Diurnal self-aggregation

Abstract

Convective self-aggregation is a modelling paradigm for convective rain cell organisation over a constant-temperature tropical sea surface. This set-up can give rise to cloud clusters developing over timescales of weeks. In reality, sea-surface temperatures do oscillate diurnally, affecting the atmospheric state and influencing rain rates significantly. Over land, surface temperatures vary more strongly. Here, we carry out a suite of cloud-resolving numerical experiments, and find that qualitatively different dynamics emerge from modest surface temperature oscillations: while the spatial distribution of rainfall is homogeneous during the first day, already on the second day, the rain field is firmly structured. In later days, this clustering becomes stronger and alternates from day to day. We show that these features are robust to changes in resolution, domain size and mean surface temperature, but can be removed by a reduction of the amplitude of diurnal surface temperature oscillation, suggesting a transition from a random to a clustered state. Maximal clustering occurs at a scale of $${l}_{\max }\approx 180\ {\rm{km}}$$ l max ≈ 180 km , which we relate to the emergence of mesoscale convective systems. At $${l}_{\max }$$ l max , rainfall is strongly enhanced and far exceeds the rainfall expected at random. Simple conceptual modelling helps interpret the transition to clustering, which is driven by the formation of mesoscale convective systems, and brings about day-to-day moisture oscillations. Our results may help clarify how continental extremes build up, and how cloud clustering over the tropical ocean could emerge as an instance of spontaneous symmetry breaking at timescales much faster than in conventional radiative–convective equilibrium self-aggregation.

Mechanical Forcing of Convection by Cold Pools: Collisions and Energy Scaling

“…Second, PE is varied, and the resulting scaling of CP properties studied—mimicking changes in precipitation intensity and resulting CP strength. Such intensity changes can occur within the diurnal cycle of precipitation, where intensifying precipitation cells might release increasingly more energy (Haerter et al., 2017, 2020).…”

“…A particular future challenge might be posed by the emergence of mesoscale convective systems (Houze Jr, 2004), which appear to hinge of the formation of “combined cold pools” (Haerter et al., 2020), formed by rapid successions of multi‐CP interactions—finally leading to the formation of a joint, deeper CP. Through their large depth and potential energy, these CPs then act more autonomously, exciting subsequent updrafts near their periphery.…”

“…Thanks to this spatiotemporal link, CPs affect not only the cloud cover's local properties but also shape its mesoscale organization. Given an ensemble of Generation 1 convective cells and corresponding CPs, Generation 2 convective cells appear to be non‐randomly connected to the present ones, leading to the emergence of complex, non‐random, organizational patterns (Feng et al., 2015; Haerter et al., 2019, 2020; Purdom, 1976; Torri & Kuang, 2019). Simple geometric models reveal that if the collision of any pair of two CPs yields a new convective cell, and thus a new CP, the number density of CPs inevitably increases and even diverges with time (Nissen & Haerter, 2020).…”

Anthropogenic intensification of short-duration rainfall extremes

“…Increased moisture advection into future MCSs and changes in the cloud microphysics are possible causes for the rapid increase in precipitation volume but further work is needed to understand the robustness of these results. The amplitude of the diurnal cycle of surface temperature, not just its mean or maximum, might also help MCSs to develop 115 .…”

Mechanical forcing of convection by cold pools: collisions and energy scaling.

“…Second, PE is varied, and the resulting scaling of CP properties studiedmimicking changes in precipitation intensity and resulting CP strength. Such intensity changes can occur within the diurnal cycle of precipitation, where intensifying precipitation cells might release increasingly more energy (Haerter et al, 2017(Haerter et al, , 2020.…”