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Kyoto University (1999)

Studies on the Structure, Evolution, and Maintenance Mechanism of a Severe Squall Line in an Arid Region

Takemi, Tetsuya

Titre : Studies on the Structure, Evolution, and Maintenance Mechanism of a Severe Squall Line in an Arid Region

Auteur : Takemi, Tetsuya

Université de soutenance : Kyoto University

Grade : Doctoral Thesis 1999

Résumé partiel
Organized convective cloud systems appear to rarely occur in arid regions because of a low moisture content. They can actually, however, develop in arid regions if a certain condition is satisfied. In the present thesis, the structure, evolution, and maintenance mechanism of a severe squall line in an arid region was investigated observationally and numerically. The observational study described the structure and evolution of a long-lived squall line that developed over the arid region in China on 5 May 1993 and caused widespread severe dust storms. This squall line evolved in a dry environment characterized by a high level of free convection (LFC) of about 5 km above the ground level. In the pre-storm environment, a mixed layer developed, and its maximum depth was estimated as about 4.3 km. The role of this deep mixed layer in the squall-line evolution was proposed. The growth of the deep mixed layer decreases the difference between the height of the mixed-layer top and the LFC and reduces the energy required to lift the mixed-layer air above the LFC. The surface cold-air pool emanated from the squall line provides the required lifting at its leading edge. Furthermore, the dry mixed layer is favorable for the evaporation of falling precipitation. Thus, the observed squall line had a strong cold pool, produced little precipitation, and evolved in the dry environment. One-dimensional modeling of rain evaporation showed that a large amount of rain was evaporated in a dry, deep mixed layer typical of the arid region in China. The amount of evaporated rain varied widely according to rain intensity and fall distance from the top of the mixed layer. With a constant amount of total rain, the amount of evaporated rain increased significantly with rain intensity. For severe squall lines, rain falling through the subcloud layer is likely to completely evaporate. Two-dimensional numerical experiments investigated the mechanisms for the development and maintenance of squall lines in dry environments. The effects of the mixed-layer depth and the vertical moisture profile in the mixed layer were focused on. The experiments showed that a deep mixed layer is indispensable for the squall-line development, and that a moisture profile nearly constant with height is favorable for the squall-line maintenance. The most persistent squall line developed in the condition of a well-mixed moisture profile within a deep mixed layer. Although in this condition the convective available potential energy (CAPE) for the surface air parcel was not large, air parcels in the upper part of the mixed layer ahead of a surface cold pool had moderate CAPE values, and the differences between the source levels of these parcels and their LFCs were very small. These parcels were lifted to the upper troposphere, thus contributing to the formation of clouds. On the other hand, the parcels in the lower mixed layer were transported rearward, contributing to the intensification of the cold pool. The vertical distribution of CAPE in the mixed layer affects the dynamics of the squall lines in dry environments.

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