Editorial Type:
Article
Article Type:
Research Article

Effect of Three-Dimensional Structure on the Stormwide Horizontal Accelerations and Momentum Budget of a Simulated Squall Line

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

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Margaret A. LeMone National Center for Atmospheric Research, * Boulder, Colorado

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William C. Skamarock National Center for Atmospheric Research, * Boulder, Colorado

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Print Publication:
01 Oct 1998
DOI:
https://doi.org/10.1175/1520-0493(1998)126<2580:EOTDSO>2.0.CO;2
Page(s):
2580–2598
Restricted access

Abstract

Past studies of the effects of mesoscale convective systems (MCSs) on the environmental flow have been limited by data coverage and resolution. In the current study the MCS-scale (stormwide) horizontal accelerations and momentum budget associated with an oceanic MCS are analyzed using output from a high-resolution three-dimensional numerical model integrated over a large domain. The simulation is based on an observed MCS that occurred on 22 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. An important aspect of both the observed and simulated MCS is its evolution from a quasi-two-dimensional to an asymmetric three-dimensional morphology, which was demonstrated in companion studies to result from the finite length of the MCS interacting with environmental vertical shear that varies in direction with height. Herein, the authors focus on the effects of the three-dimensional structure on MCS-scale horizontal accelerations.

The horizontal accelerations over the central portion of the MCS, where its leading edge is perpendicular to the low-level environmental vertical shear, resemble those from available observations and two-dimensional models of linear squall-type MCSs. However, the vertical structure of horizontal accelerations is quite different on the MCS scale. Zonal accelerations, which are aligned along the environmental low-level vertical shear, generally exceed meridional accelerations in the lower and upper troposphere, and are dominated by the vertical flux convergence term at low levels, and by the horizontal flux convergence term at upper levels. In contrast, zonal accelerations are weaker than meridional accelerations at midlevels, owing to strong cancellation of zonal accelerations in the central portion with those along the northern periphery of the MCS, where both the alignment of the convective band relative to the environmental vertical shear and its mesoscale organization are different. This compensation between different regions of the MCS results in modifications to the environmental vertical shear by mesoscale convection that differ substantially from those typically reported in idealized studies of two-dimensional squall lines. Since three-dimensional organization often occurs in MCSs that lack persistent external linear forcing, the current findings may have implications for the parameterization of the momentum effects of mesoscale deep convection in large-scale models.

Corresponding author address: Dr. Stanley B. Trier, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000.

Abstract

Past studies of the effects of mesoscale convective systems (MCSs) on the environmental flow have been limited by data coverage and resolution. In the current study the MCS-scale (stormwide) horizontal accelerations and momentum budget associated with an oceanic MCS are analyzed using output from a high-resolution three-dimensional numerical model integrated over a large domain. The simulation is based on an observed MCS that occurred on 22 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. An important aspect of both the observed and simulated MCS is its evolution from a quasi-two-dimensional to an asymmetric three-dimensional morphology, which was demonstrated in companion studies to result from the finite length of the MCS interacting with environmental vertical shear that varies in direction with height. Herein, the authors focus on the effects of the three-dimensional structure on MCS-scale horizontal accelerations.

The horizontal accelerations over the central portion of the MCS, where its leading edge is perpendicular to the low-level environmental vertical shear, resemble those from available observations and two-dimensional models of linear squall-type MCSs. However, the vertical structure of horizontal accelerations is quite different on the MCS scale. Zonal accelerations, which are aligned along the environmental low-level vertical shear, generally exceed meridional accelerations in the lower and upper troposphere, and are dominated by the vertical flux convergence term at low levels, and by the horizontal flux convergence term at upper levels. In contrast, zonal accelerations are weaker than meridional accelerations at midlevels, owing to strong cancellation of zonal accelerations in the central portion with those along the northern periphery of the MCS, where both the alignment of the convective band relative to the environmental vertical shear and its mesoscale organization are different. This compensation between different regions of the MCS results in modifications to the environmental vertical shear by mesoscale convection that differ substantially from those typically reported in idealized studies of two-dimensional squall lines. Since three-dimensional organization often occurs in MCSs that lack persistent external linear forcing, the current findings may have implications for the parameterization of the momentum effects of mesoscale deep convection in large-scale models.

Corresponding author address: Dr. Stanley B. Trier, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000.

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