THE ROLE OF MIDTROPOSPHERIC WINDS IN THE EVOLUTION AND MAINTENANCE OFLOW-LEVEL MESOCYCLONES

Using a three-dimensional numerical model, supercell simulations initi alized in environments characterized by hodographs with large curvatur e in the lowest 3 km and a range of linear midlevel shears are investi gated. For low values of the midlevel shear (0.005 s(-1)), the storm d evelops a mesocyclone at the lowest model level within the first hour of the simulation. The gust front starts to move ahead of the main upd raft and cuts off the inflow to the storm by approximately 2 h, result ing in decay of the initial storm and growth of a new rotating storm o n the outflow. As the midlevel shear increases to approximately 0.010 s(-1), the initial development of the low-level mesocyclone is delayed , but the,mesocyclone that develops is more persistent, lasting for ov er 2 h. Further increases of the shear to 0.015 s(-1) result in the su ppression of any low-level mesocyclone, despite the presence of intens e rotation at midlevels of the storm. We hypothesize that differences in the distribution of precipitation within the storms, resulting from the changes in storm-relative winds, are responsible for the changes in low-level mesocyclone development. In the weak-shear regime, storm- relative midlevel winds are weak and much of the rain is carried by th e midlevel mesocyclonic flow to fall west of the updraft. As this rain evaporates, baroclinic generation of vorticity in the downdraft leads to mesocyclogenesis at low levels of the storm. The outflow from the cold air associated with the rain eventually undercuts the inflow to t he storm. As the midlevel shear increases, the storm-relative winds in crease and more of the rain generated by the storm falls well away fro m the updraft. As a result, baroclinic generation of vorticity in the downdraft immediately west of the updraft is slower. Once a low-level mesocyclone is generated, however, the weaker outflow allows the mesoc yclone to persist.