Abstract
This paper aims at characterizing how different key cloud properties (cloud fraction, cloud vertical distribution, cloud reflectance, a surrogate of the cloud optical depth) vary as a function of the others over the tropical oceans. The correlations between the different cloud properties are built from 2 years of collocated A-train observations (CALIPSO-GOCCP and MODIS) at a scale close to cloud processes; it results in a characterization of the physical processes in tropical clouds, that can be used to better understand cloud behaviors, and constitute a powerful tool to develop and evaluate cloud parameterizations in climate models. First, we examine a case study of shallow cumulus cloud observed simultaneously by the two sensors (CALIPSO, MODIS), and develop a methodology that allows to build global scale statistics by keeping the separation between clear and cloudy areas at the pixel level (250, 330 m). Then we build statistical instantaneous relationships between the cloud cover, the cloud vertical distribution and the cloud reflectance. The vertical cloud distribution indicates that the optically thin clouds (optical thickness <1.5) dominate the boundary layer over the trade wind regions. Optically thick clouds (optical thickness >3.4) are composed of high and mid-level clouds associated with deep convection along the ITCZ and SPCZ and over the warm pool, and by stratocumulus low level clouds located along the East coast of tropical oceans. The cloud properties are analyzed as a function of the large scale circulation regime. Optically thick high clouds are dominant in convective regions (CF > 80 %), while low level clouds with low optical thickness (<3.5) are present in regimes of subsidence but in convective regimes as well, associated principally to low cloud fractions (CF < 50 %). A focus on low-level clouds allows us to quantify how the cloud optical depth increases with cloud top altitude and with cloud fraction.
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Acknowledgments
Thanks are due to NASA and CNES for CALIPSO, MODIS observations. ICARE is acknowledged for data access, and for doing the collocation between CALIPSO (330 m) and MODIS (250 m) reflectance along the lidar track. Thanks are due to Climserv for computing facility. We would like to thank the anonymous reviewers for their useful comments that helped to improve the manuscript.
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Appendices
Appendix 1: Description of sensors, cloud products and variables
Table 1 describes the different sensors and variables used in this study, their spatial resolution, and some of their technical characteristics. The analysis is based on 2 years of observations (January 2007–December 2008).
Appendix 2: Conversion of cloud reflectance in cloud optical depth
The cloud reflectance (CRef) in a fixed viewing direction is used in this study as a surrogate of the cloud optical thickness (Cτ). The relationship between the cloud reflectance and the cloud optical thickness depends on the solar zenith angle and the satellite viewing zenithal and azimuthal angles, and on the phase function of the clouds particles. For the A-train in the tropics, θs varies between 19° and 60°, and 18° < θv-modis < 65°, and 0° < ϕs-ϕv < 150°. Changes of cloud reflectances values due to solar zenith angle variations are lower than 0.1 in the tropical regions (30°S–30°N, 18° < θs < 60°) considering a given phase function. Hence latitudinal reflectance’s variations larger than 0.1 in the Tropics can not be attributed to variations of θs, they are caused by changes in the atmosphere composition (clouds). The sensitivity of the reflectance to the cloud particles scattering phase function is of 0.07 in the tropics. Figure 14 shows the cloud reflectance as a function of the cloud optical depth for a cloud composed of spherical particles and a cloud composed of non spherical particles in the viewing direction of MODIS in the tropics. This curves have been computed following Chepfer et al. (2001) in using an adding doubling radiative transfer code (De Haan et al. 1986), the optical properties of spherical particles are obtained with the Mie theory, and the non-spherical ones with ray-tracing computations (Noël et al. 2001) (Fig. 14).
Appendix 3: Link between cloud cover and cloud reflectance for low and high clouds only
To eliminate the existence of clouds at other levels when studying the relation between cloud fraction and cloud reflectance for low and high clouds mainly, we use the criterion of CF-low > 0.9*CF & CF-mid + CF-high < 0.1*CF (CF-high > 0.9*CF & CF-low + CF-mid < 0.1*CF for high clouds only) referring to low clouds only with no clouds above (and high clouds with no clouds beneath respectively). For low clouds only (Fig. 15a) this criterion eliminated the upper right part of Fig. 6a (CF > 0.7) corresponding to low clouds with high reflectances and high cloud fractions, found at the East part of tropical oceans (when looking at their geographical distribution). We identified that the high values of cloud reflectance correspond to low optically thick clouds, these latter are eliminated when using the criterion of ‘low clouds only’ probably due to noise caused by the low and very reflective clouds. The population of high clouds ‘only’ (Fig. 15b) is much less numerous and misses the clouds with CRef > 0.2, the optically thick clouds that may co-exist with low and mid level clouds (Fig. 15).
Appendix 4: Link between the cloud cover, the cloud reflectance, and the cloud vertical distribution for identified cloud types
Three small regions corresponding to identified cloud types (Marchand et al. 2009; Bodas-Salcedo et al. 2008; Chepfer et al. 2010) are examined in more detail (Fig. 16): Tropical Western Pacific, Californian Stratus and North Pacific. The optically thick atmospheric columns (Cτ > 3.4, Fig. 8c, i) are not uniformly distributed over the regions: optically thick low level clouds are encountered along the Californian coast composed of stratus with cloud cover between 40 and 60 % depending on the season and deep convective high clouds are encountered in the warm pool (35–50 %). Optically thick clouds are also observed over the North Pacific for boundary layer clouds (CF ≈ 25–30 %) and for high frontal clouds mainly during the winter (CF ≈ 22 %) (Fig. 16).
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Konsta, D., Chepfer, H. & Dufresne, JL. A process oriented characterization of tropical oceanic clouds for climate model evaluation, based on a statistical analysis of daytime A-train observations. Clim Dyn 39, 2091–2108 (2012). https://doi.org/10.1007/s00382-012-1533-7
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DOI: https://doi.org/10.1007/s00382-012-1533-7