Idealized study of the interaction between squall lines and convective cells

Squall lines are mesoscale lines consisting of numerous convective cells with an along-line extent of at least 100 km and a smaller across-line extent on the order of tenths of kilometers. Typically, squall lines form in a sheared environment by interactions of convective cells that are near to each other. Over time, the convective cells form a continuous cold pool that drives the motion of the squall line.

Initiating single convective cells in a sheared environment leads to the production of vertical vorticity due to tilting. Hence, these cells are connected to positive and negative vertical vorticity patches. While a squall line forms a band of positive and negative vorticity patches along the line, single cells will form a vorticity dipole.

In this work, we investigate the interaction between squall lines and convective cells initiated in the vicinity of the squall line. This interaction can be understood as an interaction between different vortices. We study different configurations using (1) idealized simulations with the non-hydrostatic, convection-permitting Cloud Model 1 (CM1) and (2) a theoretical point vortex model representative of the vorticity ensembles.

Squall line simulations with CM1 are initially run for 3 hours until a mature squall line forms. After 3 hours, convective cells are initiated at different positions near the squall line. The impact of the convection on the squall line is studied by an analysis of the squall line intensity and motion. In addition, the setup is studied using a point vortex model of a comparable vortex ensemble. A point vortex model is an idealized, mathematical model that describes vortex dynamics of a two-dimensional, non-divergent, inviscid atmosphere. Despite these strong limitations, it can be used to understand vortex dynamics and interactions in a relatively simple, but descriptive way. Moreover, the model describes well the vortex dynamics on the larger, synoptic scale. For example, a vortex couple of zero total circulation with the positive vortex south of the negative one starts to move towards the west which counteracts the typical westerly flow of the midlatitudes and, hence, leads to a slowed-down motion of the couplet compared to the westerlies.

The general aim of this work is to better understand the interaction between single cells and convective lines. The results will contribute towards improved squall line forecasting.