From Cold Pool Interaction to Extreme Precipitation

Cold pools play an important role in the formation and self-organization of deep convective thunderstorms. They are the dynamical driver for the interaction between multiple deep convective storms and may be implicated in the intensification of thunderstorms in the late afternoon. Thus, their accurate representation in global circulation models is required. Parameterizations can only be realized with a better understanding of the physical processes and properties of the cold pools.

Here, we uncover the entire causal chain of events formed by rain cell-cold pool interactions. Therefore, we combine a new tracer based, 3D cold pool tracking method with an extended rain, graupel and cloud water tracking that can describe the whole lifecycle of the rain cells beginning with the primary initiated clouds. The cold pool tracking is applied on idealized LES-simulation of diurnal convection conducted with the UCLA-LES model. The new developed tracking method provides a tool to determine the radius of influence of cold pools. The probability of cold pools interacting is an increasing function of their radius of influence and decreases with the mean spacing between cells. Those interactions can be caused by collision of the respective cold pool guest fronts or the convergence of air masses in between several cold pools. The tracer method can quantify how many cold pools contribute to new convective events. Furthermore, the time delay between the collision process of cold pools and the resulting precipitation cell caused by the cloud microphysical processes is investigated.

Immediately after the transition from shallow to deep convection, the convection is mainly thermodynamically driven. An enhanced number of cold pools contributing to new convective events induce the transition into a self-organized state. While cold pool collisions at the early stage do not necessarily form deep convection, in the late afternoon collisions of most likely three cold pools trigger convection with a certain appreciable probability and start to organize. A key quantity we identify is the probability of forming new convective events by cold pool interaction. This quantity, together with the propagation speed of cold pools, will serve us as a starting point for prediction of extreme convective precipitation events.