The fact that the direction and size of this albedo asymmetry is identical from satellite to satellite suggests that one common physical mechanism is governing the global albedo patterns of all three moons. The most plausible such mechanism is the impact of macroscopic meteoroids that originated outside the jovian system. These impacts, which eject the dust that forms Jupiter's ring system (M. E. Ockert-Bell et al., 1999, Icarus 138, 188–213; J. A. Burns et al., 1999, Science 284, 1146–1150), are probably also responsible for brightening the leading sides of these small satellites.

Average path-length parameter of diffuse light in scattering media

We use Monte Carlo simulations to study in detail the propagation of light in a plane-parallel medium containing scattering particles. In particular, we compute the forward and backward average path-length parameters (FAPP and BAPP, respectively) of four-flux radiative transfer models as functions of the optical depth. Strong dependence on the single scattering albedo and phase function asymmetry is found for both quantities. In general the values of the FAPP decrease with increasing absorption, whereas the opposite occurs for the BAPP. A similar effect is produced when changing from isotropic phase functions to phase functions with a large asymmetry in the forward direction. We present analytical results for the asymptotic values of the FAPP and BAPP as functions of albedo for the particular case of isotropic scattering. Our results differ markedly from the predictions obtained recently with two multiple-scattering models by Vargas and Niklasson [J. Opt. Soc. Am. A 14, 2243 (1997); Appl. Opt. 36, 3735 (1997)]. The differences found point out the intrinsic limitations of these models.

Diurnal asymmetry in surface albedo

Observed surface albedos do not always exhibit the classical diurnal symmetry associated with the diurnal variation of zenith angle. Morning/afternoon albedo differences observed in even vegetated terrain are found to range on the daily average from 0.01 to 0.05 in magnitude in this study, and they have been found to be as high as 0.1 at a given solar zenith angle in other studies, which influence both reflected and absorbed solar radiation during the course of a day. This diurnal asymmetry in albedo may be explained by the prevailing wind direction, which causes the canopy to lean. The albedo asymmetry also is simulated, taking into account the canopy reclination angle and direction.

Condensation of Nitrogen: Implications for Pluto and Triton

We have observed in the laboratory that nitrogen condenses initially as a transparent layer if the deposition rate is less than about 3 μm/hr. To correlate our laboratory results with the Voyager observations of Triton and ground-based observations of Pluto, we have calculated the nitrogen deposition rates on these bodies. The maximum deposition rate for Pluto (with 40K icy surface temperature) is calculated to be about 1.65 μm/hr and for Triton is ≈1.38 μm/hr at 38 K. This implies that nitrogen on these bodies may preferentially condense as a transparent layer. Voyager 2 observations in August 1989 revealed that Triton's northern hemisphere had an overall lower albedo than the southern hemisphere. This was surprising since at that time deposition of fresh N2frost should have taken place over most of Triton's northern hemisphere (subsolar point was 45°S). In contrast, the subliming southern cap was seen as a bright feature. Our new approach to this problem is based on the combination of the energy balance calculations and the laboratory results. We explain the darker northern hemisphere by the condensation of an initially transparent layer on a dark substrate, which might be dark organics produced by UV photolysis of CH4ice. The bright southern cap can be a result of N2shattering due to the earlier passage of the cubic–hexagonal phase transition fronts. If there is a similar puzzling north/south albedo asymmetry on Pluto, as indicated by two independent sets of observations (M. W. Buie, D. J. Tholen, and K. Horne, 1992,Icarus97, 211–227; E. F. Young and R. P. Binzel, 1993,Icarus102, 134–149), our results offer an explanation for it. We also infer an original grain size of N2ice on Triton and, possibly, on Pluto. The role of impurities is discussed.

Asymmetry in the diurnal variation of surface albedo

Remote sensing of surface properties and estimation of clear-sky and surface albedo generally assume that the albedo depends only on the solar zenith angle. The effects of dew, frost, and precipitation as well as evaporation and wind can lead to some systematic diurnal variability resulting in an asymmetric diurnal cycle of albedo. This paper examines the symmetry of both surface-observed and top-of-the-atmosphere (TOA) albedos derived from satellite data. Broadband surface albedos were measured at the Department of Energy Atmospheric Radiation Measurement (ARM) Program Southern Great Plains Central Facility near Lamont, Oklahoma and several extended facilities. GOES satellite radiance data are converted to broadband albedo using bidirectional reflectance functions and an empirical narrowband-to-broadband relationship. The surface and top-of-atmosphere albedos vary in a consistent fashion during both the morning and afternoon. The initial results indicate that surface moisture, probably in the form of dew, has a significant effect and can change the albedo by 10% at a given solar zenith angle between the morning and afternoon. Wind speed is well correlated with the diurnal albedo asymmetry. Light winds and small dew point depressions are associated with the greatest morning/afternoon albedo differences. Aerosols tend to moderate those differences. Changes in the surface properties from dew may alter the bidirectional reflectance characteristics of the scene, affecting the interpretation of remote sensing data. Errors in the diurnally averaged albedos derived from Sun-synchronous satellite measurements that arise from albedo asymmetry are generally less than 3%. Further examination of surface albedo asymmetry is needed to assess its influence on satellite measurements and the surface energy budget over a range of land surface types.

Spectrophotometry and Organic Matter on Iapetus: 2. Models of Interhemispheric Asymmetry

The albedo asymmetry of Iapetus is unique in the Solar System. Many models have been proposed to explain why the leading hemisphere has a reflectance 10–20 times lower than the trailing hemisphere and the poles. Compositional and observational constraints appear to rule out many of these models. Little attempt has been made to explain why, of all the moons in the Solar System, only Iapetus displays such properties. The photometric differences between Phoebe and the dark material on Iapetus, and the expectation that accreted dust from Phoebe would darken the poles more than the antapex of orbital motion on Iapetus, make the Phoebe dust model inconsistent with observations. Other models that instead have Iapetus accreting circumsaturnian material coming from either Hyperion or Iapetus itself are also beset by an inability to explain the bright poles. The endogenous extrusion of dark organics, by itself, is ruled out, since it is extremely unlikely that this model would produce elliptical albedo contours centered on the apex of orbital motion. The modern impact flux does not deliver enough kinetic energy to Iapetus's surface to vaporize significant quantities of water ice and produce a lag deposit. Also, impact vaporization is expected to form a similar lag deposit on the trailing hemisphere. Our most optimistic estimate of the amount of organic material that accumulates on the surface fromin situradiation synthesis of solid organics from methane clathrate is more than sufficient to explain the albedo and spectrum. This process, however, is more effective on the trailing than leading hemisphere, so the reverse asymmetry is expected.Iapetus's asymmetry is best explained by a thick primordial low-albedo subsurface layer of organics exhumed by impact erosion. The layer may be the consequence of ancient thermochemistry in Iapetus's interior, processing methane and/or HCN into dark organic matter which was then extruded over the entire surface. The model requires a thin layer of nearly pure water ice, either exogenous or endogenous, deposited over the dark organics. Subsequent impacts would have preferentially eroded the ice on the leading hemisphere, revealing the underlying dark material. The uniqueness of Iapetus's albedo asymmetry can be understood by its impact-induced erosion rate, size, and formation distance from the Sun and Saturn—a combination of parameters duplicated nowhere else in the Solar System. Tests of these conclusions can be made by Cassini.