The Momentum Budget of an Intense Midlatitude Squall Line

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Abstract
Rawinsonde data from OK PRE-STORM are used to calculate the momentum budgets of an intense midlatitude squall-line system with a trailing stratiform region. Budgets have been computed at three separate times during the mature through dissipating stages of the squall line using composited data over 3-hour intervals based on 90-minute releases. Budgets have been done in both ground-relative and system-relative reference frames, although system-relative terms are emphasized for comparison with other recent studies. Low-level radar data are used to partition the system into convective line and stratiform regions. The system contained a strong midlevel mesolow that contributed to rear-to-front pressure gradient acceleration in the vicinity of a rear-inflow jet. The mesolow, located within a larger region of low pressure sloping rearward with height, also led to strong front-to-rear acceleration elsewhere through a majority of the system. Convective-scale effects produced the primary accelerations opposing the pressure gradient. Advections (large during the mature stage) and local momentum tendencies weakened significantly during dissipation. The line-normal vertical momentum flux was generally negative, which might be expected from downgradient arguments applied to presquall vertical wind profiles. However, wind profiles averaged over the convective line region reversed during the mature and dissipating stages so that the calculated flux was countergradient at the budget times, in agreement with other momentum budget studies. Vertical momentum transport in the along-line direction was weakly positive in the convective line and negative elsewhere. Because the along-line winds generally increased with height at the budget times, the positive transport in the convective line was countergradient, contrary to the downgradient transport often observed for the along-line flow in convective systems. This result is attributed to an alongline asymmetry in the pressure field, namely, a midlevel mesolow in the northern portion of the squall-line system.

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