INTERACTION focused on how thunderstorm clouds influence each other through their hydrodynamics and thermodynamics. Hydrodynamically, a convective cell is characterized by initially rapid updrafts, subsequent rain formation and finally a deep convective cold pool (CP). CPs are caused by rain evaporation in the subcloud layer and often give rise to density currents spreading along the surface. The momentum transport can cause subsequent lifting and convection in the vicinity. Another mechanism, thermodynamically driven, is that of buoyancy transport through CPs.
Work within INTERACTION addressed the different effects mediated by CPs:
- the small-scale interaction mechanisms between CPs and the resulting self-organized structures.
- isolated interaction effects, studying the resolution dependence of CPs and their dynamical properties as well as the collision mechanism.
- tracking of CPs in models and observations, e.g. from satellite, using particle-based dynamical methods and machine-learning based statistical methods to distinguish CP from non-CP regions.
- emergent self-organization: INTERACTION employed a range of approaches, from conceptual models, e.g. a "circle model" where cold pools are effectively modeled as spreading circles that are able to collide, to more "coarse-grained" models where the collision field between cold pools is described to cloud-resolving simulations of CPs organizing under the diurnal cycle and over ocean. Also wind shear was imposed to mimic a realistic large-scale forcing for the tropical or subtropical atmosphere. Important findings included "diurnal self-aggregation", a novel insight into so-called "convective self-aggregation".
In INTERACTION, several new routes to convective self-aggregation were identified: (a) that interacting CPs together with a mesoscale energetic constraint can already give rise to strongly aggregated atmospheres where only a part of the domain is covered by convection whereas other parts are cloud free; (b) that convective self-aggregation can be induced under conditions of relatively high horizontal resolution (~1km) when combined with a surface temperature diurnal cycle, giving rise to strong CP effects and organization into mesoscale convective systems. This self-organization does not take place when the diurnal cycle is removed. In addition, once organized, strong clustering persists when the diurnal cycle is switched off - mimicking a transition from land to sea.
Since 2023, INTERACTION has contributed to a dedicated field campaign "High-resolution weather observations east of Dakar (DakE)" in Senegal, a region well-known for its convective self-organization effects: mesoscale convective systems are common during the rainy season and drive extreme rainfall - leading to flood risk in metropolitan areas, e.g. Dakar. The team has been implementing an observational network east of Dakar with more than 15 automatic weather stations as well as ~100 additional measurement devices including soil, flood and humidity sensors.
Several conferences were organized in INTERACTION: Niels Bohr Institute, U Copenhagen (May 5-7, 2001), Utrecht (2002) and ICTP Trieste (2003 & 2004). The "Workshop of Convective Organization" is now a yearly recurring event drawing ~100 scientists from around the world. In 2026 the event is taking place in Brazil.
