Hysteresis in self-organized mesoscale convective systems driven by diurnal temperature oscillations

Self-aggregation of convective cloud activity has attracted a lot of attention due to its role in the emergence of large scale weather phenomena such as the formation of hurricanes. Simulations with uniform boundary conditions show self-aggregation ocurring on the time-scale of a few weeks [1]. Recently, numerical experiments demonstrated that spatial clustering can form withing a few days, when the system is driven by diurnal temperature oscillations [2]. These simulations indicate that there may be a discontinuous phase transition between the clustered and the non-clustered state, i.e. that a threshold for the amplitudes of diurnal temperature oscillations exist, below which clustering does not emerge. A conceptual model has been proposed, suggesting that the phase transition might give rise to hysteresis in the sense that clustering emerging at high temperature amplitudes might persist if the amplitude is subsequently reduced to a level below the critical threshold.

Here we test the hysteresis–hypothesis explicitly by performing cloud-resolving simulations with a high-amplitude transient period followed by a period with a low diurnal temperature amplitude. In reality, diurnal temperature oscillations have significantly larger amplitudes over land than over the ocean. The existence of hysteresis effects in the convective cloud clustering could have profound implications: clusters formed over land, where diurnal temperature variations are large, could persist over the ocean when transported there by large-scale wind advection. Once present, the clusters could even intensify over the ocean—with possible implications for cyclogenesis.

[1] CJ Muller and IM Held, Journal of the Atmospheric Sciences, 69(8):2551–2565, 2012.
[2] arXiv:2001.04740 [physics.ao-ph]