Tropical high cloud characteristics derived from SAGE II extinction measurements

Abstract

This work investigates the characteristics of tropical high clouds based on the 1989 Stratospheric Aerosol and Gas Experiment (SAGE II) particulate extinction measurements and the National Meteorological Center (NMC) temperature data. A discussion is also provided of the cloud measurement constraints of the SAGE II instrument, including optical limitation, sensitivity of the measurement to particle sizes, and uncertainty due to the spatial resolution of the sampling volume. Because of the viewing geometry of the solar occultation technique, the SAGE II instrument is very sensitive to thin cirrus clouds. The results of this study indicate that (1) the tropical high cloud data can be distinguished approximately from aerosol data by using the ratio of the extinction at 0.525 μm to that at 1.02 μm as an indicator, the 1989 cloud data being characterized by an extinction ratio less than 2.1; (2) the background aerosol size distribution in the tropical tropopause region is bimodal with the small mode extending even below a radius of 0.075 μm and the large mode centered at a radius of 0.6 μm; (3) SAGE II cloud extinction data reveal various degrees of wavelength dependency, which suggests specific progressive changes in particle size distribution during cloud development; and (4) aerosol nucleation and ice nucleation as well as coagulation growth of small particles are important processes in the tropical cirrus cloud evolution. The specific characteristics of the aerosol-cloud interaction as manifested in the multiwavelength SAGE II high cloud extinctions in the tropical tropopause region can be useful information for model studies of the cirrus cloud evolution.