A Simple Lagrangian Parcel Model for the Initiation of Summertime Mesoscale Convective Systems over the Central United States

Mesoscale convective systems (MCSs) account for more than 50% of summertime precipitation over the central United States and have a significant impact on local weather and hydrologic cycle. It is hypothesized that the inadequate treatment of MCSs is responsible for the long-standing warm and dry bias over the central United States in coarse-resolution general circulationmodel (GCM) simulations. In particular, a better understanding ofMCS initiation is still lacking. Here a single-column Lagrangian parcel model is first developed to simulate the basic features of a rising parcel. This simple model demonstrates the collective effects of boundary layermoistening and dynamical lifting in triggering convective initiation and reproduces successfully its early afternoon peak with surface equivalent potential temperature as a controlling factor. It also predicts that convection is harder to trigger in the future climate under global warming, consistent with the results from convection-permitting regional climate simulations. Then, amulticolumn model that includes an array of single-column models aligned in the east-west direction and incorporates idealized cold pool interactionmechanisms is developed. Themulticolumn model captures readily the cold pool-induced upscale growth feature inMCS genesis from initially scattered convection that is organized into a mesoscale cluster in a few hours. It also highlights the crucial role of lifting effects due to cold pool collision and spreading, subsidence effect, and gust front propagation speed in controlling the final size ofmesoscale clusters and cold pool regions. This simplemodel should be useful for understanding fundamental mechanisms of MCS initiation and providing guidance for improvingMCS simulations in GCMs.