Abstract:

Extreme low-temperature frosts occurred in eastern Aksu Prefecture, Xinjiang, China on April 23 and May 6, 2023 (“4.23” process and “5.6” process for short, respectively). Particularly, the latest frost in history appeared in Kuqa City during the “5.6” process. In this study, based on conventional observation data, L-band sounding data, and 6-h reanalysis data of NCEP/NCAR 1°×1°, we contrastively analyze the circulation background, circulation configurations of high and low altitudes, cold air intensity, cold advection intensity, and frontogenesis during two extreme low-temperature processes using weather dynamics. Moreover, the causes of extreme low-temperature frosts are discussed. We realized both common characteristics and differences between the two processes. (1) The common characteristics are as follows: The temperature of eastern Aksu at 500 hPa is affected by northwest flow with cold air at the back of the low trough (low vortex), with a great temperature gradient and strong frontal zone during the two processes. Deep cold advection flows from the upper level of western Aksu to the lower level of eastern Aksu along the northern flow. Temperature inversion and obvious frontogenesis occur in the near-surface layer during the low-temperature period. Frontogenesis corresponds to the path of cold air entering the southern Xinjiang basin and the low-temperature duration. In addition, the upper-level energy frontal zone is stronger, and the mid-low-level energy frontal zone gradually strengthens with baroclinic instability. The cold advection intensity increases to a maximum of 6 h before frost. The cold advection can significantly decrease the temperature of the Aksu area from above 8 ℃ to 3-4 ℃ at 850 hPa, indicating strong frost. (2) The differences are as follows: The cold air invades the Aksu Prefecture from the northwest path in the “4.23” process. The air temperature in the Aksu Prefecture is affected by both “Xifan (flowing over the mountains from west)” and “Dongguan (injecting from east)” cold air. The temperature inversion in the near-surface layer is enhanced by radiation and advection cooling. However, the cold air passes over Tianshan Mountains with a strongly cold high pressure and then invades the southern Xinjiang basin from the northern path in the “5.6” process, accompanied by “Dongguan” cold air. The influence range is large, and the temperature inversion is weak. However, the temperature of the entire layer is significantly reduced, mainly via radiation cooling. (3) The low vortex of the “5.6” process lasts for a long time in Xinjiang. The cold air and frontal zone intensity, potential pseudo-equivalent temperature gradient, and influence time are stronger than those of the “4.23” process, resulting in a more severe disaster.

Key words: late spring, extreme low temperature, comparative analysis, eastern Aksu