Height Variation in the Summer Quasi-Zero Wind Layer over Dunhuang, Northwest China: A Diagnostic Study

This study investigates the variation in the stratospheric quasi-zero wind layer (QZWL) over Dunhuang, Gansu Province, China, on 9 August 2020 using sounding observations from the Dunhuang national reference station and the fifth generation of ECMWF atmospheric reanalysis data (ERA5). The QZWL over Dunhuang was located between 18.6 and 20.4 km on 9 August 2020. The South Asian high (SAH) and subtropical westerly jet jointly affected the QZWL. As the SAH retreated westward, the upper-level westerly jet over Dunhuang strengthened, and the jet axis height increased. As a result, the zonal westerly wind was lifted to a higher altitude, and the wind speed of 100–70 hPa increased, raising the QZWL. In addition, the east–west oscillation of the SAH occurred earlier than the adjustment of the QZWL altitude, which can be used as a forecasting indicator for the QZWL. To further explore the mechanism responsible for the QZWL adjustment, the forcing terms in the equations for zonal wind, kinetic energy, and vertical wind shear were analyzed. The results showed that the upper-level geopotential gradient was the basic physical factor forcing the local change in zonal wind and kinetic energy. The change in zonal wind and kinetic energy led to the uplift of the QZWL. The results revealed that the vertical shear of horizontal wind could adequately indicate the stratospheric QZWL location.