Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> The numbers of clouds of a given size is a defining feature of the earth's atmosphere. As well as cloud area, cloud perimeter <em>p</em> is interesting because it represents the length of the shared interface between clouds and clear-skies across which air and buoyant energy are dissipated. A recent study introduced a first-principles expression for the steady-state distribution of cloud perimeters, measured within a quasi-horizontal moist isentropic layer, that is a scale invariant power-law <em>n </em>(<em>p</em>) ∝ <em>p<sup>–</sup></em><sup>(1+<em>β</em>)</sup>, where <em>n</em> (<em>p</em>) is the number density of cloud perimeters within [<em>p</em>, <em>p </em>+ <em>dp</em>] and <em>β</em> = 1. This value of <em>β</em> was found to be in close agreement with output from a high-resolution, large eddy simulation of tropical convection. To further test this formulation, the current study evaluates <em>n</em> (<em>p</em>) within near-global imagery from nine full-disk and polar-orbiting satellites. A power-law is found to apply to measurements of <em>n</em> (<em>p</em>), and the value of <em>β</em> is observed to be remarkably robust to latitude, season, and land/ocean contrasts suggesting that, at least statistically speaking, cloud perimeter distributions are determined more by atmospheric stability than Coriolis forces, surface temperature, or contrasts in aerosol loading between continental and marine environments. However, the measured value of <em>β</em> is found to be 1.29 ± 0.05 rather than <em>β</em> = 1, indicating a relative scarcity of large clouds in satellite observations compared to theory and high-resolution cloud modeling. The reason for this discrepancy is unclear but may owe to the difference in perspective between evaluating <em>n (p)</em> along quasi-horizontal moist isentropes rather than looking down from space. As a test of this hypothesis, numerical simulation output shows that, while <em>β</em> ∼ 1 within isentropes, higher values of <em>β</em> are reproduced for a simulated satellite view. However, the simulated value is a function of the cloud detection sensitivity, but little such sensitivity is seen in satellite observations, suggesting a possible misrepresentation of the physics controlling cloud sizes in simulations. A power-law also applies to satellite observations of cloud areas covering a range between ∼ 3 km<sup>2</sup> and ∼ 3 × 10<sup>5 </sup>km<sup>2</sup>, a much wider range of scales than has been previously described in studies that we argue inappropriately treated the statistics of clouds truncated by the edge of a measurement domain.
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