7.6 Influence of BAMEX mesoscale convective vortices on secondary deep convection

Tuesday, 7 November 2006: 5:45 PM
St. Louis AB (Adam's Mark Hotel)
S. B. Trier, NCAR, Boulder, CO; and C. A. Davis

Observations from the Bow-Echo and MCV Experiment (BAMEX) are used to examine the role of the five mesoscale convective vortices (MCVs) on heavy precipitation during the daytime heating cycle. Persistent widespread stratiform rain without deep convection occurs for two strong MCVs in conditionally stable environments with strong vertical shear. Two other MCVs in moderate-to-strong vertical shear have localized redevelopment of deep convection, termed secondary convection, on their downshear side, where conditional instability exists. The strongest of the five MCVs occurs in weak vertical shear and has widespread secondary convection, which is most intense on its conditionally unstable southeast periphery.

The two MCVs with only localized secondary convection have well-defined mesoscale vertical motion couplets with d ownshear ascent and upshear descent above the planetary boundary layer (PBL). Although the amplitude is significantly greater, the kinematically derived vertical motion dipole resembles that implied by steady, vortex-relative isentropic flow, consistent with previous idealized (dry) simulations and diagnoses based on operational model analyses. In the other three cases with either widespread precipitation or weak environmental vertical shear, the kinematic and isentropic vertical motion patterns are poorly correlated.

Vertical motions above the PBL focus secondary convection through adiabatic cooling downshear and adiabatic warming upshear of the MCV center. The MCVs occur within surface frontal zones with large temperature and moisture gradients across the environmental vertical shear vector. Thus, the effect of vertical motions on conditional instability is reinforced by horizontal advections of high equivalent potential temperature, air downshear and low- air upshear within the PBL. On average, the quadrant immediately right of downshear (typically southeast of the MCV center) best supports deep convection due to the juxtaposition of greatest mesoscale ascent, high- PBL air, and MCV-induced enhancement of the vertical shear.

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