Objective Analysis and Numerical Simulations of Elevated Heavy-Rain-Producing Mesoscale Convective Systems

This research employs objective analysis techniques to refine the Training-Line/Adjoining Stratiform (TL/AS) mesoscale convective system (MCS) archetype into two sub-categories. Fifty heavy-rain producing TL/AS events were subjectively identified through visual analysis of composite radar reflectivity. Rotated principal component analysis was performed on atmospheric fields associated with these events. Cluster analysis within the subspace defined by the leading two principal components (PCs) revealed a distinct bimodal distribution of events. These two subtypes were termed synoptic-type and warm season-type events. Composite atmospheric fields for each sub-category, along with numerical simulations of individual cases were then analyzed.

Synoptic type events typically occur in the spring and fall months upstream of a progressive upper level trough. Training of convection and heavy rainfall occurs to the north of a warm front and associated southerly low-level jet, within a region of maximized low-level convergence and isentropic up-glide. Training behavior results from a balance of low-level rain-cooled convective outflow and opposing inflow from the low level jet, which continuously supplies elevated conditionally unstable air to the MCS.

Warm season type events occur predominantly during the summer months in association with the right entrance region of an anticyclonically curved jet stream/streak. Training of convection again occurs north of a quasi-stationary west-east oriented boundary and associated southerly low-level jet. Gravity waves generated by latent heating from initial convective development become near-stationary upstream of the MCS, vertically agitating an elevated conditionally unstable layer supplied by the low-level jet and generating moist-absolutely-unstable (MAUL) layers. Convection erupts from MAUL layers on the upstream side of the MCS where a convectively induced horizontal perturbation pressure gradient field modifies the environmental wind field favorably for low-level convergence and continuous convective regeneration.

These results illustrate that while the two types of convective systems identified here are commonly grouped into the same MCS archetype, their associated synoptic scale environments and governing dynamics exhibit noteworthy differences, highlighting the utility of principle component analysis in refining pre-existing subjective MCS archetypes.