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François-Marie Bréon

Abstract

The transfer of solar irradiance in plane parallel and broken cloud fields is simulated using a Monte Carlo method. The angular distribution pattern of radiances exiting the cloud layer is studied with varying cloud geometries, optical thicknesses, cloudiness, and solar zenith angles. A rather large anisotropy of the reflected flux is found, usually increasing with solar zenith angle and with patterns that strongly depend on cloud geometry. The main features are 1) a local maximum of reflected intensity in the forward direction for all cases, 2) a limb darkening for the plane parallel case, and 3) a limb brightening and a local maximum of reflected intensity in the backward direction for broken clouds. A parameterization for the azimuth-averaged reflectance function is developed. It reproduces the Monte Carlo simulation with a reasonable accuracy and illustrates that, when azimuthally averaged, the reflectance function is dominated by side viewing and intercloud shadowing effects.

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François-Marie Bréon and Stéphane Colzy

Abstract

This paper describes the cloud screening algorithms that have been developed for the processing of Polarization and Directionality of the Earth Reflectances (POLDER) measurements over land surfaces. Four tests are applied to the measurements. The first one is a threshold on the 0.44-μm reflectance after atmospheric correction. The second one is similar but with a smaller threshold and is applied only over targets with significant spectral variation. The third one compares the surface pressure to an estimate derived from two POLDER channels centered on an oxygen absorption band. The fourth one makes use of POLDER polarization capabilities and seeks the presence of a rainbow generated by water clouds.

The performance of the method is evaluated using a large dataset of nearly coincident POLDER measurements and surface observations of cloud cover. The validation dataset is fully independent of the spaceborne measurements and allows the sampling of a wide range of situations. The results demonstrate the capability of the algorithm to distinguish clear and cloudy pixels, although a large fraction of pixels with a small cloud cover are incorrectly declared clear. This may be due in part to the different field of view of the spaceborne and surface observations. For a given cloud cover, the detection is less efficient for high clouds than for low- or medium-altitude clouds, which may result from their lower-optical thickness.

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François-Marie Bréon and Sophie Bouffiés

Abstract

The POLDER (polarization and directionality of the earth reflectances) instrument to be launched in 1996 carries two channels that cover the oxygen A absorption band (near IR). The authors investigate the possibility of using these measurements to achieve cloud detection: An estimate of the surface pressure is made from the two measurement ratios. This apparent pressure can then be compared to what is expected in clear conditions. For this objective, the authors analyze here the uncertainty on the apparent pressure in clear conditions through radiative transfer simulations.

It is found that the radiometric noise yields an uncertainty on the order of 20 hPa. The variability in the temperature profile has a negligible influence on the apparent pressure. On the other hand, the spectral shape of the surface reflectance yields a variability of 160 hPa in the apparent pressure. The aerosol effect on the apparent profile depends on their altitude. A decrease in apparent pressure of 14 hPa is found for a typical stratosphere aerosol layer of optical thickness 0.05 and altitude 20 km. A boundary layer aerosol has a smaller influence.

In clear conditions, the uncertainty on the apparent pressure is on the order of 100 hPa. Therefore, a cloud-detection test based on the oxygen absorption will only be able to detect medium and high clouds.

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François-Marie Bréon and Bérengère Dubrulle

Abstract

Horizontally oriented plates in clouds generate a sharp specular reflectance signal in the glint direction, often referred to as “subsun.” This signal (amplitude and width) may be used to analyze the relative area fraction of oriented plates in the cloud-top layer and their characteristic tilt angle to the horizontal. Use is made of spaceborne measurements from the Polarization and Directionality of the Earth Reflectances (POLDER) instrument to provide a statistical analysis of these parameters. More than half of the clouds show a detectable maximum reflectance in the glint direction, although this maximum may be rather faint. The typical effective fraction (area weighted) of oriented plates in clouds lies between 10−3 and 10−2. For those oriented plates, the characteristic tilt angle is less than 1° in most cases. These low fractions imply that the impact of oriented plates on the cloud albedo is insignificant. The largest proportion of clouds with horizontally oriented plates is found in the range 500– 700 hPa, in agreement with typical in situ observation of plates in clouds.

A simple aerodynamic model is proposed that accounts for the orienting torque of the flow as the plate falls under its own gravity and the disorienting effects of Brownian motion and atmospheric turbulence. The model indicates that the horizontal plate diameters are in the range 0.1 to a few millimeters. For such sizes, Brownian forces have a negligible impact on the plate orientation. On the other hand, typical levels of atmospheric turbulence lead to tilt angles that are similar to those estimated from the glint observation.

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François-Marie Breon, Robert Frouin, and Catherine Gautier

Abstract

A method is proposed to compute the net solar (shortwave) irradiance at the earth's surface from Earth Radiation Budget Experiment (ERBE) data in the S4 format. The S4 data are monthly averaged broadband planetary albedo collected at selected times during the day. Net surface shortwave irradiance is obtained from the shortwave irradiance incident at the top of the atmosphere (known) by subtracting both the shortwave energy flux reflected by the earth-atmosphere system (measured) and the energy flux absorbed by the atmosphere (modeled). Precalculated atmospheric- and surface-dependent functions that characterize scattering and absorption in the atmosphere are used, which makes the method easily applicable and computationally efficient. Four surface types are distinguished, namely, ocean, vegetation, desert, and snow/ice. Over the tropical Pacific Ocean, the estimates based on ERBE data compare well with those obtained from International Satellite Cloud Climatology Project (ISCCP) B3 data. For the 9 months analyzed the linear correlation coefficient and the standard difference between the two datasets are 0.95 and 14 W m−2 (about 6% of the average shortwave irradiance), respectively, and the bias is 15 W m−2 (higher ERBE values). The bias, a strong function of ISCCP satellite viewing zenith angle, is mostly in the ISCCP-based estimates. Over snow/ice, vegetation, and desert no comparison is made with other satellite-based estimates, but theoretical calculations using the discrete ordinate method suggest that over highly reflective surfaces (snow/ice, desert) the model, which accounts crudely for multiple reflection between the surface and clouds, may substantially overestimate the absorbed solar energy flux at the surface, especially when clouds are optically thick. The monthly surface shortwave irradiance fields produced for 1986 exhibit the main features characteristic of the earth's climate. As found in other studies, our values are generally higher than Esbensen and Kushnir's by as much as 80 W m−2 in the tropical oceans. A cloud parameter, defined as the difference between clear-sky and actual irradiances normalized to top-of-atmosphere clear-sky irradiance, is also examined. This parameter, minimally affected by sun zenith angle, is higher in the midlatitude regions of storm tracks than in the intertropical convergence zone (ITCZ), suggesting that, on average, the higher cloud coverage in midlatitudes is more effective at reducing surface shortwave irradiance than opaque, convective, yet sparser clouds in the ITCZ. Surface albedo estimates are realistic, generally not exceeding 0.06 in the ocean, as high as 0.9 in polar regions, and reaching 0.5 in the Sahara and Arabian deserts.

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Francois-Marie Bréon, Darren Jackson, and John Bates

Abstract

The GMS-5 geostationary satellite carries a channel centered at 6.7 μm for the measurement of upper-tropospheric humidity. This channel’s spectral response shows structures that are similar to those shown by the atmospheric transmission. This note shows that these structures probably result from water vapor absorption between the calibration source and the instrument while making the response measurement. A corrected filter is proposed after normalization by the inferred atmospheric transmission. The brightness temperatures computed by a radiative transfer model using the spurious response exhibit a warm bias of about 1 K.

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Francois-Marie Bréon, Robert Frouin, and Catherine Gautier

Abstract

Two sets of ocean surface longwave irradiance measurements collected during the FASINEX and MILDEX experiments are analyzed for quality and variability studies. Using concomitant radiosonde data, the clear-sky contribution to the downward flux at the surface is computed and, subsequently, the effect of clouds from the surface measurements is deduced. The longwave irradiance computations are performed using a broadband model, a simpler parameterization, and an empirical formula. The three schemes are chosen because they represent different, possible approaches for computing surface longwave irradiance. They are intercompared, separating clear and cloud components, and verified against in situ measurements.

During both experiments, which took place in midlatitudes during different seasons, variations in the downward longwave flux associated with clear-sky variations (air temperature, humidity changes) and cloud effects are found to be of the same order of magnitude (≈70 W m−2). Applied to radiosonde profiles, the two more physical schemes provide consistent results with a 4 W m−2 the standard deviation of the differences is only 4 W m−2. These schemes, however, exhibit a 4 W m−2 relative bias, which is comparable to the desired accuracy for monthly-mean longwave flux estimates. Intercomparisons with the empirical formula yield larger standard deviations, demonstrating the formula's inability to reproduce adequately the clear-sky flux variability. For all three schemes, the scatter around the measured values is rather large (20–25 W m−2); surprisingly, the empirical formula gives the best results. This is explained by the large uncertainty in the cloud parameters used as input to the schemes; when no reliable estimates of these critical variables can be made, it may be more accurate to take a simple parameterization for the cloud effect on the longwave flux.

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