How weakened cold pools open for convective self-aggregation

Mapping Intimacies ◽

10.5194/egusphere-egu21-1520 ◽

2021 ◽

Author(s):

Silas Boye Nissen ◽

Jan O. Haerter

Keyword(s):

Crucial Role ◽

Generation Time ◽

Spatial Organization ◽

Large Eddy Simulations ◽

Cell Generation ◽

Cold Pools ◽

Large Eddy ◽

Simulated Rain ◽

Self Aggregation ◽

Large Domains

In radiative-convective equilibrium (RCE) simulations, convective self-aggregation (CSA) is the spontaneous organization into segregated cloudy and cloud-free regions. Evidence exists for how CSA is stabilized, but how it arises favorably on large domains is not settled. Using large-eddy simulations (LES), we link the spatial organization emerging from the interaction of cold pools (CPs) to CSA. We systematically weaken simulated rain evaporation to reduce maximal CP radii, Rmax, and find reducing Rmax causes CSA to occur earlier. We further identify a typical rain cell generation time and a minimum radius, Rmin, around a given rain cell, within which the formation of subsequent rain cells is suppressed. Incorporating Rmin and Rmax, we propose a toy model that captures how CSA arises earlier on large domains: when two CPs of radii ri,j &#8712; [Rmin, Rmax] collide, they form a new convective event. These findings imply that CPs play a crucial role in RCE simulations by preventing the onset of CSA.

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Self-aggregation conceptualized by cold pool organization

10.5194/egusphere-egu2020-19042 ◽

2020 ◽

Author(s):

Silas Boye Nissen ◽

Jan O. Haerter

Keyword(s):

Tropical Cyclones ◽

Large Scale ◽

Spatial Organization ◽

Cold Pool ◽

Mathematical Framework ◽

Madden Julian Oscillation ◽

Cold Pools ◽

Convective Cells ◽

Negatively Buoyant ◽

Self Aggregation

In radiative-convective equilibrium (RCE) simulations, self-aggregation is the spontaneous emergence of one or several long-lasting convective clusters from an apparently homogenous atmosphere (Wing, 2019). This phenomenon may implicate the formation of tropical cyclones (Wing et al., 2016; Muller et al., 2018) and large-scale events such as the Madden-Julian Oscillation (Arnold et al., 2015; Satoh et al., 2016; Khairoutdinov et al., 2018). However, it remains poorly understood how cold pools (CPs) contribute to self-aggregation. Using a suite of cloud-resolving numerical simulations, we link the life-cycle and the spatial organization of CPs to the evolution of self-aggregation. By tracking CPs, we determine the maximal CP radius Rmax &#8776; 20 km and show that cloud-free regions exceeding such radii always grow indefinitely. Besides, we identify a minimum CP radius Rmin &#8776; 8 km below which CPs are too cold, hence negatively buoyant, to initialize new convective cells. Finally, we suggest a simple mathematical framework that describes a mechanism, where cloud-free areas are likely to form when CPs have small Rmax, whereas large Rmax hampers cavity formation. Our findings imply that interactions between CPs crucially control the dynamics of self-aggregation, and known feedbacks may only be required in stabilizing the final, fully-aggregated state.

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