Shift and intensification processes of the Japan-Sea polar-airmass convergence zone associated with the passage of a mid-tropospheric cold core

Abstract

A mid-tropospheric mesoscale cold core passed over the Sea of Japan on 14 January 2001 during a cold air outbreak. In association with the passage of the cold core, intensification and northward and southward shifts of the Japan-Sea Polar-Airmass Convergence Zone (JPCZ) were observed. In order to clarify a response of the JPCZ to a change of upper-level environment, the shift and intensification processes associated with the passage of the cold core were studied mainly using a non-hydrostatic cloud-resolving model. When the cold core approached the JPCZ, the JPCZ was intensified and shifted northward. The intensification of the JPCZ is caused by a change in a decrease of convective stability associated with the passage of the cold core. As a ridge of potential temperature is formed along the intensified updraft of the JPCZ, a leeward region of the JPCZ is locally heated by the horizontal advection in a westerly wind, which is predominant in the upper levels. This hydrostatically reduces the low-level pressure in the north of the JPCZ, and increases the wind speed blowing into the JPCZ from the south. The divergence equation indicates that the abrupt change in the pressure as well as the adjusted wind fields cause the northward shift of the JPCZ. In contrast, the JPCZ decayed and shifted southward when the cold core moved away from the JPCZ. The pressure decrease, which occurred in the north of the JPCZ, is not maintained during the southward shift, and the pressure gradient across the JPCZ becomes relatively uniform. This results in the decrease of the wind speed blowing into the JPCZ from the south, and the JPCZ shifted southward due to then. advection. © 2007, Meteorological Society of Japan.