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Saturn's Great White Spots are rare planetary-scale storms that have been observed only six times since 1876. The most recent Great White Spot appeared in December 2010 and has been studied from both ground-based and spacecraft observations. The storm developed into an enormous disturbance extending over 10,000 km at cloud level, emitted intense electrostatic discharges over several months, and caused long-standing localized warming in the high stratosphere of about 60 K. Here we analyse the dynamics of the storm's head using high-resolution imagery obtained by the Cassini spacecraft on 26 February 2011. We find strong winds with speeds up to 160 m s−1 and organized into a divergent open anticyclone where massive cumulus-like cloud clusters interact with the ambient zonal flow to generate a storm front. The cloud clusters evolved over a timescale of hours, with cloud tops reaching 44 km above the undisturbed environment. Simulations using a general circulation model, which includes Saturn's zonal winds, reproduce the observations when a persistent heat source is introduced, causing a high-pressure anomaly. We conclude that the complex phenomenology of a mature Great White Spot represents a natural response of the saturnian atmosphere to severe sustained convection in a sheared background flow. [ABSTRACT FROM AUTHOR]

Convective storms occur regularly in Saturn's atmosphere. Huge storms known as Great White Spots, which are ten times larger than the regular storms, are rarer and occur about once per Saturnian year (29.5 Earth years). Current models propose that the outbreak of a Great White Spot is due to moist convection induced by water. However, the generation of the global disturbance and its effect on Saturn's permanent winds have hitherto been unconstrained by data, because there was insufficient spatial resolution and temporal sampling to infer the dynamics of Saturn's weather layer (the layer in the troposphere where the cloud forms). Theoretically, it has been suggested that this phenomenon is seasonally controlled. Here we report observations of a storm at northern latitudes in the peak of a weak westward jet during the beginning of northern springtime, in accord with the seasonal cycle but earlier than expected. The storm head moved faster than the jet, was active during the two-month observation period, and triggered a planetary-scale disturbance that circled Saturn but did not significantly alter the ambient zonal winds. Numerical simulations of the phenomenon show that, as on Jupiter, Saturn's winds extend without decay deep down into the weather layer, at least to the water-cloud base at pressures of 10-12?bar, which is much deeper than solar radiation penetrates. [ABSTRACT FROM AUTHOR]