Diurnal Self-Aggregation

Abstract

Convective self-aggregation is a modelling paradigm for thunderstorm organisation over a constant-temperature tropical sea surface. This setup can give rise to cloud clusters over timescales of weeks. In reality, sea surface temperatures do oscillate diurnally, affecting the atmospheric state. Over land, surface temperatures vary more strongly, and rain rate is significantly influenced. Here, we carry out a substantial suite of cloud-resolving numerical experiments, and find that even weak surface temperature oscillations enable qualitatively different dynamics to emerge: the spatial distribution of rainfall is only homogeneous during the first day. Already on the second day, the rain field is firmly structured. In later days, the clustering becomes stronger and alternates from day-to-day. We show that these features are robust to changes in resolution, domain size, and surface temperature, but can be removed by a reduction of the amplitude of oscillation, suggesting a transition to a clustered state. Maximal clustering occurs at a scale of l max ≈ 180 km, a scale we relate to the emergence of mesoscale convective systems. At l max rainfall is strongly enhanced and far exceeds the rainfall expected at random. We explain the transition to clustering using simple conceptual modelling. Our results may help clarify how continental extremes build up and how cloud clustering over the tropical ocean could emerge much faster than through conventional self-aggregation alone. Currently, general circulation models cannot simulate organised deep convection as these models describe convection as a collection of non-interacting convective cells. Yet, the increase in tropical rainfall was stated to predominantly stem from organised deep convection. 1 Similarly, in midlatitudes, the majority of flood-producing rainfall was attributed to mesoscale convective systems (MCSs), 2,3 that is, long-lived complexes of thunderstorms spanning ∼ 100 km in diameter. 4 In self-aggregation studies pronounced clustering occurs on the timescale of several weeks. 5-7 There, the radiative convective equilibrium scheme (RCE) 8,9 is usually employed, assuming spatially and temporally constant surface temperature (∼ 300 K). In self-aggregation, radiation feedbacks have emerged as the "smoking gun" for sustaining and increasing clustering. 5 Still, factors such as sea surface feedbacks, 10 domain size, geometry, and resolution, 11 as well as cold pool effects, 12,13 all contribute.Prescribing constant boundary conditions is an elegant model simplification, but not always realistic. Especially under weak surface wind conditions and strong insolation, sea surface temperature amplitudes were observed to be as large as two 14,15 to five kelvin 16 and suggested to modify atmospheric properties. 16 Such temperature variations may even affect the Madden-Julian Oscillation arXiv:2001.04740v1 [physics.ao-ph]

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).…”

Cold pools over the Netherlands: A statistical study from tower and radar observations

“…Numerous recent studies have highlighted the importance of CP effects in structuring the convective cloud and precipitation field over space and time (Rio et al ., 2009; Böing et al ., 2012; Schlemmer and Hohenegger, 2014; Böing, 2016; de Szoeke et al ., 2017; Haerter and Schlemmer, 2018; Haerter et al ., 2019; 2020). Further observational studies along the lines presented here may help clarify pressing modelling questions, such as how parameterized CPs are affected by large‐scale weather conditions, or the numerical grid resolution required to resolve the gust fronts of spreading CPs appropriately.…”

“…CPs may therefore be a key ingredient to the understanding of how clouds organize spatially and temporally into larger‐scale precipitating systems (Böing, 2016; Haerter, 2019; Haerter et al ., 2019). The “upscale communication” between small‐scale individual CPs ( horizontally and temporally) and the larger‐scale spatial organization of the cloud field (, respectively , such as mesoscale convective systems) has been explored in both theoretical (Rotunno et al ., 1988; Jeevanjee and Romps, 2013; Haerter et al ., 2019; Haerter et al ., 2020) and observational studies (Zipser, 1977; Feng et al ., 2015; Zuidema et al ., 2017). All the more, there is a need for better understanding of the processes that determine the temporal evolution of CP properties, such as their spatial extent, lifetime, and strength (Drager and van den Heever, 2017).…”