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Surface Pressure and Precipitation Life Cycle Characteristics of PRE-STORM Mesoscale Convective Systems

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

Extensive observations of the May–June 1985 Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central (OK PRE-STORM) are used to examine the life cycle characteristics of 16 mesoscale convective systems (MCSs). The primary focus is on the surface pressure, flow, and precipitation structures of the mess.

It is found that despite the wide variety of initial precipitation structures of the MCSs, reputable patterns developed in 75% of the cases during their mature-to-dissipating stages. The precipitation structure at this later stage of the life cycle can be described as asymmetric, following the definition of Houze et al. Due to the variety of directions of movement, the systems are examined in a motion-relative quadrant perspective. One axis is along the direction of system motion and the other perpendicular to it, crossing at the center of the convective line. The asymmetry is characterized by 1) a leading convective line containing relatively weak cells on the northern end and a trailing stratiform region in the left-rear quadrant (although in three of the cases there was also leading stratiform rain in the left-front quadrant) and 2) a leading convective line with progressively more intense cells on the southern end and a distinct lack of trailing stratiform precipitation in the right-rear quadrant. All MCSs possessed a surface presquall mesolow, mesohigh, and wake low during some period in their life cycles. Notably. the asymmetric pattern developed at the stage of the maximum pressure gradient between the mesohigh and the wake low. At this stage, the mesohigh often extended well into the stratiform region and the wake low was at its deepest point. Very intense pressure gradients were observed, averaging 2 mb (10 km)−1, but ranging up to 5 mb (10 km)−1. The flow in several cases passed through the wake low at or near severe limits (25 m s−1).

A major finding of this paper is that while the symmetric and asymmetric MCS classification of Houze et al. is confirmed, these patterns characterize the precipitation structures at different stages of the life cycle rather than representing specific types of MCSs. The symmetric structure, if present occurs during the earlier stages of development. The asymmetric structure predominates during the latter stages of the life cycle. There were four general paths taken by the MCSs from their widely varying initial structures to the asymmetrical structure. These paths are referred to as disorganized, back-building, linear, and intersecting convective bands. Some cases exhibited combinations of these paths. Conceptual models of the symmetric and asymmetric patterns, along with their corresponding surface pressure fields, are presented.

Abstract

Extensive observations of the May–June 1985 Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central (OK PRE-STORM) are used to examine the life cycle characteristics of 16 mesoscale convective systems (MCSs). The primary focus is on the surface pressure, flow, and precipitation structures of the mess.

It is found that despite the wide variety of initial precipitation structures of the MCSs, reputable patterns developed in 75% of the cases during their mature-to-dissipating stages. The precipitation structure at this later stage of the life cycle can be described as asymmetric, following the definition of Houze et al. Due to the variety of directions of movement, the systems are examined in a motion-relative quadrant perspective. One axis is along the direction of system motion and the other perpendicular to it, crossing at the center of the convective line. The asymmetry is characterized by 1) a leading convective line containing relatively weak cells on the northern end and a trailing stratiform region in the left-rear quadrant (although in three of the cases there was also leading stratiform rain in the left-front quadrant) and 2) a leading convective line with progressively more intense cells on the southern end and a distinct lack of trailing stratiform precipitation in the right-rear quadrant. All MCSs possessed a surface presquall mesolow, mesohigh, and wake low during some period in their life cycles. Notably. the asymmetric pattern developed at the stage of the maximum pressure gradient between the mesohigh and the wake low. At this stage, the mesohigh often extended well into the stratiform region and the wake low was at its deepest point. Very intense pressure gradients were observed, averaging 2 mb (10 km)−1, but ranging up to 5 mb (10 km)−1. The flow in several cases passed through the wake low at or near severe limits (25 m s−1).

A major finding of this paper is that while the symmetric and asymmetric MCS classification of Houze et al. is confirmed, these patterns characterize the precipitation structures at different stages of the life cycle rather than representing specific types of MCSs. The symmetric structure, if present occurs during the earlier stages of development. The asymmetric structure predominates during the latter stages of the life cycle. There were four general paths taken by the MCSs from their widely varying initial structures to the asymmetrical structure. These paths are referred to as disorganized, back-building, linear, and intersecting convective bands. Some cases exhibited combinations of these paths. Conceptual models of the symmetric and asymmetric patterns, along with their corresponding surface pressure fields, are presented.

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