Toward an improved understanding of the synoptic and mesoscale dynamics governing nocturnal heavy-rain-producing mesocale convective systems

Abstract

In the first stage of this research, rotated principal component analysis was applied to the atmospheric fields associated with a large sample of heavy-rain-producing mesoscale convective systems (MCSs) that exhibited the training-line adjoining stratiform (TL/AS) morphology. Cluster analysis in the subspace defined by the leading two resulting principal components revealed two sub-types with distinct synoptic and mesoscale characteristics, which are referred to as warm-season type and synoptic type events respectively. Synoptic type events, which tended to exhibit greater horizontal extent than warm-season type events, typically occurred downstream of a progressive upper-level trough, along a low-level potential temperature gradient with the warmest air to the south and southeast. Warm-season type events on the other hand occurred within the right entrance region of a minimally-to-anticyclonically curved upper level jet streak, along a low-level potential temperature gradient with the warmest low-level air to the southwest. Synoptic-scale forcing for ascent was stronger in synoptic type events, while low-level moisture was greater in warm-season type events. Warm-season type events were frequently preceded by the passage of a trailing stratiform (TS) type MCS, while synoptic type events often occurred prior to the passage of a TS type system. An idealized modeling framework was developed to simulate a quasi-stationary heavy-rain-producing MCSs. A composite progression of atmospheric fields from warm season TL/AS MCSs was used as initial and lateral boundary conditions for a numerical simulation of this MCS archetype. A realistic TL/AS MCS initiated and evolved within a simulated mesoscale environment that featured a low-level jet terminus, maximized low-level warm air advection, and elevated maximum in convective available potential energy. The first stage of MCS evolution featured an eastward moving trailing-stratiform type MCS that generated a surface cold pool. The initial system was followed by rearward off-boundary development (ROD), where a new line of convective cells simultaneously re-developed north of the surface cold pool boundary. Backbuilding persisted on the western end of the new line, with individual convective cells training over a fixed geographic region. The final stage was characterized by a deepening and southward surge of the cold pool, resulting in the weakening and slow southward movement of the training line. The dynamics of warm season TL/AS MCSs are elucidated through the analysis of the idealized simulation, along with a simulation of an observed case. The environmental conditions external to the MCS contributed to the development of a new convective line west of the initial MCS, and displaced northward of the southwestern flank of the surface OFB. Southwesterly low-level flow was thermodynamically stabilized as it lifted over the southwestern OFB from a pattern of adiabatic cooling below latent heating. This flow traveled 80-100 km northeastward beyond the surface OFB to the point where large-scale lifting sufficiently destabilized the flow for deep convection. These factors explain the geographic offset of the second convective line from the surface OFB left by the forward-propagating MCS. Eventually the surface cold pool became sufficiently deep so that gradual ascent of parcels with moisture and instability over the feature began triggering new convection close to the OFB (rather than 80-100 km away from it), which eventually drove the system southward. These results suggest that large-scale environmental factors were predominantly responsible for the quasi-stationary behavior of the simulated MCS, though upscale convective feedbacks played an important role in the complexity of the convective evolution.