干旱区生态环境知识资源中心

The mechanisms responsible for the development and maintenance of long-lived squall lines in dry environments are investigated through two-dimensional numerical experiments by using a nonhydrostatic cloud model. The squall line environments are characterized by a low convective available potential energy (CAPE), low moisture content, and a high level of free convection (LFC), which are based on observations of a squall line over an arid region in China. Although these environments seem to be unfavorable for the development of convective systems, a long-lived squall line is simulated in the environment of a well-mixed moisture profile within a deep, mixed boundary layer. During the mature stage of this simulated squall line, the air parcels originating in the upper part of the mixed layer ahead of a surface cold-air pool are lifted to the upper troposphere. On the other hand, the air parcels originating in the lower part of the mixed layer are forced to go rearward, never reaching the upper levels. The low pressure just above the surface cold pool plays an important role in determining these parcel trajectories.

The sensitivity experiments in which the mixed-layer height and vertical profile of moisture within the mixed layer are varied illuminate the mechanisms for the development and maintenance of the simulated squall lines in the dry environments. First, the presence of a deep mixed layer is indispensable for the squall line development. Second, a moisture profile that is more uniform with height is favorable for the long-lived squall lines, although the CAPE value for the surface air parcel is small (250 J kg(-1)). In this moisture condition, air parcels in the upper part of the mixed layer have moderate CAPE values of 50-150 J kg(-1), and the differences between the source levels of these parcels and their LFCs are very small. These parcels are easily lifted to their LFCs without experiencing substantial inhibition of convection, thus releasing their CAPE. Therefore, squall lines persist in the dry condition. The role of the cold pool and the vertical distribution of CAPE values in the mixed layer is very important for the squall line maintenance in dry environments.