Structure and Evolution of Numerically Simulated Squall Lines

Using a three-dimensional numerical cloud model, we investigate the effects of vertical wind shear on squall-line structure and evolution over a wide range of shear magnitudes, depths, and orientations relative to the line. We find that the simulated squall lines are most sensitive to the magnitude of the component of shear perpendicular to the line, and that we may reproduce much of the range of observed structures by varying this single parameter. For weak shear, a line of initially upright-to-downshear-tilted short-lived cells quickly tilts upshear, producing a wide band of weaker cells extending behind the surface outflow boundary. For moderate-to-strong shear, the circulation remains upright-to-downshear tilted for longer periods of time, with vigorous, short-lived cells confined to a relatively narrow band along the system's leading edge. At later times, however, these systems may also weaken as the circulation tilts upshear. For strong, deep shears oriented obliquely to the line, the squall line may be composed of quasi-steady, three-dimensional supercells. The squall-line lifecyle that occurs in most of the simulations is dependent on both the strength of the developing cold pool, which induces an upshear-tilted circulation, and the strength of the ambient low-level shear ahead of the line, which promotes a circulation tilting the system downshear. When these two factors are in balance, the overall system circulation remains upright, and we obtain the optimal conditions for deep lifting that promotes the regeneration of strong cells along the outflow boundary. In the current experiments, this optimal state occurs with 15–25 m s−1 of velocity change over the lowest 2.5 km AGL.