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- Author or Editor: Rolf F. A. Hertenstein x
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Abstract
This study involves observations and model simulations of potential vorticity anomalies in the wake of midlatitude squall lines. Using data from the Oklahoma–Kansas PRE-STORM experiment, we analyze potential vorticity fields near two squall lines—one with and one without a trailing stratiform region. From this observational analysis we suggest that squall lines with trailing stratiform regions can leave large, positive, midtropospheric potential vorticity anomalies in their wake. To further interpret these observations we consider a two-dimensional version of semigeostrophic theory formulated in isentropic and geostrophic coordinates, which results in a simple potential pseudodensity (inverse potential vorticity) equation. Using apparent heat source fields that model those computed diagnostically from PRE-STORM data, we find that theory does indeed predict large, midtropospheric potential vorticity anomalies for model squall lines with a trailing stratiform region but not for model squall lines that lack this feature. An interpretation of this result comes directly from the potential vorticity equation, which states that the material derivative of the potential vorticity depends on the derivative, along the vorticity vector, of the apparent heat source. In a squall line with a trailing stratiform region, large values of this derivative are found in the midtroposphere, above the lower-tropospheric evaporative cooling and below the upper-tropospheric stratiform condensational heating. This large derivative of the heating, coupled with the longer influence time associated with the width of the stratiform region, allows the potential vorticity signature of the stratiform region to dominate over the signature of the convective line. Thus, the midtropospheric mesoscale vortices often generated in the wake of squall lines are due in large part to the unique apparent heat source/sink pattern associated with the trailing stratiform region.
Abstract
This study involves observations and model simulations of potential vorticity anomalies in the wake of midlatitude squall lines. Using data from the Oklahoma–Kansas PRE-STORM experiment, we analyze potential vorticity fields near two squall lines—one with and one without a trailing stratiform region. From this observational analysis we suggest that squall lines with trailing stratiform regions can leave large, positive, midtropospheric potential vorticity anomalies in their wake. To further interpret these observations we consider a two-dimensional version of semigeostrophic theory formulated in isentropic and geostrophic coordinates, which results in a simple potential pseudodensity (inverse potential vorticity) equation. Using apparent heat source fields that model those computed diagnostically from PRE-STORM data, we find that theory does indeed predict large, midtropospheric potential vorticity anomalies for model squall lines with a trailing stratiform region but not for model squall lines that lack this feature. An interpretation of this result comes directly from the potential vorticity equation, which states that the material derivative of the potential vorticity depends on the derivative, along the vorticity vector, of the apparent heat source. In a squall line with a trailing stratiform region, large values of this derivative are found in the midtroposphere, above the lower-tropospheric evaporative cooling and below the upper-tropospheric stratiform condensational heating. This large derivative of the heating, coupled with the longer influence time associated with the width of the stratiform region, allows the potential vorticity signature of the stratiform region to dominate over the signature of the convective line. Thus, the midtropospheric mesoscale vortices often generated in the wake of squall lines are due in large part to the unique apparent heat source/sink pattern associated with the trailing stratiform region.
Abstract
The generation and organization of mesoscale convective vortices (MCVs) is a recurring theme in midlatitude and tropical meteorology during the warm season. In this work a simulation of a finite-length idealized convective line in a westerly shear environment is investigated in the absence of ambient vertical vorticity. An asymmetry in average vertical vorticity forms rapidly at early times in the present simulation. This study focuses on the formation and organization of vertical vorticity at these early simulation times. Previous simulations suggest that tilting of either ambient or storm-generated horizontal vorticity is the primary mechanism responsible for the formation, organization, and maintenance of MCVs. This study confirms recent work regarding the generation of vertical vorticity at early times in the simulation. A Lagrangian budget analysis of the vertical vorticity equation, however, shows that vorticity convergence becomes a comparable, and at times dominant, mechanism for the enhancement and long-term organization of vertical vorticity early in the simulation. Despite differences in the initial ambient horizontal vorticity, hodograph, and convective available potential energy, the Lagrangian budget analysis in the present midlatitude case is consistent with the Lagrangian budget results of a previous tropical squall line simulation. The study of idealized convective lines in midlatitude environmental conditions therefore provide valuable insight into understanding vertical vorticity production in tropical squall lines and their potential relevance to tropical cyclogenesis.
Abstract
The generation and organization of mesoscale convective vortices (MCVs) is a recurring theme in midlatitude and tropical meteorology during the warm season. In this work a simulation of a finite-length idealized convective line in a westerly shear environment is investigated in the absence of ambient vertical vorticity. An asymmetry in average vertical vorticity forms rapidly at early times in the present simulation. This study focuses on the formation and organization of vertical vorticity at these early simulation times. Previous simulations suggest that tilting of either ambient or storm-generated horizontal vorticity is the primary mechanism responsible for the formation, organization, and maintenance of MCVs. This study confirms recent work regarding the generation of vertical vorticity at early times in the simulation. A Lagrangian budget analysis of the vertical vorticity equation, however, shows that vorticity convergence becomes a comparable, and at times dominant, mechanism for the enhancement and long-term organization of vertical vorticity early in the simulation. Despite differences in the initial ambient horizontal vorticity, hodograph, and convective available potential energy, the Lagrangian budget analysis in the present midlatitude case is consistent with the Lagrangian budget results of a previous tropical squall line simulation. The study of idealized convective lines in midlatitude environmental conditions therefore provide valuable insight into understanding vertical vorticity production in tropical squall lines and their potential relevance to tropical cyclogenesis.
