Abstract
This paper describes an analysis of the structure of a tropical squall line by the use of radar, satellite, and upper air sounding data. The observation was made in the west of the western tropical Pacific during Summer MONEX in 1979. The results of the analysis were compared with the structure of tropical squall lines observed by other authors.
The squall line in this study showed a significantly organized mesoscale structure; that is, cumulus-scale convections and organized convections were combined within it. The existence of organized convection was the particular feature of this squall line. Anvil clouds covered these convections and extended back from the leading edge for more than 200 km. The line-shaped leading edge of the squall line propagated at a speed larger than that of ambient winds. Some portions of the leading edge were not dissipated and grew into organized convections, which maintained size and intensity for several hours.
The squall line was formed in the boundary region between the monsoon southwesterly and the trade wind easterly in the lower troposphere. Just before the squall line was formed, a relatively large positive vorticity had existed in the middle and lower troposphere, and a large divergence of air above 400 mb and moderate convergence below 400 mb. The overturning of the atmosphere was found by upper sounding data, and the moist static energy located in the lower troposphere ahead of the squall line was thought to be an energy source of the formation of the squall line.
Several common properties were seen in tropical squall lines in our case and in other tropical regions, but the cloud structure of the squall line in our case should be discussed three-dimensionally because of the existence of the organized convections.
