The wind's response to transient mesoscale pressure fields associated with squall lines
Date
1990-05
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Abstract
A simple one-dimensional slab model has been developed to examine the wind's response to transient mesoscale pressure fields that frequently accompany mature midlatitude squall lines. The model numerically solves a form of the momentum equation that includes the pressure gradient force, advection, and the frictional force. The Coriolis force is neglected since attention is focused on time periods of 1-2 hrs. The pressure field is specified by a sine wave with a constant phase speed. The amplitude of this wave is initially zero, but then increases linearly with every timestep until t = 2 hrs. At this time, the wave reaches its predetermined maximum amplitude. This pressure wave represents a mesohigh and wake low that have the same amplitude and phase speed. Since these features are transient, the air lacks sufficient time to achieve a balanced state. Therefore, winds are directed forward through the mesohigh, and rearward through the wake low, at right angles to the isobars. This airflow pattern produces an axis of divergence to the rear of the mesohigh, with convergence occurring near the back edge of the wake low. The model is able to accurately simulate the airflow near these pressure features. Pressure waves with various maximum amplitudes and phase speeds are used in the model for the purpose of comparison. Model results vary slightly as the phase speed and maximum amplitude of the pressure wave is changed. Air parcel trajectories are used to help explain these variations. Model results are compared to the observed airflow near the mesohigh and wake low associated with an intense squall line that moved through Oklahoma and Kansas on 10-11 June 1985. The time period of interest is 0100–0400 UTC on the 11th of June, during which the squall line, as well as the mesoscale pressure fields, reached their maximum intensity. Cross sections through the center of the mesohigh and wake low indicate that the pressure field were roughly sinusoidal with an amplitude of approximately 2.5 mb. Therefore, model results obtained using a 2.5 mb maximum amplitude pressure wave are used for the comparison. The model derived wind field is similar to the observed wind field in many respects. Differences can be attributed to several factors, one of which is the fact that the pressure field, specified in the model, is only an approximation to the observed pressure field. Finally, a discussion of the frictional force is presented. To assess the relative importance of the surface friction term, a scale analysis of the momentum equation is performed. This analysis shows that this term is only one order of magnitude smaller than the next smallest term in the momentum equation. Therefore, surface friction cannot be neglected. Slight variations in model results occur as the magnitude of the term varies. The effects of momentum transport from above on the surface wind field are also discussed. It appears that such transport is only important near the leading convective line where convective scale updrafts and downdrafts are occurring. Behind the convective line, the rain cooled air is much too stable, and the convective scale motion is too weak, for significant momentum transport into the boundary layer.
Description
May 1990.
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Subject
Squall lines
Atmospheric pressure