Abstract
Abstract. The recently recognized continuous transition zone between detectable clouds and cloud-free atmosphere ("the twilight zone") is affected by undetectable clouds and humidified aerosol. In this study, we suggest to distinguish cloud fields (including the detectable clouds and the surrounding twilight zone) from cloud-free areas, which are not affected by clouds. For this classification, a robust and simple-to-implement cloud field masking algorithm which uses only the spatial distribution of clouds, is presented in detail. A global analysis, estimating Earth's cloud field coverage (50° S–50° N) for 28 July 2008, using the Moderate Resolution Imaging Spectroradiometer (MODIS) data, finds that while the declared cloud fraction is 51%, the global cloud field coverage reaches 88%. The results reveal the low likelihood for finding a cloud-free pixel and suggest that this likelihood may decrease as the pixel size becomes larger. A global latitudinal analysis of cloud fields finds that unlike oceans, which are more uniformly covered by cloud fields, land areas located under the subsidence zones of the Hadley cell (the desert belts), contain proper areas for investigating cloud-free atmosphere as there is 40–80% probability to detect clear sky over them. Usually these golden-pixels, with higher likelihood to be free of clouds, are over deserts. Independent global statistical analysis, using MODIS aerosol and cloud products, reveals a sharp exponential decay of the global mean aerosol optical depth (AOD) as a function of the distance from the nearest detectable cloud, both above ocean and land. Similar statistical analysis finds an exponential growth of mean aerosol fine-mode fraction (FMF) over oceans when the distance from the nearest cloud increases. A 30 km scale break clearly appears in several analyses here, suggesting this is a typical natural scale of cloud fields. This work shows different microphysical and optical properties of cloud fields, urging to separately investigate cloud fields and cloud-free atmosphere in future climate research.
Highlights
Clouds and aerosols play key roles in the climate system as major components in the Earth’s energy system and water cycle (Kiehl and Trenberth, 1997; Trenberth et al, 2009)
A comparison was done between the nearest neighbor cumulative distribution function (NNCDF) of cloud fields and the theoretical Poisson NNCDF in order to find the level of spatial randomness or regularity of cloud fields
The cloud mask input data are based on the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask product (Ackerman et al, 1998; Platnick et al, 2003), the aerosol properties data are based on the MODIS aerosol product (Remer et al, 2005; Levy et al, 2007), and both sea/land mask and geo-location data are based on MODIS Geolocation product
Summary
Clouds and aerosols play key roles in the climate system as major components in the Earth’s energy system and water cycle (Kiehl and Trenberth, 1997; Trenberth et al, 2009). An extensive research has been conducted in order to estimate the regularity and clustering properties of cloud fields, using morphological techniques (Weger et al, 1992, 1993; Zhu et al, 1992; Lee et al, 1994; Nair et al, 1998) In these studies, a comparison was done between the nearest neighbor cumulative distribution function (NNCDF) of cloud fields and the theoretical Poisson NNCDF in order to find the level of spatial randomness or regularity of cloud fields.
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