Large-Scale Influences Upon the Generation of a Mesoscale Disturbance

J. M. Lewis Laboratory for Atmospheric Research, University of Illinois, Urbana, 61801

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Y. Ogura Laboratory for Atmospheric Research, University of Illinois, Urbana, 61801

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L. Gidel Laboratory for Atmospheric Research, University of Illinois, Urbana, 61801

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Abstract

A case of squall line generation in the National Severe Storms Laboratory (NSSL) network has been examined with the intention of capturing synoptic-scale influences. A telescopic analysis approach was used whereby observations from both synoptic and mesoscale networks were combined.

The squall line formed in the warm air behind the surface position of the cold front. Large-scale circulation was responsible for creating a shallow layer (∼1-km thick) of convectively unstable air immediately above this front. Horizontal gradient of low-level moisture, pronounced low-level wind shear, and surface convergence were the large-scale factors that combined to produce the unstable region.

Mesoscale analysis showed that vertical velocity in the low levels exhibited a persistent small-scale variation prior to convective activity. The horizontal variation in vertical velocity was ultimately responsible for creating a favored position within the mesonetwork.

Conservation of potential temperature and specific humidity is examined as well as the relative importance of horizontal and vertical advection.

Abstract

A case of squall line generation in the National Severe Storms Laboratory (NSSL) network has been examined with the intention of capturing synoptic-scale influences. A telescopic analysis approach was used whereby observations from both synoptic and mesoscale networks were combined.

The squall line formed in the warm air behind the surface position of the cold front. Large-scale circulation was responsible for creating a shallow layer (∼1-km thick) of convectively unstable air immediately above this front. Horizontal gradient of low-level moisture, pronounced low-level wind shear, and surface convergence were the large-scale factors that combined to produce the unstable region.

Mesoscale analysis showed that vertical velocity in the low levels exhibited a persistent small-scale variation prior to convective activity. The horizontal variation in vertical velocity was ultimately responsible for creating a favored position within the mesonetwork.

Conservation of potential temperature and specific humidity is examined as well as the relative importance of horizontal and vertical advection.

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