Long-lived mesoconvective vortices and their environment. Part I: Observations from the central United States during the 1998 warm season

Observations from the modernized United States National Weather Service (NWS) data network are used to assess the frequency and general characteristics of midtropospheric cyclonic vortices (MCVs) generated by mesoscale convective systems (MCSs). Results from the 1998 convective season (15 May-15 September) over the central United States suggest that long-lived MCVs, which persist after the dissipation of the initiating MCS, are more common than previously documented. These MCVs occur in weaker ambient vertical shear (both in the lower troposphere and through a nominal vortex layer) than MCSs from which no detectable MCVs are spawned. An important aspect of MCVs is that they may focus subsequent convective development within long-lived discontinuous heavy precipitation episodes. Subsequent deep convection is observed in the vicinity of MCVs in slightly greater than 1/2 of the MCV cases. This subsequent convection occurs in thermodynamic environments of moderate-to-large convective available potential energy and small convective inhibition, and is located in a region from the center of the MCV circulation outward to its downshear periphery. This location is consistent with lower-tropospheric ascent arising from the interaction of a quasi-balanced vortex with the ambient vertical shear. Long-lived organized convection near the MCV center is likely crucial in either sustaining or reinvigorating vortices during the relatively rare MCV events that persist longer than a diurnal cycle. Examples from the 1998 convective season are used to illustrate differences in the relationship between the MCV circulation and the organization of subsequent convection among different MCV cases.