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Processes Leading to the Formation of Mesoscale Waves in the Midwest Cyclone of 15 December 1987

Peter J. PokrandtDepartment of atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin

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Gregory J. TripoliDepartment of atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin

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David D. HoughtonDepartment of atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin

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Abstract

On 15 December 1987, several long-lived, large-amplitude mesoscale wave disturbances accompanied a rapidly developing extratropical cyclone in the midwest United States. Previous observational and modeling studies have suggested that the disturbances had large amplitudes and long lifetimes as a result of a wave-CISK-type instability occurring within an imperfect wave duct and were initiated by convection. However. infrared (IR) satellite imagery and radar echoes shortly before the wave disturbances formed suggest that convection was not the primary feature in the wave genesis region at that time. Instead, a meso-β-scale comma-shaped cloud was present and appeared to evolve into the wave disturbances. The origins of the comma cloud can be traced back to a cloud streak and precipitation maximum in the left exit region of an approaching jet streak over northern Mexico 15 h earlier. In this study, satellite observations are examined in conjunction with numerical simulations of the case to explore a new hypothesis for the formation of the wave disturbances. Specifically, the transverse circulation about an approaching jet streak transports potential vorticity from a reservoir in the stable cold low-level air to produce a meso-β-scale potential vorticity anomaly at midlevels, which is subsequently rotated relative to the upper-level flow to force mesoscale waves.

Abstract

On 15 December 1987, several long-lived, large-amplitude mesoscale wave disturbances accompanied a rapidly developing extratropical cyclone in the midwest United States. Previous observational and modeling studies have suggested that the disturbances had large amplitudes and long lifetimes as a result of a wave-CISK-type instability occurring within an imperfect wave duct and were initiated by convection. However. infrared (IR) satellite imagery and radar echoes shortly before the wave disturbances formed suggest that convection was not the primary feature in the wave genesis region at that time. Instead, a meso-β-scale comma-shaped cloud was present and appeared to evolve into the wave disturbances. The origins of the comma cloud can be traced back to a cloud streak and precipitation maximum in the left exit region of an approaching jet streak over northern Mexico 15 h earlier. In this study, satellite observations are examined in conjunction with numerical simulations of the case to explore a new hypothesis for the formation of the wave disturbances. Specifically, the transverse circulation about an approaching jet streak transports potential vorticity from a reservoir in the stable cold low-level air to produce a meso-β-scale potential vorticity anomaly at midlevels, which is subsequently rotated relative to the upper-level flow to force mesoscale waves.

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