Vertical Cloud Structure in Saturn's 1990 Equatorial Storm

We present a model study of the evolution of the vertical cloud structure and aerosol optical characteristics of the giant storm that erupted in Saturn's atmosphere in September 1990. This analysis is based on the photometric measurements of the reflected intensity of the equatorial region in the optical range (336-890 nm, continuum and methane absorption bands) that were previously published (A. Sanchez-Lavegaet al.1994,Icarus108, 158-168). Different stages of the storm evolution are analyzed: preoutburst, onset and mature (October-November 1990), and evolved (July 1991). Good fits between the observations and radiative transfer calculations are found using a three-layer model (a gas layer, a haze layer and a semiinfinite cloud deck). Two-term Henyey-Greenstein and Mie phase functions are used for the scattering particles. According to the models, the main change during the evolution of the disturbance took place in the altitude of the cloud layer. Cloud tops elevated by 1.2-1.5 scale heights during the onset and mature storm stage relative to the undisturbed atmosphere, descending later during the evolved phase by 0.7 scale heights relative to the altitude of the mature clouds. The altitude location of the haze layer did not change during the storm development but the single scattering albedo of the haze particles increased significantly during the onset and mature stages in the UV-blue wavelengths. If the haze particles are modeled as Mie spheres, their mean radius varied from ∼0.83 to 0.64 μm with real refractive index 1.5 and extreme values of the imaginary index 0.01 (370 nm) and 0.0005 (830 nm).