Broadband Albedo Research Articles

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.

Effects of Aerosol and Horizontal Inhomogeneity on the Broadband Albedo of Marine Stratus: Numerical Simulations

Recent estimates of the effect of increasing amounts of anthropogenic sulfate aerosol on the radiative forcing of the atmosphere have indicated that its impact may be comparable in magnitude to the effect from increases in CO2. Much of this impact is expected from the effects of the aerosol on cloud microphysics and the subsequent impact on cloud albedo. However, internal horizontal variations in cloud optical properties are also known to affect cloud albedo. A solar broadband version of a 2D radiative transfer model was used to quantify the impact of enhanced aerosol concentrations and horizontal inhomogeneity on the solar broadband albedo of marine stratus. The 2D cross sections of cloud physics data taken from a set of 3D RAMS/LES simulations of marine stratus provided realistic optical property data for radiative transfer simulations. A control run using typical marine CCN concentrations and a sensitivity study using enhanced concentrations of CCN were examined. The results of the radiative transfer calculations indicated that in unbroken marine stratus clouds the net horizontal transport of photons over a domain of a few kilometers was nearly zero, and the domain-average broadband albedo computed in a 2D cross section was nearly identical to the domain average calculated from a series of independent pixel approximation (IPA) calculations of the same cross section. However, the horizontal inhomogeneity does affect the cloud albedo compared to plane-parallel approximation (PPA) computations due to the nonlinear relationship between albedo and optical depth. The reduction in cloud albedo could be related to the variability of the distribution of log (cloud optical depth). These results extend the findings of Cahalan et al. to broadband solar albedos in a more realistic cloud model and suggest that accurate computation of domain-averaged broadband albedos in unbroken (or nearly unbroken) marine stratus can be made using IPA calculations with ID radiative transfer models. Computations of the mean albedo over portions of the 3D RAMS domain show the relative increase in cloud albedo due to a 67% increase in the boundary-layer average CCN concentration was between 6% and 9%. The effects of cloud inhomogeneity on the broadband albedo as measured from the PPA bias ranged from 3% to 5%.

The surface albedo of the Greenland ice sheet: satellite-derived and in situ measurements in the Søndre Strømfjord area during the 1991 melt season

AbstractThe temporal and spatial variation in the surface albedo of the Greenland ice sheet during the ablation season of 1991 is investigated. The study focuses on an area east of Søndre Strømfjord measuring 200 km by 200 km and centred at 67°5′ N, 48° 13′W. The analysis is based on satellite radiance measurements carried out by the Advanced Very High Resolution Radiometer (AVHRR). The broad-band albedo is estimated from the albedos in channel 1 (visible) and channel 2 (near-infrared). The results are calibrated with the surface albedo of sea and dry snow.Satellite-derived albedos are compared with GIMEX ground measurements at three stations. There is a high degree of consistency in temporal variation at two of the three stations. Large systematic differences are attributed to albedo variations on sub-pixel scale.In the course of the ablation season four zones appear, each parallel to the ice edge. It is proposed that these are, in order of increasing altitude: (I) clean and dry ice, (II) ice with surface water, (III) superimposed ice, and (IV) snow. An extensive description of these zones is given on the basis of the situation on 25 July 1991. Zones I, III and IV reveal fairly constant albedos (0.46, 0.65 and 0.75 on average), whereas zone II is characterised by an albedo minimum (0.34). Survey of the western margin of the Greenland ice sheet (up to 71° N) shows that the zonation occurs between 66° and 70° N.

The surface albedo of the Greenland ice sheet: satellite-derived and in situ measurements in the Søndre Strømfjord area during the 1991 melt season

AbstractThe temporal and spatial variation in the surface albedo of the Greenland ice sheet during the ablation season of 1991 is investigated. The study focuses on an area east of Søndre Strømfjord measuring 200 km by 200 km and centred at 67°5′ N, 48° 13′W. The analysis is based on satellite radiance measurements carried out by the Advanced Very High Resolution Radiometer (AVHRR). The broad-band albedo is estimated from the albedos in channel 1 (visible) and channel 2 (near-infrared). The results are calibrated with the surface albedo of sea and dry snow.Satellite-derived albedos are compared with GIMEX ground measurements at three stations. There is a high degree of consistency in temporal variation at two of the three stations. Large systematic differences are attributed to albedo variations on sub-pixel scale.In the course of the ablation season four zones appear, each parallel to the ice edge. It is proposed that these are, in order of increasing altitude: (I) clean and dry ice, (II) ice with surface water, (III) superimposed ice, and (IV) snow. An extensive description of these zones is given on the basis of the situation on 25 July 1991. Zones I, III and IV reveal fairly constant albedos (0.46, 0.65 and 0.75 on average), whereas zone II is characterised by an albedo minimum (0.34). Survey of the western margin of the Greenland ice sheet (up to 71° N) shows that the zonation occurs between 66° and 70° N.

Impact of Scene Dependence on AVHRR Albedo Models

Abstract Narrowband observations from NOAA's Advanced Very High Resolution Radiometer (AVHRR) are used operationally by NOAA to estimate the earth's broadband planetary albedo. Since May of 1988; these broadband albedo estimates have been derived using the two-channel (visible and near-infrared), scene-independent regression model of Wydick et al. The occurrence of relatively large regional bias errors using this model has led to a study of scene-dependent models that substantially reduce these errors. Three classes of scene stratification are considered: 1) by surface geography type alone (SFC); and 2) and 3) by surface geography type in combination with cloud amount category (SCN) and normalized difference albedo index (NDAI) using AVHRR channels 1 and 2. These and the Wydick model are applied to independent AVHRR global data of 2 July 1985 (hereafter July) and 9 January 1986 (hereafter January). Using ERBE (Earth Radiation Budget Experiment) data as a reference, errors in reflected flux are computed fo...

Satellite Determination of Stratus Cloud Microphysical Properties

Abstract Satellite measurements of liquid water path from SSM/I, broadband albedo from ERBE, and cloud characteristics from ISCCP are used to study stratus regions. An average cloud liquid water path of 0.120 ± 0.032 kg m−2 is derived by dividing the average liquid water path for stratus areas by the fractional area coverage of cloud in the region. The diurnal range in this average cloud liquid water path is about 25%. Stratus cloud liquid water is positively correlated with cloud amount and is negatively correlated with low cloud-top temperature. Cloud liquid water path (LWP) and cloud albedo measurements are used to derive an effective droplet radius using the plane-parallel cloud albedo model of Slingo. The 2.5;dg by 2.5;dg grid boxes are first screened for completely overcast scenes in an attempt to justify the plane-parallel assumption. The mean effective droplet radius for this sample is 10.1 ± 4.4 μm. This serves as an upper bound since small-scale LWP variability is estimated to affect the average...

Narrow-band to broad-band conversion for Meteosat-visiible channel and broad-band albedo using both AVHRR-1 and -2 channels

Abstract Satellite-derived broad-band albedo offers a useful tool for monitoring surface conditions. Given the limited wavelength window of most satellite radiometers, satellite albedo studies need to define narrow-band to a broad-band transformations. Signals from the AVHRR channels on board the NOAA-11 satellite, the Meteosat visible channel and a rectangular spectral band from 0-3 to 2.5 μm were simulated for a set of 20 representative land surfaces. The radiative transfer code described in Tanre et al. (1990) was used to obtain top of the atmosphere radiances. The derived signals were then correlated to predict broadband albedo and the Meteosal response via the two AVHRR signals. The results indicated that the use of the two AVHRR bands makes the narrow-band to broad-band conversion independent of the surface type. Finally, the regression was applied to two concurrent AVHRR and Meteosat images so as to compare a broad-band AVHRR-derived top of the atmosphere (TOA) albedo for the Meteosat-band with a c...

Estimation of surface albedo from space: A parameterization for global application

This study proposes a simple parameterization to derive surface broadband albedo from satellite observations, using the results of detailed radiative transfer computations for a variety of atmospheric and surface conditions. The end result is a single equation that directly yields surface albedo from observed albedo at the top of the atmosphere, solar zenith angle, and total precipitable water. It was demonstrated that the parameterization is valid for retrieval of both instantaneous and daily mean surface albedo. Sensitivity tests were conducted for precipitable water, aerosol, CO2, O3, and temperature profile. Preliminary validation using collocated satellite and tower measurements indicates that the absolute accuracy requirement of 5% for climate studies is well satisfied. A global monthly climatology (excluding polar areas) of surface albedo is then developed from 5 years of Earth Radiation Budget Experiment clear‐sky satellite data and European Centre for Medium‐Range Weather Forecasts humidity analysis data. Examination of month‐to‐month differences for specific 2.5°×2.5° areas indicate that the absolute random error on monthly estimates is less than 1%. Seasonal variation of surface albedo exceeding 1% can thus be detected. Comparisons with other satellite estimates show much closer agreement than with the values used in some general circulation models and numerical weather prediction models especially over snow/ice and deserts.

Shortwave, longwave, and net cloud‐radiative forcing as determined from Nimbus 7 observations

We studied the effect of clouds on the Earth's radiation budget at the top of the atmosphere. Both the northern hemisphere summer and winter are considered in terms of cloud‐radiative forcing, which describes the total effect of clouds on the Earth's radiation budget components. The cloud forcing is obtained by using statistically generated models correlating simultaneous observations of cloud properties and radiation budget parameters obtained from observations from Nimbus 7 satellite observations. The models estimate the broadband albedo and longwave flux throughout the range of variation, including periods of extensive to near‐total cloud cover where more simplistic representations fail. As such, the models are used to estimate period‐averaged clear‐sky radiation budget components. Knowledge of the cloudy‐sky and clear‐sky radiation budget components permits the computation of cloud‐radiative forcing. We show that the net effect of clouds is to cool the Earth. On the average, for an Earth observed to be 52.1% cloud covered, the net cloud forcing is −26.8 Wm−2. This net cooling is obtained from a longwave warming of 24.1 Wm−2 and a shortwave cooling of −50.9 Wm−2. Over much of the Earth there is a general compensation between the warming and cooling effects of clouds. However, over the middle‐ and high‐latitude oceans the albedo effect of clouds is dominant. This is particularly true in each summer hemisphere, where the effects of persistent cloudiness, low oceanic albedos, and high solar insolation combine to drive the net cloud forcing strongly negative. The net effect of cloud forcing cools the Earth, primarily due to the effect of clouds in the summer hemisphere oceans.

Diurnal Variations of Outgoing Longwave Radiation and Albedo from ERBE Scanner Data

The scanning instruments of the Earth Radiation Budget Experiment provide measurements of instantaneous broadband albedo and outgoing longwave radiation (OLR) with a spatial resolution of about 50 km. Data from the Earth Radiation Budget Satellite (ERBS), which is in an orbit that precesses through local time at the rate of one hour every three days, can be used to describe the mean, hourly diurnal variations in the distribution of OLR and albedo on this scale. Much of this variation is caused by cloud type and amount changes. Two-dimensional histograms show the coevolution of OLR and albedo with the diurnal cycle, and the distribution of albedo-OLR pairings associated with the cloud distribution in a particular region and season. The albedo-OLR pairing characterizes a cloud type and determines its net effect on the energy balance at the top of the atmosphere. Diurnal variations in cloud type and amount in many regions are sufficient to cause substantial errors in radiation budget quantities and cloud properties estimated from observations taken from a single sun-synchronous orbit. Errors in estimated net radiation can be as large as 50 W/sq m for oceanic stratus regions and for land regions during summer.

Application of angular models to AVHRR data for determination of the clear‐sky planetary albedo over land surfaces

Clear‐sky planetary albedos are derived from NOAA 9 Advanced Very High Resolution Radiometer (AVHRR) data during 1986 over the U.S. Great Plains with several distinct surface types (shrubland, grassland, deciduous, and coniferous forests). The cloud‐free observations are selected using a combination of absolute threshold and spatial coherence cloud‐screening procedures. The broadband albedo is approximated from the narrow‐band visible and near‐infrared albedo. Bidirectional reflectivity models are applied to account for the effect of Sun‐target‐sensor geometry. The corrected albedos are converted to daily means using a directional model to account for the dependence upon Sun angle. The results indicate that the present Earth radiation budget models can be efficiently applied to AVHRR data to determine the clear‐sky planetary albedos and to remove the apparent albedo discontinuity on images of mapped data.

Determination of the clear-sky planetary albedo from AVHRR using a bi-directional reflectance model

Clear-sky planetary albedos are derived from NOAA-9 AVHRR data during 1986 over the U.S. Great Plains with several distinct surface types (shrubland, grassland, deciduous and coniferous forests). The cloud-free observations are selected using a combination of absolute threshold and spatial coherence cloud-screening procedures. The broadband albedo is approximated from the narrow-band visible and near-infrared albedos. Bi-directional reflectivity models are applied to account for the effect of Sun-target-sensor geometry. The corrected albedos are converted to daily means using a directional model to account for the dependence upon sun angle. The results indicate a significant reduction in variability of the observed albedos. The efficiency of the model application is demonstrated on two examples: 1) using time series of cloud-free observations over one target and 2) using one daily image of mapped data in July, 1986.

The Relative Merits of Narrowband Channels for Estimating Broadband Albedos

Abstract Observations made with the current and proposed narrowband shortwave channels aboard the NOAA series of satellites were simulated for a number of different surfaces (ocean, vegetative land, desert, cloud and snow) using the ATRAD radiation model to study the relative merit of each channel and, in various combinations to predict the broadband albedo. Solar zenith angles were varied over the range from 0 to 60 degrees. The results indicated that for all of the surfaces considered there would be no significant difference in predicting the broadband albedo with either the current (0.58–0.68 μn) or proposed (0.58–0.68 μm) channel 1 of the AVHRR. The proposed narrower channel 2(0.84–0.87 μm), however, would be a better predictor than the current wider channel 2(0.725–1.0 μm). Channel 1 is better than channel 2 for surfaces of low or moderate reflectivity, while over snow, the error in using channel 2 would be less than half of that for channel 1. Combining channels 1 and 2 would reduce the error by abo...

A Simple Method for Estimating Monthly Mean Albedo of Land Surfaces From AVHRR Data

It is suggested that the observed periodicity of cloud-free, visible and near-infrared data from the Advanced Very High Resolution Radiometer (AVHRR) onboard NOAA-9 for a particular target can be efficiently used for deriving the monthly mean clear-sky planetary albedo. The broadband albedo is approximated by a linear combination of visible and near-infrared albedos. The new method of 9-day compositing distributes the clear-sky observations over the sun-target-sensor geometry combinations and weights them by the occurrence of each combination during a particular month. It is shown that the monthly mean clear-sky planetary albedo can be estimated solely from the visible and near-infrared data in a model-independent manner. The surface albedo is then obtained by applying a simple atmospheric correction to the derived clear-sky planetary albedo. A comparison of the results of the present method and those obtained by using the “minimum albedo” method indicates that the latter may lead to an underestimation of monthly values because of the angular variation in clear-sky observed albedos. The derived surface albedos are compared with those reported in climatological studies based on ground observations.

Simulations of the GOES visible sensor to changing surface and atmospheric conditions

Numerical experiments have been conducted to simulate the GOES Visible and Infrared Spin Scan Radiometer (VISSR) visible sensor's response under varying surface and atmospheric conditions, as a function of solar zenith angle. The possible bias in the information obtained with this limited spectral response sensor was assessed by comparing the narrow‐band filtered clear‐sky planetary albedo, as observable with the VISSR, with the broadband (0.3–2.5 μm) unfiltered planetary albedo under the same environmental conditions. Four cases of wavelength‐dependent surface albedo (snow, meadow, dry sand, and water) and three atmospheric conditions have been simulated. It was demonstrated that the relationship between the filtered and the broadband planetary albedo depends primarily on the assumptions made about the magnitude and wavelength dependence of the surface albedo and, to a lesser extent, on the atmospheric conditions. Under certain circumstances (e.g., dry sand, water), the two were found to be in close agreement. For snow the filtered albedo exceeds the broadband albedo for all zenith angles; for meadow the broadband albedo will exceed the filtered albedo. Regression equations were derived to transform the VISSR filtered albedo to unfiltered broadband albedo for each surface type.

Are the Viking Lander sites representative of the surface of Mars?

Global remote sensing data of the Martian surface, collected by earth- and satellite-based instruments, are compared with data from the two Viking Landers to determine if the Lander data are representative of the Martian surface. The landing sites are boulder-strewn and feature abundant fine material and evidence of strong eolian forces. One site (VL-1) is in a plains-covered basin which is associated with volcanic activity; the VL-2 site is in the northern plains. Thermal IR, broadband albedo, color imaging and radar remote sensing has been carried out of the global Martian surface. The VL-1 data do not fit a general correlation observed between increases in 70-cm radar cross-sections and thermal inertia. A better fit is found with 12.5-cm cross sections, implying the presence of a thinner or discontinuous duricrust at the VL-1 site, compared to other higher-inertia regions. A thin dust layer is also present at the VL-2 site, based on the Lander reflectance data. The Lander sites are concluded to be among the three observed regions of anomalous reflectivity, which can be expected in low regions selected for the landings. Recommendations are furnished for landing sites of future surface probes in order to choose sites more typical of the global Martian surface.

Surface properties of the Pettit wind streak on Mars: Implications for sediment transport

Pettit crater (12°N, 174°W) is associated with a complex wind streak consisting of both Type I (bright) streak and Type II (dark) streak materials. Broadband albedo measurements of the streak indicate a minimum of .20 and a maximum of .32, with .27 on the surrounding plains. The darkest materials have the highest thermal inertias, which can be related to an effective grain diameter typical of medium sand. Most of the dark streak materials have thermal inertias representative of fine sand, the material most easily set in motion by the wind on Mars. The bright portions of the streak have thermal inertias higher than that of the surrounding plains; the bright streak values are representative of very fine sand. Differences between temperatures measured at 11 and 20 μm indicate there is no difference in the areal abundance of unresolved high thermal inertia materials (e.g., blocks, bedrock) between the Pettit streak and the surrounding plains. The dark streak material is consistent with models involving sand saltation and transport but the bright streak thermal inertia is not consistent with a volumetrically significant enhancement of unconsolidated dust in the streak.

The influence of satellite spectral sensor response on the analysis of satellite imagery at high latitudes

Abstract Many polar-orbiting satellites used in high-latitude work employ instruments which sense only narrow regions of the solar spectrum. A radiative transfer scheme is used to illustrate the differences between broad-band albedos and those calculated using the spectral response of the NOAA-6 AVHRR channel 1 and DMSP visible sensors. The albedo contrast between snow/no snow is shown to be exaggerated by the satellite sensors and the impact of cloudiness over snow on the albedo is drastically reduced, by a factor of 10 in the case of the NOAA-6 sensor. A significant solar zenith angle dependence is also shown for the NOAA-6 sensor

The influence of the spectral response of satellite sensors on estimates of broadband albedo

AbstractThe radiative impact of cloudiness variations on climate depends on the relative magnitudes of changes in the planetary albedo and the outgoing longwave radiation. R. D. Cess and co‐workers showed in 1982 that the calculated direction of the cloud‐climate feedback (i.e. whether the earth‐atmosphere system warms or cools with changing cloud amount) depended on the satellite data sets used in their analysis. It was suggested that the different spectral resolutions of the satellite sensors might be responsible. This note examines that suggestion. It is shown that the change in planetary albedo measured by the narrow band scanning radiometers on board NOAAs 2 to 5 as a result of cloudiness variation can be an overestimate of the actual change by a factor of up to 1.38 at low latitudes. The overestimate is not sufficient to explain completely the differences amongst the satellite data. Nevertheless, the results indicate that satellite spectral response must be carefully considered if the strength (and possibly even the sign) of the cloud‐climate feedback is to be correctly assessed from narrow band satellite data. Over snow‐covered surfaces it is shown that the NOAA data may underestimate the effect of cloud cover changes by over 70%, and overestimate the planetary albedo by over 0.2.

The influence of the spectral response of satellite sensors on estimates of broadband albedo

The influence of the spectral response of satellite sensors on estimates of broadband albedo