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Dynamic Aspects of a Distinctly Three-Dimensional Mesoscale Convective System

Carl E. HaneNOAA/ERL/National Severe Storms Laboratory, Mesoscale Research and Applications Division, Norman, Oklahoma

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David P. JorgensenNOAA/ERL/National Severe Storms Laboratory, Mesoscale Research and Applications Division, Boulder, Colorado

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Abstract

The dynamics of a mesoscale convective system that matured in central Kansas on 3–4 June 1985 is investigated based upon data from ground-based dual-Doppler radar and other sources. The system was distinctly three-dimensional, evolving to a wavelike shape owing to the intersection of two convective bands. The two bands, one oriented north–south and the other east-north–west-southwest, are compared and contrasted with respect to their velocity, pressure, and buoyancy structure and the consequent attributes of their momentum transports and budgets.

Dynamic retrieval of pressure and buoyancy from the wind fields provides insight into system structure and allows for the calculation of pressure gradients needed for the horizontal momentum budget. Several independent checks are carried out to ensure the quality of the pressure solution. The solution is found to be more accurate if the velocity time derivatives are included in the retrieval process. Dissimilar structure of the two bands is highlighted by a reversal of the low-level pressure gradient in the line-normal direction that can be related to the presence of a baroclinic zone along the more northern band.

In both convective lines momentum fluxes at all levels are negative in the line-normal direction and positive along the line in agreement with the results of past studies. Line-parallel fluxes are comparable in magnitude to line-normal fluxes owing to the strong line-parallel shear. The calculated wind component tendencics resulting from the line-normal momentum budget for the north–south line include increases in rear-to-front momentum, at all altitudes, that is most pronounced at low and high levels, similar to results of past studies of lines oriented normal to the environmental shear. For the northeast–southwest line, increases in front-to-rear momentum are found at all levels except for a thin layer near the system top. This difference stems from a pronounced low-level pressure decrease toward the rear of the line.

In the north–south line, budget calculations show an increase in along-line momentum with time at low and high levels and a decrease at midlevels. The results for the northeast–southwest band are quite different in that a decrease with time in along-line momentum is calculated at all levels, with a pronounced decrease at high levels owing to outward flux through the downwind boundaries. Along-line pressure gradients are quite weak in both convective bands. The significant influence of the horizontal flux divergence is attributed in part to the three-dimensional character of the system. The momentum budget results for each band are discussed in relation to maintenance of trailing stratiform precipitation, provision of favorable environments for severe weather, and their potential effects upon system evolution.

Abstract

The dynamics of a mesoscale convective system that matured in central Kansas on 3–4 June 1985 is investigated based upon data from ground-based dual-Doppler radar and other sources. The system was distinctly three-dimensional, evolving to a wavelike shape owing to the intersection of two convective bands. The two bands, one oriented north–south and the other east-north–west-southwest, are compared and contrasted with respect to their velocity, pressure, and buoyancy structure and the consequent attributes of their momentum transports and budgets.

Dynamic retrieval of pressure and buoyancy from the wind fields provides insight into system structure and allows for the calculation of pressure gradients needed for the horizontal momentum budget. Several independent checks are carried out to ensure the quality of the pressure solution. The solution is found to be more accurate if the velocity time derivatives are included in the retrieval process. Dissimilar structure of the two bands is highlighted by a reversal of the low-level pressure gradient in the line-normal direction that can be related to the presence of a baroclinic zone along the more northern band.

In both convective lines momentum fluxes at all levels are negative in the line-normal direction and positive along the line in agreement with the results of past studies. Line-parallel fluxes are comparable in magnitude to line-normal fluxes owing to the strong line-parallel shear. The calculated wind component tendencics resulting from the line-normal momentum budget for the north–south line include increases in rear-to-front momentum, at all altitudes, that is most pronounced at low and high levels, similar to results of past studies of lines oriented normal to the environmental shear. For the northeast–southwest line, increases in front-to-rear momentum are found at all levels except for a thin layer near the system top. This difference stems from a pronounced low-level pressure decrease toward the rear of the line.

In the north–south line, budget calculations show an increase in along-line momentum with time at low and high levels and a decrease at midlevels. The results for the northeast–southwest band are quite different in that a decrease with time in along-line momentum is calculated at all levels, with a pronounced decrease at high levels owing to outward flux through the downwind boundaries. Along-line pressure gradients are quite weak in both convective bands. The significant influence of the horizontal flux divergence is attributed in part to the three-dimensional character of the system. The momentum budget results for each band are discussed in relation to maintenance of trailing stratiform precipitation, provision of favorable environments for severe weather, and their potential effects upon system evolution.

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