Distribution, characteristics, and evolution of methane storms on Titan

Methane precipitation is a key component of the climate on Titan, and provides a link between atmospheric processes and surface geomorphology. Observations of methane clouds and inferred precipitation events indicate a strong seasonality and a highly sporadic behavior—a so-called “relaxation-oscillator” model. We use simulations with the Titan Atmospheric Model (TAM), a general circulation model (GCM) of Titan’s climate system, to self-consistently reproduce various aspects of Titan’s hydroclimate, and suggest that the character of precipitation is strongly tied to the distribution of surface (and subsurface) liquids available to evaporate into the atmosphere. Our simulations therefore provide constraints on both the source of replenishment of atmospheric methane as well as how this methane cycles through the atmosphere. Moreover, we investigate the evolution and population characteristics of simulated methane storms over 40 Titan years, building robust statistics. We identify and track storms using the density-based spatial clustering of applications with noise (DBSCAN) algorithm, which allows us to follow them through time and space. In addition to expected geographic and seasonal distributions, we find that the population of storms is bimodal in traits corresponding to intensity, area, and duration, with a large population of small, short-lived, and weakly precipitating storms and a smaller population of exceptionally large, long-lasting, and intense storms. These largest storms tend to evolve similarly over their lifetimes, peaking early in intensity and in the middle of their lives in area. We also find temporal clustering of storms, in alignment with observations and the proposed relaxation-oscillation model of Titan’s methane precipitation. These storm clusters emerge quasi-periodically following long dry spells during which evaporation of surface methane recharges atmosphere. Approximately five clusters occur per Titan year, and their locations are strongly seasonal. Overall, our quantitative descriptions of storms and storm clusters over a long timescale provide additional insight into Titan’s methane cycle and the formation of its surface features.