Mesoscale Convective Vortices from BAMEX: An Overlooked Aspect (Invited Presentation)

The seminal paper by Raymond and Jiang (1990) pointed out the basic pattern of vertical motion that accompanies a mesoscale convective vortex (MCV) in vertical wind shear. MCVs over continental areas typically maximize their potenial vorticity between 600 and 500 hPa, and when superposed in deep vertical wind shear, they produce lifting on the downshear side. Such lifting can overcome convective inhibition to initiate deep, moist convection, thereby sustaining a mesoscale convective system. The subsequent convection may reinvigorate (or reform) the original MCV, and so initiate a strong diurnal cycle of deep convection and vorticity (and potential vorticity) generation.

One element of the adiabatic vertical motion has been apparently overlooked. Given the seeming omnipresence of large CAPE in continental environments (especially over the central United States) in the warm season, and the related presence of convective inhibition, the primary focus of MCV studies has been on convective initiation. Yet, the environments of MCVs are often not like the typical, large-CAPE environments that endow severe convection. Rather, CAPE is often modest or near zero when entrainment and condensate loading are considered. Conditions often resemble tropical maritime environments more than severe-weather producing continental environments. In particular, lifting associated with an MCV in vertical shear increases column saturation fraction in addition to reducing convective inhibition.

Based on a re-examination of dropsondes and rawinsondes collected during 5 intensive observing periods (IOPs) from the 2003 Bow Echo and MCV Experiment (BAMEX), we demonstrate that the increase in column saturation fraction, in the presence of small but non-zero CAPE, enables sustained moderate to heavy rainfall. The location and timing of rainfall favors regions of high saturation fraction in the evening, when radiative destabilization could enhance the favorability for widespread rainfall. These results imply that while MCVs may trigger deep convection in environments with significant CAPE and convective inhibition, the pattern of sustained, heavy rainfall adheres more to areas of high saturation fraction, small CAPE (owing partly to vorticity influences on thermodynamics) and small inhibition with respect to reversible lifting.