Particle‐Based Tracking of Cold Pool Gust Fronts

Abstract

The gust fronts of convective cold pools (CPs) are increasingly recognized as loci of enhanced triggering for subsequent convective cells. It has so far been difficult to track these gust fronts in high-resolution data, such as large eddy simulations (LES)-rendering mechanistic analysis of CP interaction incomplete. Here, a simple tracking method is defined, tested, and applied, which uses horizontal advection and a condition on horizontal divergence, to emit tracers at the perimeter of surface precipitation patches. Tracers are then reliably transported to the gust front, yielding closed bands marking the CP boundary. The method thereby allows analysis of the dynamics also along the gust front, which allows to identify point-like loci of pronounced updrafts. The tracking works well for a single idealized CP and reliably tracks a population of CPs in a midlatitude diurnal cycle. As the method uniquely links CPs and their tracers to a specific parent precipitation cell, it may be useful for the analysis of interactions in evolving CP populations.Plain Language Summary Cold pools form when rain under thunderstorm clouds evaporates before reaching the ground. These cold pools constitute heavier air, that sinks to the ground and spreads along the surface. It has been found that places, where multiple cold pools collide, constitute hot spots for new thunderstorms-cold pools hence act to "communicate" information between thunderstorms. To understand cold pool dynamics better in numerical simulations, their edges need to be identified. We provide a method that does just that, by placing particles at the edges of the initial thunderstorm and allowing these particles to be transported along the surface by the wind. The method is shown to work well and we describe why. It may be useful for the further exploration of cold pools, how they organize rainfall, and in particular, how extreme events can come about.

Dynamic and thermodynamic impacts of climate change on organized convection in Alaska

“…Nevertheless, most of these studies focus on midlatitude or tropical environments and little is known about continental high latitude regions. The main takeaway is that cold pools can decrease low-level buoyancy under a storm and lead to its dissipation, but they can also trigger new convection through the mechanical circulation induced by their movement and collisions (Henneberg et al 2020).…”

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

“…This radius R and the corresponding propagation speed of the CP, U ≡ dR / dt, are determined using a tracer method introduced in Henneberg et al. (2020). The CPs spread to radii of approximately 10 km within an hour, which is in line with other numerical and observational studies (Feng et al., 2015; Romps & Jeevanjee, 2016; Tompkins, 2001b).…”

“…The initial configuration and namelist files will be made available upon publication. The cold pool tracking code used for measuring the cold pool radius is described in Henneberg et al (2020) and available online (http://doi.org/10.5281/zenodo.3665813).…”

“…The cold pool tracking code used for measuring the cold pool radius is described in Henneberg et al. (2020) and available online (http://doi.org/10.5281/zenodo.3665813).…”

Cold Pools as Conveyor Belts of Moisture

“…Idealized large‐eddy simulations (LES) are run using the University of California, Los Angeles (UCLA) LES code (Stevens et al, 2005). Parameterizations of cloud microphysics and surface energy fluxes (supporting information Text S1, Pincus & Stevens, 2009; Seifert & Beheng, 2006; Smagorinsky, 1963; Stevens, 2010) make the model suitable for studying cold pools (e.g., Haerter et al, 2017; Henneberg et al, 2020; Moseley et al, 2016). The main aim of this study is to better understand the processes leading to deep convective triggering under RCE conditions.…”