Isentropic Analysis of Regional Cold Events over Northern China

From the perspective of cold air mass (CAM) analysis, we examine the characteristics and mechanisms of regional cold events (RCEs) over northwestern and northeastern China in the past 58 years (1958/59-2015/16). The RCEs in northwestern (northeastern) China are shown to have an average duration of 6.8 (4.7) days with a moderate (sharp) temperature drop. We quantitatively estimate the RCE-related CAM, for the first time, using an isentropic analysis method. Before an RCE in northwestern China, CAM is accumulated in western Siberia with convergent CAM flux under a blocking pattern in the Urals region. During RCE outbreak, CAM penetrates the valleys of the Tianshan-Altay Mountains to the Tarim Basin and Hexi Corridor. The CAM moves slowly because of the blocking pattern and orographic effect, which explains the relatively long duration of RCEs. Comparatively, during RCEs in northeastern China, the CAM depth anomaly originates more to the east and quickly passes the Mongolian Plateau guided by an eastward-moving trough. Diagnostic analyses further show that adiabatic processes play a crucial role in regulating the local change of CAM depth during the two kinds of RCEs. The advection term of adiabatic processes mainly increases the CAM depth during RCE outbreak, while the convergence term increases (reduces) CAM depth before (after) RCE outbreak. Both terms are relatively strong during RCEs in northeastern China, resulting in the rate of change in CAM depth being similar to 50% larger than for those in northwestern China. Therefore, the variations of RCEs in duration and intensity can be well explained by the different evolution of CAM depth and flux.