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Circulations between Mesoscale Convective Systems along a Cold Front

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

A frontal squall line that passed over the central United States on 14–15 June 1985 consisted of several primary mesoscale convective systems (MCSs) with a prominent gap (several hundred kilometers wide) between two of them over central Kansas and Oklahoma. An upper-level jet streak crossed the front at right angles, with the jet exit region centered in the gap. Surface, upper-air, radar, and satellite data are used to examine conditions contributing to this squall-line gap.

All surface indicators (e.g., a sharp wind shift line and strong low-level moisture convergence) pointed toward the development of new deep convection in the gap. Computations of vertical velocity using sounding data showed that the low-level vertical motion was indeed upward there; however, strong subsidence existed aloft. The demarcation between subsidence aloft and ascent at low levels was near the 0°C level, where there was a concurrent inflow into the MCSs. A strong inversion existed near this level, which effectively capped vigorous congestus clouds that formed in the gap.

The circulations in the gap appeared to be first influenced by the upper-level jet and then the convection itself. Initial convection with the first MCS developed in the left-exit region of the jet. The second MCS evolved from initial cells that formed over strongly heated, higher terrain approximately 500 km to the southwest. The subsequent circulations between the MCSs—converging outflows aloft and at the surface, and diverging flow near the 0°C level—resemble these generated by gravity waves emanating from an MCS-like convective heat source. The gravity wave-induced subsidence aloft acted in opposition to the jet streak forcing, maintaining a strong cap and preventing deep convection in the squall-line gap. The distinction between highly broken squall lines (such as this one) and those with a nearly contiguous band of cells in the presence of strong low-level convergence appears to be related to the degree of convective inhibition in the environment and how it is modulated by a variety of processes including the convection itself.

Abstract

A frontal squall line that passed over the central United States on 14–15 June 1985 consisted of several primary mesoscale convective systems (MCSs) with a prominent gap (several hundred kilometers wide) between two of them over central Kansas and Oklahoma. An upper-level jet streak crossed the front at right angles, with the jet exit region centered in the gap. Surface, upper-air, radar, and satellite data are used to examine conditions contributing to this squall-line gap.

All surface indicators (e.g., a sharp wind shift line and strong low-level moisture convergence) pointed toward the development of new deep convection in the gap. Computations of vertical velocity using sounding data showed that the low-level vertical motion was indeed upward there; however, strong subsidence existed aloft. The demarcation between subsidence aloft and ascent at low levels was near the 0°C level, where there was a concurrent inflow into the MCSs. A strong inversion existed near this level, which effectively capped vigorous congestus clouds that formed in the gap.

The circulations in the gap appeared to be first influenced by the upper-level jet and then the convection itself. Initial convection with the first MCS developed in the left-exit region of the jet. The second MCS evolved from initial cells that formed over strongly heated, higher terrain approximately 500 km to the southwest. The subsequent circulations between the MCSs—converging outflows aloft and at the surface, and diverging flow near the 0°C level—resemble these generated by gravity waves emanating from an MCS-like convective heat source. The gravity wave-induced subsidence aloft acted in opposition to the jet streak forcing, maintaining a strong cap and preventing deep convection in the squall-line gap. The distinction between highly broken squall lines (such as this one) and those with a nearly contiguous band of cells in the presence of strong low-level convergence appears to be related to the degree of convective inhibition in the environment and how it is modulated by a variety of processes including the convection itself.

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