On the Initiation Point of the Japan-Sea Polar-Airmass Convergence Zone: A Case Study

A convergence zone called Japan-Sea Polar-Airmass Convergence Zone (JPCZ), which is often accompanied by intense convection, is frequently observed during cold air outbreaks over the Sea of Japan. Although the formation of JPCZ is strongly affected by topography, the initiation point of the JPCZ varies between cases. A JPCZ case in which its initiation points changed from one convergence line to two convergence lines was investigated to reveal the mechanism behind this shift. A numerical simulation successfully reproduced the change in the initiation points of the JPCZ. Backward trajectory analyses revealed that this change is caused by the flow around the Changbai Mountains (CMs). When the northwestern part of the JPCZ was a single convergence line, it formed between the northwesterly in the southwestern side and the northerly in the northeastern side. These two currents passed the southern and northern sides of the CMs. When two convergence line existed, another current that passed over the valley of the CMs penetrated between these two currents. The flow over the mountain was caused by the increase in the Froude number associated with the decrease in the atmospheric stability and the increase in the wind speed upstream of the CMs.