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.