Clouds cover about 70% of the Earth's surface and playa dominant role in the energy and water cycle of our planet. Only satellite observations provide a continuous survey of the state of the atmosphere over the whole globe and across the wide range of spatial and temporal scales that comprise weather and climate variability. Satellite cloud data records now exceed more than 25 years in length. However, climatologies compiled from different satellite datasets can exhibit systematic biases. Questions therefore arise as to the accuracy and Capsule:Cloud properties derived from space observations are immensely valuable for climate studies and model evaluati~n; this assessment has revealed how their statistics may be affected by instrument capabilities and/or retrieval methods but also highlight those well determined.2

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[1] This study presents a comprehensive statistical overview of the macrophysical properties of trade wind cumulus clouds over the tropical western Atlantic using 152 scenes taken from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) between September and December 2004. The size distribution, shapes, and spatial distribution of cumulus clouds were examined with ASTER nearinfrared data at 15 m resolution. The height distribution of these cumulus clouds was derived from ASTER thermal infrared data at 90 m resolution. The size distribution of cumuli exhibited a power law form and an exponent of 2.19 with a correlation coefficient of 0.99 using a direct power law fit method. The total cloud fraction of trade wind cumulus was 0.086, half of which was contributed from clouds smaller than 2 km in equivalent area diameter. An area-perimeter power law was observed with a dimension of 1.28 and a correlation coefficient of 0.87. The majority of cloudy pixels had cloud top altitudes around 1 km and increasing altitude with increasing cloud equivalent area diameter. Seventy-five percent of clouds have a nearest neighbor within a distance of 10 times their area-equivalent radius. Our results are compared to other studies of small cumulus taken over different parts of the world observed using different instruments. The statistics of cumuli observed in this study are poorly related to synoptic scale meteorological conditions from reanalysis data.Citation: Zhao, G., and L. Di Girolamo (2007), Statistics on the macrophysical properties of trade wind cumuli over the tropical western Atlantic,

[1] Errors in the standard cloud fraction products produced by the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Multi-angle Imaging SpectroRadiometer (MISR) on EOS-Terra were examined using 15 m resolution data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Relative to 124 ASTER scenes containing only trade wind cumuli and having an average cloud fraction of 0.08, MODIS and MISR overestimated cloud fraction by 0.18 and 0.36, respectively. For non-sunglint scenes, MODIS and MISR overestimated cloud fractions by 0.02 and 0.24, respectively. Systematic dependences in the MODIS and MISR cloud fractions with ASTER cloud fraction were observed. Large RMS errors in MODIS and MISR cloud fractions were observed because of variations in the spatial distribution of clouds, suggesting it may be difficult to decouple long-term changes in cloud fraction from satellites from true changes in the spatial distribution of clouds.

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