Influence of Balanced Motions on Heavy Precipitation within a Long-Lived Convectively Generated Vortex

Stanley B. Trier National Center for Atmospheric Research, Boulder, Colorado

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Christopher A. Davis National Center for Atmospheric Research, Boulder, Colorado

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Abstract

The forcing of heavy precipitation within a long-lived convectively generated mesoscale vortex (MCV) is investigated with the aid of diagnoses from Rapid Update Cycle gridded analyses. Organized convection within the MCV followed a distinct diurnal cycle, which featured organized mesoscale convective systems (MCSs) that matured overnight near the MCV center on successive days. The MCV was typically most intense in the middle troposphere, but intensified within the lower troposphere during the episodes of organized nocturnal convection.

The lower-tropospheric vertical shear was an important organizing factor in MCS development and sustenance, in the sense that its interaction with the cold temperature anomaly beneath the MCV center determined where balanced lower-tropospheric ascent occurred. From trajectory analyses, we estimate that balanced ascent accounted for approximately half of the total vertical displacement of the thermodynamically unstable air that eventually composed elevated saturated layers immediately upstream of areas of active deep convection within the MCS. Flooding occurred overnight during a portion of the MCV life cycle when the balanced ascent became located toward the rear flank of the MCS (i.e., opposite to the orientation of the mean flow). This evolution served to focus the propagation of the region of intense convection toward a direction opposite to the overall MCS movement, thereby slowing the envelope of heavy precipitation.

Corresponding author address: Dr. Stanley B. Trier, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307-3000. Email: trier@ucar.edu

Abstract

The forcing of heavy precipitation within a long-lived convectively generated mesoscale vortex (MCV) is investigated with the aid of diagnoses from Rapid Update Cycle gridded analyses. Organized convection within the MCV followed a distinct diurnal cycle, which featured organized mesoscale convective systems (MCSs) that matured overnight near the MCV center on successive days. The MCV was typically most intense in the middle troposphere, but intensified within the lower troposphere during the episodes of organized nocturnal convection.

The lower-tropospheric vertical shear was an important organizing factor in MCS development and sustenance, in the sense that its interaction with the cold temperature anomaly beneath the MCV center determined where balanced lower-tropospheric ascent occurred. From trajectory analyses, we estimate that balanced ascent accounted for approximately half of the total vertical displacement of the thermodynamically unstable air that eventually composed elevated saturated layers immediately upstream of areas of active deep convection within the MCS. Flooding occurred overnight during a portion of the MCV life cycle when the balanced ascent became located toward the rear flank of the MCS (i.e., opposite to the orientation of the mean flow). This evolution served to focus the propagation of the region of intense convection toward a direction opposite to the overall MCS movement, thereby slowing the envelope of heavy precipitation.

Corresponding author address: Dr. Stanley B. Trier, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307-3000. Email: trier@ucar.edu

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