Forcing and Organization of Convective Systems

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  • a NOAA/National Severe Storms Laboratory, Boulder, Colorado
  • b National Center for Atmospheric Research, Boulder, Colorado
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Abstract

From its initial deployment as a research tool following the second World War, radar has played a fundamental role in revealing the forces that initiate and organize severe storms and larger mesoscale convective systems composed of a conglomeration of convective storm cells. Early radar observations were primarily descriptive and showed the tremendous variety of precipitating moist convection types and sizes. Examples include single convective storms, longer-lived multicellular storms, fast-moving squall lines, slower-moving linear and nonlinear convective systems, and long-lived supercell storms. Certain modes or types of convective systems were shown to possess a variety of hazardous weather that includes very heavy rain, large hail, straight-line damaging winds, tornadoes, and lightning. It was soon recognized that the type of convective system was strongly dependent on the environment in which it was embedded. Researchers determined that two variables were particularly important in describing convective behavior: the vertical profile of the horizontal wind and potential instability of the air feeding the system [convective available potential energy (CAPE)]. The types of convective systems are discussed here according to their typical shear and CAPE values. In addition to the knowledge gained from observational radar studies, considerable advancement in understanding of convective system dynamics has resulted from high-resolution numerical simulations.

In addition to being a critical factor in determining the particular structure and organization that convective systems assume once convection is initiated, radar (particularly in clear air mode) has been a leading tool in identifying forcing mechanisms for convective initiation. In particular, the role of “boundary layer forcing” in initiating convection has received much attention in recent years. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or discontinuities in Doppler velocity. Some of these mesoscale boundary layer mechanisms for producing upward motion include horizontal convective roles, sea-breeze circulations, drylines, gust fronts, orographic circulations (e.g., mountain–valley), and circulations resulting from horizontal inhomogeneities in surface character. Convection initiation sometimes does not occur continuously along boundaries but only at preferred along-boundary locations. Location preferences can sometimes be identified with boundary intersections, such as colliding gust fronts, sea-breeze fronts and rolls, and drylines and rolls. It is not always clear, however, why convection forms at certain locations along boundaries and not others. It is possible that low-level waves, bores, and other features, which may not always be apparent in radar data, may also play an important role in convection initiation processes.

Abstract

From its initial deployment as a research tool following the second World War, radar has played a fundamental role in revealing the forces that initiate and organize severe storms and larger mesoscale convective systems composed of a conglomeration of convective storm cells. Early radar observations were primarily descriptive and showed the tremendous variety of precipitating moist convection types and sizes. Examples include single convective storms, longer-lived multicellular storms, fast-moving squall lines, slower-moving linear and nonlinear convective systems, and long-lived supercell storms. Certain modes or types of convective systems were shown to possess a variety of hazardous weather that includes very heavy rain, large hail, straight-line damaging winds, tornadoes, and lightning. It was soon recognized that the type of convective system was strongly dependent on the environment in which it was embedded. Researchers determined that two variables were particularly important in describing convective behavior: the vertical profile of the horizontal wind and potential instability of the air feeding the system [convective available potential energy (CAPE)]. The types of convective systems are discussed here according to their typical shear and CAPE values. In addition to the knowledge gained from observational radar studies, considerable advancement in understanding of convective system dynamics has resulted from high-resolution numerical simulations.

In addition to being a critical factor in determining the particular structure and organization that convective systems assume once convection is initiated, radar (particularly in clear air mode) has been a leading tool in identifying forcing mechanisms for convective initiation. In particular, the role of “boundary layer forcing” in initiating convection has received much attention in recent years. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or discontinuities in Doppler velocity. Some of these mesoscale boundary layer mechanisms for producing upward motion include horizontal convective roles, sea-breeze circulations, drylines, gust fronts, orographic circulations (e.g., mountain–valley), and circulations resulting from horizontal inhomogeneities in surface character. Convection initiation sometimes does not occur continuously along boundaries but only at preferred along-boundary locations. Location preferences can sometimes be identified with boundary intersections, such as colliding gust fronts, sea-breeze fronts and rolls, and drylines and rolls. It is not always clear, however, why convection forms at certain locations along boundaries and not others. It is possible that low-level waves, bores, and other features, which may not always be apparent in radar data, may also play an important role in convection initiation processes.

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