The Squall Line Over West Africa and Tropical Eastern Atlantic Ocean during Gate.

The mean kinematic and thermodynamic structures of the atmosphere are studied, through the use of considerably improved data networks of radiosondes over the African continent and a completely new data network over the ocean, during the monsoon season of 1974. From this necessary background the climatological aspects of West African squall lines (S/L) during the three phases of GATE on both synoptic scale and mesoscales are presented. A total of one hundred and seventy-six squall lines have been identified (fourteen only over the ocean), utilizing the excellent data from the SMS-1 geostationary satellite. S/L develop mainly over land, south of the ITCZ surface position where the monsoon layer and the overriding dry air favors the production of downdrafts through the evaporation-precipitation process. S/L genesis is also dependent upon low-level convergence associated with traveling easterly waves and divergence at the upper level associated with the Tropical Easterly Jet. This part of the research complements strongly the work from Aspliden et al. (1976). A composite study of the 91 longest-lived S/L, grouped according to their direction of propagation, is presented. The S/L move faster than the basic flow at all levels. A squall-line-wave is found to accompany the S/L throughout its lifespan. Moreover, the orientation of squall-line-wave axis is linked to the direction in which S/L propagates. A set of rules is developed in order to improve the forecasts of S/L formation and direction of propagation by the use of the windfield only. Study of the windfields and thermodynamic structure inside S/L shows that the new cumulonimbus clouds form on a preferred side, associated with the area where the sloping downdrafts from around 800 mb reach the surface and, thereby, with the direction of the propagation. A three-dimensional structure of the average S/L, in vertical cross-sections taken through different sides of the S/L, emphasizes the spatial aspects of the phenomenon.