Role of Topography in Triggering Elevated Thunderstorms Associated With Winter Cold Fronts Over the Eastern Yunnan‐Guizhou Plateau

Abstract

AbstractUsing observations and reanalysis data, the circulation characteristics of elevated thunderstorms behind winter cold fronts (ETCFs) over the eastern Yunnan‐Guizhou Plateau are summarized, and a new triggering mechanism in the cold season related to topography is proposed. ETCFs always occur on the ridge and eastern side of the Dalou Mountain and are triggered at night. Pattern classifications using obliquely rotated T‐mode principal component analysis show that low‐level large‐scale circulation patterns have the following two characteristics: a southwestern airflow at 850 hPa, changing from southwest wind to east wind; and a northeast airflow at 900 hPa, changing from northeast wind to east wind. ETCFs occur in the area of this low easterly wind. A case study of 21 January 2020, shows that it is the intersection of the topography and cold front that provides unique geographical and circulation conditions for triggering elevated thunderstorms. Lateral friction along the mountains causes horizontal wind shear to flow parallel to them, forming positive and negative vortices above and below the front, respectively. This pair of vortices is strengthened by inertial oscillation at night, producing a stronger easterly wind. In the combined effects of frontal lift and topographical lift of the easterly flow, the elevated convection is triggered in the convergence zone above the front. These results provide a new insight into how topography helps to trigger convection in the cold season.