A study of mobile trough genesis over the Yellow Sea - East China Sea region

Abstract

The purpose of this study was to understand the mechanisms responsible for the formation of mobile troughs over a prolific source region in the Yellow Sea and East China Sea. Two mobile troughs which intensified significantly after formation were analyzed. The troughs were selected from the objective climatology of mobile troughs by Lefevre and Nielsen-Gammon (1995). A quasigeostrophic potential vorticity (QGPV) and associated piecewise tendency diagnosis (PTD) technique developed by NielsenGammon and Lefevre (1995) was used to quantitatively analyze the dynamics of mobile trough formation. The PTD technique involves the inversion of QGPV and QGPV advection. A qualitative approach using dynamical tropopause maps of constant Ertel's potential vorticity (EPV) was utilized in conjunction with the QGPV method. It was determined that downstream development was the primary mechanism in initiating both mobile troughs. Type A cyclogenesis was not evident in either trough genesis event. A surface cyclone aided the development of one of the mobile troughs, but the cyclone had developed prior to the formation of the mobile trough. Baroclinic processes and large-scale interactions played lesser roles in one trough, but were not important in the other trough. Barotropic deformation modulated the strength of the second mobile trough by changing the shape of the associated QGPV anomaly. Vertical superposition was important in strengthening the latter stages of the life of the mobile trough, due to two jets superimposing over the mobile trough. Subjective analysis of tropopause maps of both trough genesis events showed the downstream development process as the growth of a new wave by cross-contour advection of potential temperature due to a pre-existing upstream disturbance.