Cloud structure of Saturn’s 2010 storm from ground-based visual imaging

We present a study of the vertical cloud structure for the initial stage of the Great White Spot (GWS), a giant storm that developed in Saturn in December 2010, using ground-based visual images. We focus in the characterization of the undisturbed atmosphere preceding the storm and the disturbed region in the wake of the GWS. The observations were taken at Calar Alto (Spain) and Pic du Midi (France) observatories on 27 December 2010 and 13 January 2011, about 1 month after the detection of the outbreak. They cover a spectral range from the ultraviolet at 375 nm to the near infrared at 954 nm, including the deep methane absorption band at 890 nm and a number of increasingly weaker methane absorption bands. Limb to limb scans of the absolute reflectivity of the regions preceding and following the storm at different wavelengths are compared to those produced by a radiative transfer model atmosphere. Our model assumes three layers of gas and particles: stratospheric and tropospheric hazes and a deep cloud. We find that the most notorious changes in the wake of the GWS occurred in the top level of the semi-infinite bottom cloud which ascended from an altitude level P > 1 bar in the undisturbed region to P = 300 + 300 - 100 mbar in the stormy area, representing a rise of more than 40 km. The density of the tropospheric haze does not change substantially but tropospheric particles are found to be more reflectant at all wavelengths, suggesting that they are coated by fresh material, putatively coming from deeper levels of the atmosphere.