Surface Mesoscale Features as Potential Storm Predictors in the Northern Great Plains—Two Case Studies

AndréA. Doneaud Institute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City 57701

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James R. Miller Jr. Institute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City 57701

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David L. Priegnitz Institute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City 57701

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Lakshmana Viswanath Institute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City 57701

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Abstract

Two mesoscale case studies in the semi-arid climate of southeastern Montana were carried out on 1 May and 3 June 1980. I May was an unstable, rainy day with two rain periods over the mesonet area, and 3 June was a potentially unstable day, with a cold frontal passage in the afternoon producing a very intense convective event.

Data from an instrumented mesoscale network (supporting the HIPLEX Montana experiment located between Miles City and Baker), a 5 cm radar, soundings, satellite (GOES), and synoptic maps were considered. The mesonet wind, temperature and moisture data were processed, computed every 15 min, and compared with radar rain patterns.

The study confirmed that convergence cell development within the surface kinematic fields precedes radar echoes and is directly related to the convective event. The areas involved in the vertical motions generating storms are much larger compared to those reported in humid climates. The “areal convergence” is a better storm predictor than the maximum convergence point value. A cloud merging effect related to the storm intensity and reduced rain efficiencies were also found.

The structure of the divergence field over the whole network experienced a cyclic evolution in both cases. This cyclic evolution is identified as a potential predictor for rain beginning 25–70 min after the last cycle before the rain phase.

Abstract

Two mesoscale case studies in the semi-arid climate of southeastern Montana were carried out on 1 May and 3 June 1980. I May was an unstable, rainy day with two rain periods over the mesonet area, and 3 June was a potentially unstable day, with a cold frontal passage in the afternoon producing a very intense convective event.

Data from an instrumented mesoscale network (supporting the HIPLEX Montana experiment located between Miles City and Baker), a 5 cm radar, soundings, satellite (GOES), and synoptic maps were considered. The mesonet wind, temperature and moisture data were processed, computed every 15 min, and compared with radar rain patterns.

The study confirmed that convergence cell development within the surface kinematic fields precedes radar echoes and is directly related to the convective event. The areas involved in the vertical motions generating storms are much larger compared to those reported in humid climates. The “areal convergence” is a better storm predictor than the maximum convergence point value. A cloud merging effect related to the storm intensity and reduced rain efficiencies were also found.

The structure of the divergence field over the whole network experienced a cyclic evolution in both cases. This cyclic evolution is identified as a potential predictor for rain beginning 25–70 min after the last cycle before the rain phase.

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