Scale effect on statistics of the macrophysical properties of trade wind cumuli over the tropical western Atlantic during RICO

Abstract

This study investigates the effect of observational domain size and pixel resolution on the statistics of macrophysical properties of trade wind cumulus clouds over the tropical western Atlantic. In order to better facilitate comparison with model simulations, the results are presented as an extension of earlier work by Zhao and Di Girolamo (2007), who have analyzed 152 Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) scenes collected between September and December 2004 to generate comprehensive statistics of cumulus clouds at the original 60 km × 60 km domain and 15 m pixel resolution of the data. The statistics change with domain size because as the domain size becomes smaller, (1) the probability of chopping clouds by the domain boundaries into smaller clouds increases, and (2) the probability of having cloudier and clearer domains increases. In addition, degrading pixel resolution results in (1) an amalgamation of smaller clouds into larger clouds, and (2) an increase in the frequency of partially filled cloudy pixels. The mean cloud fraction in our ASTER data set increases more than fourfold, with the total number of clouds reducing 26‐fold, when pixel resolution degrades from 15 m to ∼1 km. The cloud fraction frequency distribution becomes wider with a shift of the peak toward higher values at coarser resolution. However, at smaller domain sizes, the shift of the peak toward larger values due to pixel effects is overcompensated by the increase in frequency of smaller cloud fractions. The change in cloud size distribution is significant at domains smaller than 10 km × 10 km. An increase in the cloud size at coarser resolution shifts the peak of the size distribution toward larger values. At 15 m resolution, 98% of clouds are distributed at distances less than 300 m, with a peak in the nearest‐neighbor distance at less than 50 m at all domain sizes. The peak of the nearest‐neighbor distribution becomes wider and shifts to larger distances at coarser resolution. These results clearly indicate the importance of considering scale effect when comparing cloud resolving model simulations of trade wind cumuli with observations.