Simulation of the stratosphere-troposphere exchange process in a typical cold vortex over Northeast China

A mesoscale weather research and forecasting (WRF) model was used to simulate a cold vortex that developed over Northeast China during June 19-23, 2010. The simulation used high vertical resolution to reproduce the key features of the cold vortex development. Characteristics of the associated stratosphere-troposphere exchange (STE), specifically the spatiotemporal distribution of the cross-tropopause mass flux (CTF), were investigated using the Wei formula. The simulation results showed that the net mass exchange induced by the cold vortex was controlled by stratosphere-to-troposphere transport (SIT) processes. In the pre-formation stage of the cold vortex (i.e., the development of the trough and ridge), active exchange was evident. Over the lifecycle of the cold vortex, STT processes prevailed at the rear of the trough and moving vortex, whereas troposphere-to-stratosphere transport (TST) processes prevailed at the front end. This spatial pattern was caused by temporal fluctuations of the tropopause. However, because of the cancellation of the upward flux by the downward flux, the contribution of the tropopause fluctuation term to the net mass exchange was only minor. In this case, horizontal motion dominated the net mass exchange. The time evolution of the CTF exhibited three characteristics: (1) the predominance of the STT during the pre-formation stage; (2) the formation and development of the cold vortex, in which the CTF varied in a fluctuating pattern from TST to STT to TST; and (3) the prevalence of the STT during the decay stage.