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

This study directly compares plane parallel model calculations with 1 year of Earth Radiation Budget Satellite shortwave observations at nadir over ocean between 30°S and 30°N. 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°. 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° when applied to a general ensemble of real clouds, and for solar zenith angles ≲63° 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.

[1]  G. M. Hale,et al.  Optical Constants of Water in the 200-nm to 200-microm Wavelength Region. , 1973, Applied optics.

[2]  T. McKee,et al.  Scattering of Visible Radiation by Finite Clouds , 1974 .

[3]  Owen B. Toon,et al.  A global average model of atmospheric aerosols for radiative transfer calculations , 1976 .

[4]  M. Aida.  Scattering of solar radiation as a function of cloud dimensions and orientation , 1977 .

[5]  R. Davies,et al.  The Effect of Finite Geometry on the Three-Dimensional Transfer of Solar Irradiance in Clouds , 1978 .

[6]  H. Quenzel,et al.  Turbidity of the atmosphere determined from satellite: Calculation of optimum viewing geometry , 1979 .

[7]  Robert J. Charlson,et al.  The atmospheric aerosol system: An overview , 1983 .

[8]  Roger Davies,et al.  Reflected solar radiances from broken cloud scenes and the interpretation of scanner measurements , 1984 .

[9]  Patrick Minnis,et al.  Diurnal variability of regional cloud and clear-sky radiative parameters derived from GOES data. Part II : November 1978 cloud distributions. , 1984 .

[10]  Roger Davies,et al.  Spectral Absorption of Solar Radiation in Cloudy Atmospheres: A 20 cm−1 Model , 1984 .

[11]  P. Minnis,et al.  Cloud bidirectional reflectance functions: a comparison of experimental and theoretical results. , 1985, Applied optics.

[12]  Leonard P. Kopia,et al.  Earth Radiation Budget Experiment Scanner Instrument , 1986 .

[13]  Roger Davies,et al.  The Effect of Cloud Sides on Reflected Solar Radiation as Deduced from Satellite Observations , 1986 .

[14]  D. Hartmann,et al.  Diurnal variation of outgoing longwave radiation in the tropics , 1986 .

[15]  F. X. Kneizys,et al.  Users Guide to LOWTRAN 7 , 1988 .

[16]  R. Chervin,et al.  Global distribution of total cloud cover and cloud type amounts over the ocean , 1988 .

[17]  K. Stamnes,et al.  Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media. , 1988, Applied optics.

[18]  Patrick Minnis,et al.  Viewing zenith angle dependence of cloudiness determined from coincident GOES East and GOES West data , 1989 .

[19]  Edwin F. Harrison,et al.  Earth Radiation Budget Experiment (ERBE) archival and April 1985 results , 1989 .

[20]  Bruce A. Wielicki,et al.  Cloud Identification for ERBE Radiative Flux Retrieval , 1989 .

[21]  J. W. Fitzgerald,et al.  Aerosol size distributions and optical properties found in the marine boundary layer over the Atlantic Ocean , 1990 .

[22]  Reflectivities of uniform and broken layered clouds , 1991 .

[23]  P. Durkee,et al.  Global analysis of aerosol particle characteristics , 1991 .

[24]  W. Rossow,et al.  ISCCP Cloud Data Products , 1991 .

[25]  Bruce A. Wielicki,et al.  On the determination of cloud cover from satellite sensors: The effect of sensor spatial resolution , 1992 .

[26]  M. Desbois,et al.  Structural Characteristics of Deep Convective Systems over Tropical Africa and the Atlantic Ocean , 1992 .

[27]  Francois-Marie Breon,et al.  Reflectance of broken cloud fields : simulation and parameterization , 1992 .

[28]  Takahisa Kobayashi,et al.  Effects Due to Cloud Geometry on Biases in the Albedo Derived from Radiance Measurements. , 1993 .

[29]  Roger Davies,et al.  Spatial autocorrelation of radiation measured by the Earth Radiation Budget Experiment: Scene inhomogeneity and reciprocity violation , 1994 .