Evolution and Structure of Tropical Squall Line Elements within a Moderate CAPE and Strong Low-Level Jet Environment

Abstract

A three-dimensional cloud model is used to simulate the early development and propagation of an arc-shaped line element similar to that found in the GARP (Global Atmospheric Research Program) Tropical Atlantic Experiment (GATE) 4‐5 September 1974 squall line system. The simulated squall line element forms in a relatively unexplored environment with moderate convective available potential energy and a strong low-level jet (bulk Richardson number 5 37) associated with an easterly wave. The simulated line element develops in a large-scale convergence region from an initial cell that splits and elongates in a manner reminiscent of some midlatitude lines. Simulation features compare favorably with observed characteristics of some of the line elements including line orientation (approximately perpendicular to the average wind shear below the low-level jet), propagation speed (11 m s21 to the southwest), length (75 km), and maximum precipitation rate (187 mm h21). In addition, the simulated line merges with cells that form ahead as observed. The simulated arc-shaped line element consists of four regions several hours after its initiation. The northern region or right flank of the line contains a long-lived cell exhibiting three-dimensional characteristics similar to midlatitude supercells including long-lasting and steady updrafts with midlevel cyclonic rotation and movement to the right of the mean winds. Although the simulated supercell characteristics cannot be confirmed because of insufficient data from the limited GATE 4‐5 September observations, updrafts with strong vertical vorticity have been observed to occur in other GATE rainbands where waterspouts have been seen. The region just to the south of this cell is the site where a cell that formed out ahead of the line segment merges with the line and also develops supercell-like characteristics. Farther to the south, convection develops with quasi twodimensional characteristics below 3‐4 km but breaking into several multicells above. A rear-inflow jet does not accompany these features and the winds in the downdraft are oblique too and nonuniform along the associated gust front. Finally, on the southernmost or left flank of the line element, cells with both long and short lifetimes develop. Sensitivity tests indicate that once the early line structure has developed, its evolution and structure are not seriously altered by the removal of large-scale forcing. Further, the formation of the cold pool and gust front between the initial separating (splitting) cells is crucial to the filling in of the line. Changes in the thermodynamic profile consistent with nonsquall observations one-half day earlier result in only modest differences. Changes in the wind profile (based on the same observations) led to significant differences, such as the lack of convection between the initial separating cells and the merger taking place to the north of the right flank creating a convective complex in this region.