The Westward and Eastward Movement Processes of the Fronts over the Yunnan–Guizhou Plateau: A Comparison

The Yunnan–Guizhou quasi-stationary front (YGQSF) governs sharp weather contrasts on its two sides over Southwest China in winter. Its movement is closely related to synoptic-scale weather on the daily timescale, but exhibits significant diurnal variations. Accurately quantifying the frontal movement is vital to improving weather forecasts in this area. Based on the fifth generation ECMWF reanalysis (ERA5) data for a chosen region of high frontal activity, this study proposes a new method using the meridionally averaged potential temperature gradient to quantify frontal movement and investigate the YGQSF characteristics in its daily westward-moving (WM) and eastward-moving (EM) processes. The results show that both the WM and EM fronts exhibit a consistent diurnal cycle: eastward movement during 0800–1600 Beijing Time (BJT), followed by westward movement thereafter. For WM fronts, the large-scale circulation usually features cold-air outbreaks, with prevailing easterly winds and cold advection over Southwest China. Stronger cold-air advection weakens the eastward movement and amplifies the subsequent westward movement. Consequently, the front exhibits net westward propagation on the diurnal timescale. For EM fronts, the large-scale circulation pattern at 850 hPa is opposite to that of WM fronts. This pattern presents a weakening cold-air intrusion, and the EM fronts often appear following the cold-air outbreak. Additionally, EM fronts generally exhibit weaker intensity and a more westward location. Compared with those of WM fronts, the cloud cover on both sides of EM fronts is lower, resulting in stronger radiative heating of the cold air east of the fronts before 1600 BJT. Warm advection at frontal locations further promotes the eastward movement of EM fronts. Together, these factors lead to a net eastward retreat of YGQSF on the daily timescale. In sum, WM fronts are dynamically driven by large-scale cold advection, whereas EM fronts are thermodynamically driven by local cloud-radiative heating. Forecasting of frontal movements must distinguish between synoptic-scale dynamical forcing and local diurnal thermal contrasts.