The Formation of a Cooling-induced Mesovortex in the Trailing Stratiform Region of a Midlatitude Squall Line

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  • 1 Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
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Abstract

There have been some ambiguities in recent observational studies as to whether midlevel mesovortices are induced by latent heating or cooling, and develop in the descending or ascending portion of mesoscale convective systems (MCS's). In this study, a comprehensive examination of a cooling-induced mesovortex in the trailing stratiform region of a midlatitude squall line that occurred on 10–11 June 1985 during the Preliminary Regional Experiment for STORM-Central (PRE-STORM) is presented using a 20-h high-resolution simulation of the squall system.

This cooling-induced midlevel vortex originates from the preexisting cyclonic vorticity associated with a traveling meso-α-scale short wave. The vortex is intensified in the descending rear-to-front (RTF) inflow as a result of continued sublimative melting and evaporative cooling in the stratiform region. It decouples from the front-to-rear (FTR) ascending and anticyclonic flow in the upper troposphere during the formative stage. The vortex tilts northward with height, resulting in a deep layer of cyclonic vorticity (up to 250 mb) near the northern end of the squall line. It has an across-line scale of 120–150 km and a longitudinal scale of more than 300 km, with its maximum intensity located above the melting level.

A three-dimensional vorticity budget shows that the cooling-induced vortex is initially maintained through the vertical stretching of its absolute vorticity associated with the short-wave trough. As the descending rear inflow develops within the system, the tilting of horizontal vorticity is about one order of magnitude larger than the stretching in determining the early intensification of the vortex. In most vortex layers, the stretching tends to destroy the vortex locally, owing to the existence of the divergent outflow in the lower troposphere. Only when the vortex propagates into the FTR-RTF flow interface does the stretching effect begin to control the final amplification of the vortex, and the tilting plays a negative role during the squall's decaying stage.

The model also reproduces well a narrow heating-induced (or warm-core) cyclonic vortex along the leading convective line and a deep anticyclonic-vorticity zone between the heating- and cooling-induced mesovortices. It is shown that the cyclonic vortex along the leading line develops through positive tilting and stretching, whereas the anticyclonic-vorticity zone is generated by tilting of horizontal vorticity by the FTR-ascending and RTF-descending flows, and later enhanced by negative stretching along the interface convergence zone. The warm-core vortex dissipates and eventually merges into the cooling-induced vortex circulation as the system advances into a convectively less favorable environment. The anticyclonic-vorticity zone rapidly diminishes as the cooling-induced vortex moves into the flow interface. At the end of the life cycle, the cooling-induced mesovortex becomes the only remaining element of the squall system that can be observed in a deep layer and at a larger scale in the low to midtroposphere. Different characteristics of heating-induced versus cooling-induced mesovortices and their relationships are discussed. The results suggest that mesovortices are ubiquitous in MCS's and that their pertinent mesoscale rotational flow may be the basic dynamic effect of MCS's on their larger-scale environments.

Abstract

There have been some ambiguities in recent observational studies as to whether midlevel mesovortices are induced by latent heating or cooling, and develop in the descending or ascending portion of mesoscale convective systems (MCS's). In this study, a comprehensive examination of a cooling-induced mesovortex in the trailing stratiform region of a midlatitude squall line that occurred on 10–11 June 1985 during the Preliminary Regional Experiment for STORM-Central (PRE-STORM) is presented using a 20-h high-resolution simulation of the squall system.

This cooling-induced midlevel vortex originates from the preexisting cyclonic vorticity associated with a traveling meso-α-scale short wave. The vortex is intensified in the descending rear-to-front (RTF) inflow as a result of continued sublimative melting and evaporative cooling in the stratiform region. It decouples from the front-to-rear (FTR) ascending and anticyclonic flow in the upper troposphere during the formative stage. The vortex tilts northward with height, resulting in a deep layer of cyclonic vorticity (up to 250 mb) near the northern end of the squall line. It has an across-line scale of 120–150 km and a longitudinal scale of more than 300 km, with its maximum intensity located above the melting level.

A three-dimensional vorticity budget shows that the cooling-induced vortex is initially maintained through the vertical stretching of its absolute vorticity associated with the short-wave trough. As the descending rear inflow develops within the system, the tilting of horizontal vorticity is about one order of magnitude larger than the stretching in determining the early intensification of the vortex. In most vortex layers, the stretching tends to destroy the vortex locally, owing to the existence of the divergent outflow in the lower troposphere. Only when the vortex propagates into the FTR-RTF flow interface does the stretching effect begin to control the final amplification of the vortex, and the tilting plays a negative role during the squall's decaying stage.

The model also reproduces well a narrow heating-induced (or warm-core) cyclonic vortex along the leading convective line and a deep anticyclonic-vorticity zone between the heating- and cooling-induced mesovortices. It is shown that the cyclonic vortex along the leading line develops through positive tilting and stretching, whereas the anticyclonic-vorticity zone is generated by tilting of horizontal vorticity by the FTR-ascending and RTF-descending flows, and later enhanced by negative stretching along the interface convergence zone. The warm-core vortex dissipates and eventually merges into the cooling-induced vortex circulation as the system advances into a convectively less favorable environment. The anticyclonic-vorticity zone rapidly diminishes as the cooling-induced vortex moves into the flow interface. At the end of the life cycle, the cooling-induced mesovortex becomes the only remaining element of the squall system that can be observed in a deep layer and at a larger scale in the low to midtroposphere. Different characteristics of heating-induced versus cooling-induced mesovortices and their relationships are discussed. The results suggest that mesovortices are ubiquitous in MCS's and that their pertinent mesoscale rotational flow may be the basic dynamic effect of MCS's on their larger-scale environments.

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