Observational evidence of plane parallel model biases: Apparent dependence of cloud optical depth on solar zenith angle

Abstract

This study directly compares plane parallel model calculations with I year of Earth Radiation Budget Satellite shortwave observations at nadir over ocean between 30øS and 30øN. When plane parallel model calculations are matched to the observations on a pixel-by-pixel basis by adjusting cloud fraction and cloud optical depth, the resulting frequency distributions of cloud optical depth show a systematic shift towards larger values with increasing solar zenith angle, regardless of the assumptions made in the calculations. This dependence is weak for thin clouds but gets progressively stronger as the clouds become thicker. For the thinnest 50% of the clouds (optical depths <• 6), it occurs only at oblique solar zenith angles, whereas it is observed at all solar zenith angles for the thickest 10% of clouds (optical depths >• 12). On average, the increase is extremely large for solar zenith angles >• 63 ø. Such behavior is unrealistic since average cloud optical depths from such an extensive data set should be almost independent of solar zenith angle. The cause is traced to a fundamental flaw in plane parallel theory applied to real clouds: the solar zenith angle dependence of model reflectance is opposite to that of the observations. The one-dimensional nadir reflectance remains within 10% of the observed reflectance for solar zenith angles <• 53 ø when applied to a general ensemble of real clouds, and for solar zenith angles <• 63 ø when applied to the thinnest 50% of such clouds. Uncertainties are found to increase rapidly as the Sun becomes more oblique, easily reaching 30% at the lowest solar elevations. Based on results from theoretical studies, it is concluded that three-dimensional cloud structures not accounted for by plane parallel theory have a statistically important effect on the radiation field. As a minimum requirement, application of one-dimensional theory to the remote sensing of cloud optical thickness from measurements of nadir reflectance should therefore be restricted to thin clouds and small solar zenith angles. address only one aspect of the dichotomy between three-dimensional reality and one-dimensional remote sensing of cloud optical properties, namely, the effect of solar zenith angle on the cloud optical depths inferred from shortwave radiance observations. ] has already shown that reflectivities from specific types of three-dimensional clouds can differ substantially from those assumed to be plane parallel. There is also increasing observational evidence [e.g., $tuhlmann et al., 1985; Coakley and Davies, 1986; Minnis, 1989; Coakley, 1991] consistent with such theory. These studies clearly demonstrate that threedimensional effects are important for certain types of clouds, so that one-dimensional retrieval of cloud optical depth may be substantially biased. Additional observational studies are now needed to quantify this bias for mixtures of cloud types that are representative of general conditions. 1621 1622 LOEB AND DAVIES: EVIDENCE OF PLANE PARALLEL MODEL BIASES In a recent study of nonsp...

A better understanding of cloud optical thickness derived from the passive sensors MODIS/AQUA and POLDER/PARASOL in the A-Train constellation

“…The increase of COT with solar zenith angle on the one hand is due to geographical sampling of an increase of thicker clouds with latitude ( Fig. 2) and on the other hand is due to 3-D radiative effects first evidenced by Loeb and Davies (1996) from ERBE observations. They were reproduced with Monte-Carlo simulations by Loeb et al (1997) and Várnai (2000), which showed a larger increase of 3-D nadir and backward reflectances with solar incidences compared to 1-D ones leading to a retrieved optical thickness, which increases with solar zenith angles.…”

A semianalytical cloud retrieval algorithm using backscattered radiation in 0.4–2.4 μm spectral region

“…The range of applicability of such an assumption for real clouds is very limited as is readily shown by observations of light from the sky on the cloudy day. For example, the retrieved cloud optical thickness t apparently depends on the viewing geometry [Loeb and Davies, 1996;Loeb and Coakley, 1998]. This, of course, would not be the case for an idealized plane-parallel cloud layer.…”

Cloud heterogeneity effects on cloud and aerosol above cloud properties retrieved from simulated total and polarized reflectances

“…The bias on effective radius can thus be positive or negative depending on sub-pixel heterogeneity of the cloud optical thickness and effective radius (Zhang et al, 2016). 25 In addition to the sub-pixel heterogeneity, Loeb and Davies (1996) detected an increase of the retrieved optical thickness from AVHRR correlated with the solar zenith angle elevation. Indeed, for oblique solar illumination, more energy is transmitting through the clouds along the cloud side (or bump), and leads to an increase of the upward reflectances and consequently to higher cloud optical thicknesses retrieved under the homogeneous cloud assumption.…”