A Numerical Study on the Formation of Organized Convective Storms : Part II. Dependence of "Broken-Line Formation" of Squall Line on its Line Direction

Abstract

The effects of the direction of the convective-cell arrangement on the efficiency of “broken-line formation" of a squall line has been studied through numerical experiments. As fundamental experiments, small thermals were established along a line at a constant interval, and their evolutions were examined by changing the line direction relative to the vertical shear vector. The results showed that the broken-line formation occurred most efficiently when the line had a certain angle with the shear vector. The analysis suggested that the key factor controlling this efficiency is the vertical shear component normal to the line. When the component is too large, the downdrafts from pre-existing cells disturb the formation of the band-shaped updraft of a squall line. When the component is too small, a squall line cannot form easily because the low-level convergence is too weak. Broken-line formation most efficiently occurs when the component is in the middle range. Similar results were found when the shear contained veering or when the cell interval was small. As a more realistic case, we put initial thermals at random locations. In this case, squall lines formed frequently in the direction along which broken-line formation occurred efficiently in the fundamental experiment. These results suggest that there is some favorable orientation for an efficient broken-line formation, and that squall lines have a tendency to appear along that direction.