Mesoscale Momentum Budget in a Midlatitude Squall Line: A Numerical Case Study

Kun Gao National Center for Atmospheric Research, Boulder, Colorado

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Da-Lin Zhang National Center for Atmospheric Research, Boulder, Colorado

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Mitchell W. Moncrieff National Center for Atmospheric Research, Boulder, Colorado

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Han-Ru Cho National Center for Atmospheric Research, Boulder, Colorado

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Abstract

A mesoβ-scale momentum budget and its effect on larger-scale mean flow in a midlatitude mesoscale convective system are investigated using a numerical simulation of an intense squall line that occurred during 10–11 June 1985 PRE-STORM. It is found that the momentum generation normal to the line associated with the latent heating and cooling contributes most significantly to the momentum budget and determines the mesoβ-scale internal structure and evolution of the squall line. The momentum generation along the line contributes to the initial development of a mesovortex but has little effect on the final vertical structure of the along-line flow. Both vertical and horizontal momentum advection have significant contributions, particularly to the vertical mixing of the along-line flow; and this component of the horizontal momentum is locally transported down-gradient. It is also found that for midlatitude convective systems, convectively generated downdrafts can play as prominent a role as updrafts in vertically transporting horizontal momentum within both convective and stratiform regions.

The momentum flux associated with the mesoβ-scale circulations of the simulated squall line is found to agree with previous observational investigations, namely, normal to the line the squall system transports horizontal momentum in a countergradient sense while parallel to the line the transport is downgradient. Implications with respect to the convective momentum parameterization are discussed in the context of the mesoβ-scale momentum budget.

Abstract

A mesoβ-scale momentum budget and its effect on larger-scale mean flow in a midlatitude mesoscale convective system are investigated using a numerical simulation of an intense squall line that occurred during 10–11 June 1985 PRE-STORM. It is found that the momentum generation normal to the line associated with the latent heating and cooling contributes most significantly to the momentum budget and determines the mesoβ-scale internal structure and evolution of the squall line. The momentum generation along the line contributes to the initial development of a mesovortex but has little effect on the final vertical structure of the along-line flow. Both vertical and horizontal momentum advection have significant contributions, particularly to the vertical mixing of the along-line flow; and this component of the horizontal momentum is locally transported down-gradient. It is also found that for midlatitude convective systems, convectively generated downdrafts can play as prominent a role as updrafts in vertically transporting horizontal momentum within both convective and stratiform regions.

The momentum flux associated with the mesoβ-scale circulations of the simulated squall line is found to agree with previous observational investigations, namely, normal to the line the squall system transports horizontal momentum in a countergradient sense while parallel to the line the transport is downgradient. Implications with respect to the convective momentum parameterization are discussed in the context of the mesoβ-scale momentum budget.

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