Structure and Mechanism of the Prefrontal Squall Line

Using upper-air soundings taken by the Thunderstorm Project and surface and serological data available through teletype distribution, a detailed three-dimensional analysis of a prefrontal squall line is presented and certain new observational features of squall-line structure are shown. It is shown that squall-line thunderstorms appear in some cases to form first over the cold-front surface and subsequently move into the warm sector. Serial ascents taken in such a case show that there is a distinct cold front at the forward edge of the thunderstorm area, which coincides with the squall line observed at the ground. It is suggested that the squall-line activity can be accounted for partly as a result of this front, and partly by the continuous generation of new thunderstorms as a result of convergence-divergence patterns produced by the vertical transfer of horizontal momentum in pre-existent thunderstorms. This is augmented by solenoidal circulations due to unbalance between the "thermal wind" and the actual vertical wind shear when the latter is diminished by vertical mixing of horizontal momentum. An essential source of energy for maintaining squall-line activity appears to be the kinetic energy of air brought down from higher levels.