A Numerical Simulation of Cyclic Mesocyclogenesis

A three-dimensional nonhydrostatic numerical model, the Advanced Regional Prediction System, is used to study the process of cyclic mesocyclogenesis in a classic supercell thunderstorm. During the 4-h simulation, the storm’s mesocyclone undergoes two distinct occlusions, with the beginning of a third indicated at the end of the simulation. The occlusion process exhibits a period of approximately 60 min and is qualitatively similar in each case. Initial midlevel (3–7 km) mesocyclogenesis proceeds according to the ‘‘classic’’ picture, that is, via tilting of streamwise environmental vorticity. The development of an evaporatively driven rear-flank downdraft (RFD) signals the beginning of the occlusion process. The developing RFD wraps cyclonically around the mesocyclone, causing the gust front to surge outward. Simultaneously, the occluding mesocyclone rapidly intensifies near the surface. Trajectory analyses demonstrate that this intensification follows from the tilting and stretching of nearground (,500 m) streamwise vorticity produced by baroclinic generation, crosswise exchange, and streamwise stretching along descending parcel trajectories in the RFD. The surging gust front also initiates updraft development on the downshear flank at midlevels, resulting in a two-celled updraft structure. As the near-ground mesocyclone becomes detached from the gust front due to the developing occlusion downdraft, the upshear updraft flank weakens as its conditionally unstable inflow is cut off at low levels; at the same time, the downshear updraft flank continues to develop eastward. The end of the occlusion process is signaled as the old near-ground mesocyclone becomes completely embedded near the surface in divergent outflow beneath the decaying updraft and is advected away by the mean flow. Near-ground mesocyclogenesis is initiated in the new updraft in a process nearly identical to that of the initial mesocyclone. However, after the first occlusion, near-ground equivalent potential temperature and buoyancy contours are fortuitously oriented such that streamwise baroclinic generation can proceed without delay. Thus, although the initial occlusion requires two hours to become fully organized, the second occurs only one hour later. In effect, the occlusion appears to set the stage for more rapid development of subsequent mesocyclones.