The Role of Cold Pools in Tropical Oceanic Convective Systems

Abstract

The processes governing organized tropical convective systems are not completely understood despite their important influences on the tropical atmosphere and global circulation. In particular, cold pools are known to influence the structure and maintenance of midlatitude systems via Rotunno–Klemp–Weisman (RKW) theory, but cold pools may interact differently with tropical convection because of differences in cold pool strength and environmental shear. In this study, the role of cold pools in organized oceanic tropical convective systems is investigated, including their influence on system intensity, mesoscale structure, and propagation. To accomplish this goal, high-resolution idealized simulations are performed for two different systems that are embedded within a weakly sheared cloud population approaching radiative–convective equilibrium. The cold pools are altered by changing evaporation rates below cloud base in a series of sensitivity tests. The simulations demonstrate surprising findings: when cold pools are weakened, the convective systems become more intense. However, their propagation speeds and mesoscale structure are largely unaffected by the cold pool changes. Passive tracers introduced into the cold pools indicate that the convection intensifies when cold pools are weakened because cold pool air is entrained into updrafts, thereby reducing updraft intensity via the cold pools’ initial negative buoyancy. Gravity waves, rather than cold pools, appear to be the important modulators of system propagation and mesoscale structure. These results reconfirm that RKW theory does not fully explain the behavior of tropical oceanic convective systems, even those that otherwise appear consistent with RKW thinking.

Particle‐Based Tracking of Cold Pool Gust Fronts

“…As in Haerter et al (), the present tracking method exploits the fact that CPs generally are confined by a band of convergence, using Lagrangian tracers. Allowing advection in all three spatial dimensions, Lagrangian particles were previously applied to analyze CPs (Grant et al, ; Torri & Kuang, ; Torri et al, ). To prevent mixing of particles into the upper boundary layer, the current particle‐based method demands that tracer particles are constrained to travel only horizontally (Figure a,b).…”

Simulating the Effects of Surface Energy Partitioning on Convective Organization: Case Study and Observations in the US Southern Great Plains

“…To better clarify the role of cold pools in convective organization, we have performed “no‐cold pool” experiments by adjusting the Thompson microphysics. The most common way to remove cold pools in model experiments is to turn off the evaporation of rain (such as Grant et al., 2018; Jeevanjee & Romps, 2013; Khairoutdinov & Randall, 2006). That method should remove the cold air, but downdrafts below the convective updrafts could still exist because the downward dynamic pressure gradient force as well as hydrometeor loading effect is not removed.…”

Does Lower‐Stratospheric Shear Influence the Mesoscale Organization of Convection?

“…Shear in the middle and upper‐troposphere is also important (e.g., Parker & Johnson, 2000). However, in situations with weaker shear or weaker cold pools, other mechanisms, like gravity waves, can have a prominent impact on MCS dynamics (e.g., Lane & Moncrieff, 2015, hereafter LM15; Grant et al., 2018, 2020). In this study, a new paradigm for convective organization is introduced, whereby stratospheric wind shear is shown to influence MCSs through gravity wave‐convection interactions.…”

“…Lane and Zhang (2011) showed that gravity waves and mesoscale cloud populations become dynamically coupled, with the gravity waves feeding back on the convective clouds through transient ascent and descent that modifies their morphology. Grant et al (2018Grant et al ( , 2020 demonstrated that when cold pools are weak, gravity waves can help maintain long-lived mesoscale systems. In this study, idealized simulations (similar in concept to Grant et al, 2018Grant et al, , 2020Lane & Zhang 2011;Tulich & Mapes, 2008;LM15 and others) are used to further explore gravity wave-convection coupling.…”