A Case Study of the November 2012 Mixed Rain-Snow Storm over North China

A mixed rain-snow storm associated with a strong burst of cold air and development of an extratropical cyclone occurred over North China from 3 to 5 November 2012. This early snowfall event was characterized by a dramatic drop in temperature, strong winds, high precipitation intensity, broad spatial extent, and coexistence of multi-phase precipitating hydrometeors. This study investigates the multi-scale interactions between the large-scale circulation background and the synoptic-scale weather systems associated with the storm. The results are as follows. (1) The Arctic Oscillation (AO) had been in its negative phase long before the event, leading to southward advection of cold air into North China in advance of the storm. (2) The large-scale atmospheric circulation experienced a decreased number of long waves upstream of North China prior to the storm, resulting in reduced wave velocity and an almost stagnant low pressure system (extratropical cyclone) over North China. (3) An Ω-shaped blocking high over East Asia and the western Pacific obstructed the eastward movement of an upstream trough, allowing the corresponding surface cyclone to stabilize and persist over Beijing and its neighboring areas. This blocking high was a major factor in making this event a historically most severe precipitation event in autumn in Beijing for the past 60 years. (4) Baroclinic instability at lower levels gave rise to rapid development of the cyclone under the classical “second type” development mechanism for extratropical cyclones. (5) Moisture originated from the Yellow Sea entered the slowly-moving cyclone in a steady stream, creating fairly favorable water vapor supply for the heavy rainfall-snowfall, especially during the later stage of the cyclone development. (6) Moisture transport and frontal lifting triggered low-level instability and updrafts. Intensification of the front enhanced the vertical wind shear, causing conditional symmetric instability (CSI) to expand upward within the unstable lower troposphere, and to eventually gear into the CSI region of the upper troposphere, which facilitated the upward development of low-level updrafts.