A numerical simulation of cyclic mesocyclogenesis

A three-dimensional nonhydrostatic numerical model, the Advanced Regional P rediction System, is used to study the process of cyclic mesocyclogenesis i n a classic supercell thunderstorm. During the 4-h simulation, the storm's mesocyclone undergoes two distinct occlusions, with the beginning of a thir d 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 "class ic" picture, that is, via tilting of streamwise environmental vorticity. Th e development of an evaporatively driven rear-flank downdraft (RFD) signals the beginning of the occlusion process. The developing RFD wraps cyclonica lly around the mesocyclone, causing the gust front to surge outward. Simult aneously, the occluding mesocyclone rapidly intensifies near the surface. T rajectory analyses demonstrate that this intensification follows from the t ilting and stretching of near-ground (<500 m) streamwise vorticity produced by baroclinic generation, crosswise exchange, and streamwise stretching al ong descending parcel trajectories in the RFD. The surging gust front also initiates updraft development on the downshear flank at midlevels, resultin g in a two-celled updraft structure. As the near-ground mesocyclone becomes detached from the gust front due to the developing occlusion downdraft, th e 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 o ld 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 n early identical to that of the initial mesocyclone. However, after the firs t occlusion, near-ground equivalent potential temperature and buoyancy cont ours are fortuitously oriented such that streamwise baroclinic generation c an 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 o f subsequent mesocyclones.