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TOWARD AN IMPROVED UNDERSTANDING OF THE SYNOPTIC AND MESOSCALE DYNAMICS GOVERNING NOCTURNAL HEAVY-RAIN-PRODUCING MESOCALE CONVECTIVE SYSTEMS

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April 21, 2015
John Peters
Hosted by Russ Schumacher (Advisor), Dick Johnson, Sue van den Heever, Morris Weisman (Affiliate), Jeffery Niemann (Civil and Environmental Engineering)

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. Moisture, convective instability (which was maximized above ground level), and lift are continuously supplied to the location of the MCS by the large-scale environment, which maintains the minimum requirements for deep moist convection over a fixed region. An initial southeastward moving convective line propagates along the southeastern outflow boundary (OFB), where wind shear conditions were favorable for robust kinematic lifting. The linear morphology of ROD is facilitated by low-level convergence generated by low-level pressure perturbations along, and left in the wake of the leading convective line. Southwesterly low-level flow is thermodynamically stabilized as it lifted over the southwestern OFB due to a pattern of adiabatic cooling below latent heating. This flow travels ~ 100 km northeastward beyond the surface OFB to the point where large-scale lifting sufficiently re-destabilized the flow for deep convection. These factors explain both the temporal offset of ROD from the initial non-stationary convection, and the geographic offset of ROD from the surface OFB.

Eventually the western flank of the cold pool (and the associated thermodynamic stabilization affect) weeks and allows new convection to develop near the OFB. This new convection locally intensifies the cold pool, which drives the MCS southward and away from the region of heavy rainfall. The MCS weakens as large-scale low-level lifting diminishes.