Impact of Typhoon Intensity on the Development of a Pre-Tropical Cyclone Squall Line

A severe pre-TC (Tropical Cyclone) squall line that preceded Typhoon Matsa (2005) was investigated using the non-hydrostatic "Weather Research and Forecast" model. The results showed that this pre-TC squall line formed in the moist transition zone between the parent TC and a subtropical high. It was found that Matsa may have contributed to convective development in the squall line by producing conditional instability and enhancing low-level moisture convergence, leading to the organization of the early discrete convection into a linear rainband. Although the pre-TC squall line caused fewer surface pressure perturbations at its mature stage than is typically observed at mid-latitudes, it exhibited stronger moisture convergence in the boundary layer and a larger area of inflow in the mid-troposphere. Sensitivity analyses showed that stronger TCs give pre-TC squall lines that are longer lived and faster moving, with more intense rainfall. The results showed that a stronger TC tends to produce more low-level vertical wind shear in the transition zone, which favors the organization of early discrete convection into a squall line. However, low level vertical wind shear in the vicinity of the pre-TC squall line decreases when the mature stage is entered. This, in addition to the likelihood of a reduced low-level moisture supply, appears to be one of the factors that is unfavorable for the pre-TC squall line being maintained.