The Dynamic Characteristics Of Typhoon Matsa And Its Impact Of Typhoon Intensity On The Development Of A Pre-TC Squall Line

A severe Typhoon Matsa that occurred over western North Pacific in August2005is investigated using the non-hydrostatic Weather Research and Forecast model. The simulation data are applied to a barotropic shallow water equation to study barotropic characteristic wave structures and their stabilities when Matsa reaches its peak. The results show there are counterclockwise-propagating external inertia-gravity waves and vortex Rossby waves inside Matsa, which have different structures and stabilities. The inertia-gravity waves occurs the outer part of Matsa, and their growth rate increases as wavelength reduces, and the wave phase speed over the pouter part is48.9-68.5m s-1. The Rossby waves appear over the200km of center area, and their wavenumber-3is most unstable, and the phase spped at the radius of100km is5m s-1. In addition, perturbation wind fields of the inertis-gravity waves are generally perpendicular to height fields, and the ratio of perturbed divergence to perturbed vorticity is over3and could be up to103, and the waves are associated with divergence. The perturbed wind fields of the Rossby waves are generally parallel to height fields, the ratio of perturbed divergence to perturbed vorticity is10-1～10-2, and the waves are related to vorticity, which is intimately associated with tangential propagation of spiral rainbands.After Matsa landingfall, barotropic equations including a friction term are derived in cylindrical coordinates. The numerical differential method is applied to the barotropic equations to construct the characteristic wave solutions. The data from a Weather Research and Forecasting model simulation of typhoon Matsa at1500LST on August6,2005are then used to calculate the barotropic characteristic wave structures to study how they are affected by friction inside typhoon Matsa. The results show that the wavenumber is45for most unstable external inertia-gravity waves, which are weakened by friction. The waves propagate counterclockwise, and the wave speed for wavenumber1at a radius of1000km is47.43m/s. Perturbed radial winds increase and convergence strengthens at radii greater than800km. The wavenumber is2for most unstable vortex Rossby waves, whose growth rate is reduced by friction. The wave speed is4.282-29.172m/s at a radius of200km; the area of large perturbed vorticity decreases, and the area of the vortex Rossby waves shrinks toward the typhoon’s center. When all the waves are considered, friction weakens the waves at wavenumber45. The wave speed of wavenumber1is26.374m/s at a radius of1000km, whereas it is5.275m/s at a radius of200km. The wave speed of wavenumber1becomes30.324m/s at a radius of1000km and6.065m/s at a radius of200km when the radial wind is considered. The friction significantly increases the perturbed radial winds and enhances the convergence.number-45. The wave speed of wave. number-1is26.374m s-1at the radius of1000km, whereas it is5.275m s-1at the radius of200km. The wave speed of wave number-1become30.324m s-1at the radius of1000km and6.065m s-1at the radius of200km when take the radial wind into account. The friction leads to the significant increase in perturbed radial winds and the enhancement in convergence.Simulations at2km grid spacing revealed that a severe pre-TC squall line ahead of Matsa. Results show that this pre-TC squall line forms in the moist transition zone between the parent TC and a subtropical high. It is found that Matsa may contribute to convective development in the squall line by producing conditional instability, and enhancing the low-level moisture convergence, leading to the organization of early discrete convection into a linear rainband. Although the pre-TC squall line at the mature stage causes less surface pressure perturbations than that typically observed in midlatitudes, it exhibits stronger moisture convergence in the boundary layer and a larger area of inflow in the midtroposphere. Sensitivity experiments show that the stronger TC it is, the more intense rainfall, a longer life span and faster movement is the pre-TC squall line. Results show that the stronger TC tends to produce larger low-level vertical shear in the transition zone, which favors the organization of the early discrete convection into the squall line. However, after entering the mature stage, the low level vertical shear in the vicinity of the pre-TC squall line decreases. This appears to be one of the factors that is unfavorable for the maintenance of the pre-TC squall line, in addition to the likely reduced low-level moisture supply.