The mechanisms for convection triggering by cold pools

Cold pools are important ingredients in deep convective regimes. They contribute to organizing convection in the sub-cloud layer and are considered to play an important role, for example, in the transition from shallow to deep convection and in sustaining deep convection by triggering new convective cells. It has long been acknowledged that cold pools promote convection through collisions: sufficiently moist parcels situated at the gust fronts of colliding cold pools can be mechanically lifted to their level of free convection. More recently, it has also been suggested that cold pools can initiate convection when enough moisture accumulates in patches along their edges. In this case, evidence indicates that convection is triggered on a longer time scale, when cold pools are already recovering. Although both triggering mechanisms have been invoked as responsible for sustaining deep convection, it is not clear which dominates in different scenarios. Here, this problem is addressed using a Lagrangian particle dispersion model and by tracking cloud parcels, particularly those triggered by cold pools, back to the point where they originate in the sub-cloud layer. Analyzing the state of the cold pool at the time of triggering provides an indication of whether the nature of the triggering is mechanical or thermodynamical or a combination of both. Statistics carried out over a collection of parcels are then discussed for various cases, over land and over the ocean.