Climatic Variations in the Boundary Layer Height of Arid and Semiarid Areas in East Asia and North Africa

Based on the ERA-20C, climatic variations of the boundary layer height (BLH) over arid and semiarid areas in East Asia and North Africa that span 1900-2010 were analyzed. In East Asia, the BLH exhibited a descending trend from arid region centers to the periphery. Over the past 111 years, the BLH has had a rising trend of 14.0 m decade−1. in the representative region (EA1) of the eastern areas with the 111-year average of 725 m and a decreasing trend of −1.6 m decade−1. in the representative region (EA2) of the western areas with the 111-year average of 792 m. From the mid-1960s to 1970s, EA1's BLH had a sharp rise that caused the average to increase by 93 m after the 1980s. In North Africa, the BLH exhibited a high spatial distribution in the western and southern areas and a relatively low spatial distribution in the eastern and northern areas. Over the past 111 years, the BLH has had a rising trend of 9.7 m decade−1. in the representative region (NA1) of the southwestern region with the 111-year average of 915 m and a decreasing trend of −6.3 m decade−1. in the representative region (NA2) of other regions with the 111-year average of 882 m. In the 1940s and the 1970s, NA1's BLH had two obvious increases that caused the average to increase by 51 m and 22 m, respectively, while NA2's BLH had two obvious declines that caused the average to decrease by 48 m and 7 m, respectively. On the spatial distribution, the BLH, sensible heat flux, latent heat flux, and volumetric soil water had a good corresponding relationship. On the temporal change, the BLH in East Asia had a stronger correlation with thermodynamic factors, whereas the BLH in North Africa had a stronger correlation with dynamic factors. Besides, the upper-level stratification also has some influence on the BLH's change.