Abstract: The atmospheric boundary layer height (BLH) affects near-grounddevelopment of energy and water.The boundary layer of the monsoon swing area in the eastern part of northwest China is significantly affected by summer monsoon activity and monsoon precipitation; however, its changes and the response characteristics of the BLH are not clear. A suitable BLH was determining by comparing calculations using many kinds of methods. These calculations used daily upper observation data collected every 10 m at 19:00 from five stations (Minqin, Yuzhong, Pingliang, Yinchuan, and Yan’an) in the summer monsoon swing areafrom May to September during 2006-2016 in addition to the prescribed and characteristic layers of daily sounding data at 07:00 in Minqin during 2006-2016 as well as daily surface observation data during the corresponding period.Then, using NCEP/EMC reanalysis data, the influence of summer monsoon on the monsoon swing area’s BLH was calculated and analyzed, and the relationships between BLH and the summer monsoon and precipitation effects during the monsoon activity period was obtained. The results showed thatthe 9-point mean temperature gradient method was the best BLH calculation methodbased on the sounding data of pressure- temperature- humidity and wind every 10 m.The BLH in this area was higher in May and June and decreased gradually from July to September. The average BLH decreased from 2 600 m in the non- monsoon region and 1800m in the monsoon swing area to less than 1500m in the monsoon affected area.The BLH is closely related to relative humidity, temperature, and wind field at the surface, with higher humidity and wind speed yielding a lower BLH.In contrast, a greater near-ground temperature difference and higher temperature yields a higher BLH when the northwest wind blows.The BLH was higher when there was no summer monsoon and lower when there was a summer monsoon. The longer the duration of the summer monsoon was, the lower was the BLH. When the duration of the summer monsoon was 0, 1- 4, and ≥ 5 pentads, the corresponding BLH values wereapproximately 2 000 m, 1 600-1 900 m, and 1300-1 400 m. There was also a significant negative correlation between the APO monsoon intensity index and BLH in the monsoon affected area.The BLH was closely related to precipitation properties and intensity during the monsoon activity period, with non- precipitation, convective precipitation, and stable precipitation having effects in turn from large to small. The BLH decreased with increasingprecipitation intensity, and the BLH was higher in the non- monsoon area, lower in the monsoon swing area, and the lowest in the monsoon affected area. Additionally, more precipitation days were associated with a lower BLH. The summer monsoon played an active role in lowering BLH and increasing and strengthening precipitation during the monsoon period.

Key words: Boundary layer height, Summer monsoon, Precipitation in flood season, Monsoon swing area, In the east of northwest China