Various aspects of mid-latitude squall lines have been widely investigated by many scientists through observations, theoretical analyses, and numerical modeling. The interaction between surface cold-air pool and environmental vertical wind shear is commonly recognized as an important mechanism for the development of squall lines. Rotunno et al. (1988) proposed a theory for the role of that interaction in the development of squall lines, and more recently Weisman and Rotunno (2002) have confirmed that the theory is valid in their systematic numerical experiments. In addition to the cold pool-vertical shear interaction, the stability is also known to significantly affect the organization modes of mesoscale convective systems (Weisman and Klemp 1984). Based on such past studies, this study focuses mainly on the moisture profile (which has strong ties with the stability) and investigates the effects of moisture profile as well as vertical wind shear on the development of squall lines in mid-latitude environments by performing three-dimensional numerical experiments with various wind and humidity profiles. The Weather Research and Forecasting (WRF) Model was used for the numerical experiments. In this study, a microphysics parameterization including ice phase was employed in order to promote convection development even in drier environments.

Setting the boundary-layer moisture content to 14 g/kg, we reproduced the similar features of squall lines to the study of Weisman and Rotunno (2002). Decreasing the boundary-layer moisture content, the amounts of water condensates produced by convection also decreased; however, an intense surface cold pool developed in the drier environments and thus maintained the squall-line system by interacting with the low-level vertical wind shear. In the drier conditions, squall lines were not sustained for a long period when the shear layer was elevated above the boundary layer. In moister conditions, the level of the shear layer did not significantly affect the development of squall lines, because minimum amount of lifting would be required in such conditions. Since the strong, upright lifting will be produced in an optimal condition in which there is a dynamical balance between the cold-pool circulation and low-level vertical shear, the interaction mechanism proposed in the Rotunno et al.'s theory is more critically important in the environments with lower CAPE and higher CIN.