A numerical simulation of precipitation enhancement as a result of storm-storm interactions

The behavior and structure of thunderstorms can change substantially when interacting with other storms or pre-existing boundaries. Numerous cases have been reported in the literature in which an ordinary thunderstorm has been observed to become severe or even tornadic following the interaction with a low-level outflow boundary, and during the Verifications of the Origins of Rotation in Tornadoes Experiment (VORTEX) it was noted that a large percentage of significant tornadoes occurred near such boundaries. The possible storm enhancement and resultant precipitation field at the ground due to storm-storm or storm-outflow boundary interactions is therefore an important forecasting problem. Numerical simulations of the interaction between a storm and the low-level outflow boundary of another storm have been conducted using the Colorado State University Regional Atmospheric Modeling System (RAMS) to investigate the enhancement of precipitation produced at the ground following the interaction.

A 200 by 200 km model grid is initialized horizontally homogeneously using a horizontal grid spacing of 1 km and a variable vertical grid spacing beginning with 100 m. The single-moment version of RAMS microphysics is employed. The water species represented include vapor, cloud droplets, rain, pristine ice, snow, aggregates, graupel and hail. A warm, moist bubble is released to initiate convection and the storm is allowed to develop for an hour. A second warm bubble is then released upshear of the first storm and the simulation is run for another two hours, allowing the second storm to interact with the low-level outflow boundary of the first storm. A control simulation was performed in which only one warm bubble was released, the initial position of which was the same as that of the second bubble in the storm-interaction simulation. The second storm in the storm-interaction simulation begins to interact with the outflow boundary of the first storm after about 55 minutes of simulation time. Preliminary results indicate that approximately 25 minutes after the interaction, condensate mixing ratios at the lowest model level are between 20 and 30 % greater in the storm-interaction case than in the control simulation. This continues to be the case for the next 30 minutes of simulation time. Differences in vertical velocity, storm structure and vertical vorticity are also observed between the control and storm-interaction simulations and these will be presented. Reasons for the precipitation enhancement as a result of the storm-storm interaction will be suggested.