The Relationship of Surface Pressure Features to the Precipitation and Airflow Structure of an Intense Midlatitude Squall Line

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  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
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Abstract

Observations from the Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central (OK PRE-STORM) have been used to document the surface pressure features accompanying an intense midlatitude squall line with trailing stratiform precipitation. Three well-known features are observed: a pre-squall mesolow, a squall mesohigh and a wake low. Particular attention is given to the wake low, its life cycle and association with the trailing stratiform portion of the squall line.

During the formative stage, the pressure field to the rear of the squall line mesohigh is relatively flat with only weak stratiform precipitation present. As the squall line enters the developing-to-mature stages, a pronounced wake low appears at the back edge of the surface stratiform precipitation area. The squall line at this time is characterized by a strong rear-inflow jet, descending from the upper troposphere, as far as 500 km behind the leading convective line, to the lower troposphere just behind the line. The trailing stratiform cloud constitutes a significant part of the squall-line water budget, contributing 29% of the total squall line precipitation over a 400 km by 500 km mesonetwork area experiencing its passage. During the mature-to-dissipating stages, the trailing stratiform region splits into two segments, as does the wake low, with each low pressure center hugging the back edge of the stratiform segments. A composite analysis of rawinsonde data at this time shows strong warming and drying in the lower troposphere at the back edge of the stratiform regions.

Based on the results of this study, it is proposed that the wake low, which can be attributed to subsidence warming, is a surface manifestation of the descending rear-inflow jet and that the warming is maximized at the back edge of the trailing stratiform precipitation area where there is insufficient sublimation and evaporative cooling to offset adiabatic warming.

Abstract

Observations from the Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central (OK PRE-STORM) have been used to document the surface pressure features accompanying an intense midlatitude squall line with trailing stratiform precipitation. Three well-known features are observed: a pre-squall mesolow, a squall mesohigh and a wake low. Particular attention is given to the wake low, its life cycle and association with the trailing stratiform portion of the squall line.

During the formative stage, the pressure field to the rear of the squall line mesohigh is relatively flat with only weak stratiform precipitation present. As the squall line enters the developing-to-mature stages, a pronounced wake low appears at the back edge of the surface stratiform precipitation area. The squall line at this time is characterized by a strong rear-inflow jet, descending from the upper troposphere, as far as 500 km behind the leading convective line, to the lower troposphere just behind the line. The trailing stratiform cloud constitutes a significant part of the squall-line water budget, contributing 29% of the total squall line precipitation over a 400 km by 500 km mesonetwork area experiencing its passage. During the mature-to-dissipating stages, the trailing stratiform region splits into two segments, as does the wake low, with each low pressure center hugging the back edge of the stratiform segments. A composite analysis of rawinsonde data at this time shows strong warming and drying in the lower troposphere at the back edge of the stratiform regions.

Based on the results of this study, it is proposed that the wake low, which can be attributed to subsidence warming, is a surface manifestation of the descending rear-inflow jet and that the warming is maximized at the back edge of the trailing stratiform precipitation area where there is insufficient sublimation and evaporative cooling to offset adiabatic warming.

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