The Time Scale of Shallow Convective Self‐Aggregation in Large‐Eddy Simulations Is Sensitive to Numerics

Abstract

A striking feature of idealized simulations of the tropical atmosphere in radiative-convective equilibrium (RCE) is the spontaneous aggregation of their column-integrated moisture and convection into large clusters (Bretherton et al., 2005;Muller & Held, 2012). Many mechanisms have been proposed to explain this, including the collision and convective triggering of horizontally expanding and colliding cold pools of evaporated precipitation (Böing, 2016;Haerter, 2019;Tompkins, 2001) and gravity wave-convection interactions (Yang, 2021). Yet, perhaps the strongest consensus is on the importance of shallow circulations (Muller et al., 2022;Shamekh et al., 2020), configured to transport moisture from dry to moist columns.These circulations can be traced to differential, radiative cooling between moist regions, which trap outgoing longwave radiation in their moisture-rich lower atmosphere and under high clouds, and dry regions, which more readily radiate their thermal energy to space (Muller & Held, 2012). Such heating anomalies give rise to ascent in moist columns and descent in dry columns, and may be framed as a moisture-radiation instability (Beucler & Cronin, 2016;Emanuel et al., 2014) with negative moist gross stability (Bretherton et al., 2005;Raymond et al., 2009). However, the circulations may also be reinforced by turbulent mixing at cloud edges, which deposits moisture in the free troposphere and thus raises the livelihood and vigor of any subsequent convection; differential convection may then itself result in a net ascent of moist, convecting regions and descent in

Numerical Diffusion and Turbulent Mixing in Convective Self‐Aggregation

“…The importance of energy dissipation in convective permitting models at coarse resolution was also found to be critical for shallow convective self‐aggregation by Janssens et al. (2022). Interestingly, the appearance of larger and more energetic structures by damping small‐scale structures (imposing larger values of the Smagorinsky constant) is also a common feature in Large Eddy Simulations of turbulent channel flows (Hwang & Cossu, 2010).…”

“…(2022) and Janssens et al. (2022) in the aggregation of deep convection in several models and by C. Bretherton and Blossey (2017) in the mesoscale aggregation of shallow cumulus (see their Figure 16). The most prominent change of SAM and WRF with the reduction of turbulent mixing is evident in the initial stages in the variability of TWP, as shown in Figure 4d.…”

“…Here we demonstrate that in CRMs, the initial large-scale variability of convection is regulated by small-scale mixing processes. The importance of energy dissipation in convective permitting models at coarse resolution was also found to be critical for shallow convective self-aggregation by Janssens et al (2022). Interestingly, the appearance of larger and more energetic structures by damping small- 1).…”

“…At this coarse grid resolution deep convection and turbulence motions inside clouds are not resolved and overturning circulations occur in a laminar mode (Bryan et al., 2003). Moreover at coarse grid resolutions, dissipation processes in cold pools (Grant & van den Heever, 2016) and shallow convection (Janssens et al., 2022), which are fundamental for the development of shallow circulations, may depend considerably on the model's choice.…”

Computational Domain Size Effects on Large-Eddy Simulations of Precipitating Shallow Cumulus Convection

“…Precipitation can alter the spatial organization of clouds, leading to the formation of large persistent structures [1]. Even in non-precipitating cumulus convection, variance accumulation at the largest scales and generation of large horizontal structures is observed [21]. The interplay between domain size and precipitation remains relatively unexplored.…”