A NUMERICAL STUDY AN THE FORMATION OF ORGANIZED CONVECTIVE STORMS .2.DEPENDENCE OF BROKEN-LINE FORMATION OF SQUALL LINE ON ITS LINE DIRECTION

The effects of the direction of the convective-cell arrangement on the efficiency of ''broken-line formation'' of a squall line has been stu died through numerical experiments. As fundamental experiments, small thermals were established along a line at a constant interval, and the ir evolutions were examined by changing the line direction relative to the vertical shear vector. The results showed that the broken-line fo rmation occurred most efficiently when the line had a certain angle wi th the shear vector. The analysis suggested that the key factor contro lling this efficiency is the vertical shear component normal to the li ne. When the component is too large, the downdrafts from pre-existing cells disturb the formation of the band-shaped updraft of a squall lin e. When the component is too small, a squall line cannot form easily b ecause the low-level convergence is too weak, Broken-line formation mo st efficiently occurs when the component is in the middle range. Simil ar results were found when the shear contained veering or when the cel l interval was small. As a more realistic case, we put initial thermal s 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 som e favorable orientation for an efficient broken-line formation, and th at squall lines have a tendency to appear along that direction.