Based on a European Centre for Medium-Range Weather Forecasts (ECMWF) product and observed radiosonde data, we investigated temporal variations of boundary layer height (BLH) over arid regions and their relationship with energy balance. Moreover, possible factors affecting BLH change are analyzed by combining the above data. We discovered seasonal and interannual variations of BLH in arid regions of northern China. In the past 44 years, BLH in summer continuously decreased. BLH in spring and autumn increased before the 1980s and decreased afterward. BLH in winter periodically decreased. There was an adjustment period of BLH in the 1980s. Change in BLH is influenced by dynamic and thermodynamic factors. Surface sensible heat flux (SSHF) is the main thermodynamic factor affecting seasonally decreasing and interannual variations. Over the past 44 years, SSHF decreased continuously in summer and winter. SSHF in spring and autumn increased before the 1980s and decreased shortly afterward, which resulted in heat exchange and BLH decrease. With respect to diurnal variation, BLH clearly varied with SSHF. During its development stage, the convective boundary layer (CBL) may begin to break through the stable boundary layer (SBL) and enter the remnant layer when SSHF is greater than 100 W/m²; the CBL develops with increasing SSHF. The CBL is more than 3 km deep when the SSHF exceeds 250 W/m². During its maintenance and decline, the BLH shrinks with decreasing SSHF. As a component of net radiance (Rn), SSHF is affected not only by Rn, but also by surface latent heat flux (SLHF). Rn decreased and SLHF increased over the past 44 years, would result in decreasing SSHF according to energy balance. In addition, differences of the surface and air temperature are the main driving forces of thermal turbulence, it has a positive correlation with SSHF. Therefore, differences of the surface and air temperature cause decreasing SSHF. Wind turbulence and velocity are the dynamic factors involved in atmospheric boundary layer development. A decreasing trend of wind turbulence and speed prompts a decrease in vertical wind shear, which weakens dynamic turbulence and ultimately decreases BLH. Therefore, wind turbulence and SSHF have a positive correlation. With regarding to diurnal variation, the greater the friction velocity, the greater the CBL and BLH during the day in summer. Friction velocity has good correlation with CBL and BLH when the latter is less than 2.5 km, which shows that vertical wind shear can accelerate BLH development. However, wind shear has little effect on deep BLH. With decreasing wind shear, the SBL decreases at night. Mechanical turbulence may support CBL development in winter. Agricultural acreage changed in the 1980s, which reduced roughness and thereby decreased thermal transmission. This decreases SSHF and ultimately BLH. Analyzed from a dynamics viewpoint, the less the surface roughness, the smaller the friction velocity and vertical wind shear, all of which are adverse to dynamic turbulence and BLH development.
Aside from the above factors, SSHF and BLH are affected by total shortwave radiance, surface albedo, longwave radiance, and others. These factors are potentially affected by land use and land cover, climate warming, radiance effects of aerosols, atmospheric circulation change, and soil humidity change caused by precipitation change. These topics require further research.