Structure and Evolution of a Severe Squall Line over the Arid Region in Northwest China

Abstract

The structure and evolution of a long-lived squall line associated with a disastrous dust storm, called a black storm, that occurred over the arid region in northwest China are described. Data used in the present study were those from routine observations in China and those acquired from surface observations of the Heife River Field Experiment. The squall line evolved within an extremely dry environment in which surface water vapor mixing ratios were less than 2.5 g kg?1 and the level of free convection (LFC) was very high. The surface cold-air outflow that emanated from the squall line propagated rapidly, producing strong horizontal convergence at the leading edge of the cold pool. The deep, dry mixed layer that developed over the desert region, owing to strong solar surface heating, played a significant role in determining the squall line evolution. The maximum depth of the mixed layer was estimated as more than 4 km, which is often found over the desert regions of China. The growth of the deep mixed layer decreased the difference in height between the mixed-layer top and the LFC, minimizing the energy required to lift mixed-layer air above the LFC. Furthermore, the dry mixed layer is favorable for the development of an evaporatively driven downdraft, leading to the development of a surface cold pool and near-zero surface rainfall.