Understanding of a sudden shift from foehn to late spring cold span on the north slope of the Middle Tianshan Mountains with meteorological observation and terrain sensitivity simulation experiments

The north slope of the Middle Tianshan Mountains (NS-MTM), characterized by a distinctive basin-mountain-canyon topography, is frequently impacted by strong foehn winds and late spring cold span (LSCS). To better understand these phenomena, this study investigates a typical foehn-to-LSCS event that occurred in the spring of 2018. Using conventional meteorological observations and ERA5 reanalysis data, we analyze the evolution of surface meteorological elements and the associated atmospheric circulation patterns. Furthermore, the role of topography is explored through WRF (Weather Research and Forecasting) model sensitivity experiments with reduced mountain terrain altitudes. The key findings are as follows: (1) The LSCS occurred immediately following the foehn event, resulting in a rapid transition from dry, hot, and strong southeasterly winds to wet, cold, and strong northwesterly winds. The foehn was primarily triggered by a pressure drop ahead of a surface cold front, whereas the LSCS was induced by a strong frontal zone at the base of a mid-to-high-level trough. (2) During the active foehn stage, the "gorge tube effect" caused the southeasterly winds to become more uniform and significantly intensified wind speeds within the canyon. In contrast, during the LSCS, the canyon topography had little influence on the northwesterly wind direction, but its enhancing effect on wind speed was even more pronounced than during the foehn period. (3) The canyon topography facilitated a decline in surface air temperature within the canyon area. Its influence on relative humidity was negative during the foehn period but became positive during the LSCS when northwesterly winds prevailed. Moreover, with the inflow of strong cold air, the increase in canyon humidity was more substantial. In conclusion, the "gorge tube effect" is a major factor driving variations in the wind, temperature, and humidity fields during the coupled foehn and LSCS processes on the NS-MTM. These results underscore the critical role of canyon terrain in modulating local meteorological changes and highlight the necessity of considering such topographic effects in weather forecasting and warning operations.