Three-Dimensional Numerical Modeling of Convection Produced by Interacting Thunderstorm Outflows. Part I: Control Simulation and Low-Level Moisture Variations

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Abstract
The Klemp–Wilhelmson three-dimensional numerical cloud model is used to investigate cloud development along intersecting thunderstorm outflow boundaries. The model initial environment is characterized by a temperature and moisture profile typically found in strong convective situations, and the initial wind field is prescribed by a constant unidirectional shear 2.9 m s−1 km−1 from 0.8 to 8.9 km, with a constant wind everywhere else. The wind shear vector is perpendicular to the line containing the two initial outflow-producing clouds (which are spaced 16 km apart and are triggered by thermal impulses centered at the top of the boundary layer). The dynamics of the outflow collision are documented using time-dependent, kinematic air parcel trajectories and thermodynamic data. We find that ambient air in the outflow collision region is literally “squeezed” out of the way as the two outflows collide. Some of this air is lifted to saturation, triggering two convective clouds. The upshear member of the pair has a head start in development, and since the two clouds are growing close together and competing for the same air, the upshear cloud is the strongest. In addition to, the downshear cell is suppressed because it grows into the region occupied by the upshear cell's downdraft and rain region. By looking at the various terms in the inviscid form of the vertical momentum equation, we find that low-level air approaching the gust front along the outflow collision line is forced to rise up and over the cold air pool due to a deflection by the pressure gradient force. A third cloud is triggered along the outflow collision line as a result of this frontal uplifting, which is in contrast to the first two cells which are triggered primarily by the forced uplifting from the outflow collision. Air parcel trajectories indicate that even though the first two cells along the outflow collision line are triggered by a different mechanism than subsequent cells, the air comprising each updraft core is virtually undiluted, and comes from the same general region (z = 0 ∼ 0.3 km). On their way to the cloud updrafts, some low-level air parcels approaching the outflow cross the cold air interface. This is a manifestation of the well-known fact that the gust front is a region of turbulent mixing. Once above the outflow, these air parcels may pass through several updrafts and downdrafts as they traverse the cloud region. The modeled clouds are found to be sensitive to the low-level (0–1 km) moisture. When the moisture in this layer is increased, the collision line clouds become stronger and the rapidity of new cell development increases markedly. Decreasing the low-level moisture has the opposite effect, to the point that only weak shallow clouds form along the outflow collision line. Furthermore, a decrease in the low-level moisture is accompanied by a decrease in the outflow temperature deficit. This in turn decreases the outflow speed, a result that is consistent with classical inviscid density current theory.

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