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The energy mechanism controlling the continuous development of a super-thick atmospheric convective boundary layer during continuous summer sunny periods in an arid area

In the arid regions of the world, due to the specific climatic and environmental background, a super-thick convective boundary layer (SCBL) often develops on sunny days in summer, whereas such phenomena rarely occur in other areas. This special boundary layer structure has an important synoptic and climatic significance, but there have been few studies of its development mechanism in arid areas, which greatly restricts the parametric improvement of the SCBL and our understanding of the interaction between weather and climate processes. This study was conducted in Dunhuang, which is located in the hinterland of northwest China. Based on data obtained from land-air interaction experiments and long-term operational sounding observations in this region, the energy mechanism controlling the development of the CBL and the developmental process of the SCBL were systematically analyzed. In arid northwest China, it is possible that the thickness of the CBL can extend to over 3 km for most of the year, except winter. Even in the early summer, when there is little rain and strong solar radiation, the thickness of the CBL may reach an extreme state of 5.4 km. The thickness of the CBL at this time is higher than that observed in midsummer, when there is slightly more precipitation in this area. This is basically consistent with the extreme thickness of the CBL recently discovered in the Sahara Desert in Africa. In the general mechanism that controls the development of the CBL, there is a close relationship between the development of the CBL and the sensible heat flux of the surface. However, the correlation between the thickness of the CBL and the surface sensible heat flux at the same time is not strong, whereas the correlation between the thickness of the CBL and the cumulative surface sensible heat flux is very strong. This indicates that the development of the CBL is the result of the continuous accumulation of the sensible heat flux on the surface, which is consistent with the energy mechanism controlling the CBL. Although the development of the CBL is closely related to the cumulative heating effect of the daytime surface sensible heat flux, the CBL would still continue to increase even if the integral value of the daytime surface sensible heat flux remained unchanged or even weakened during the continuous clear sky period. The energy provided through sensible heat does not fully explain the energy required to develop the CBL. This is mainly because the deep near-neutral residual layer (RL) background plays an important role in the development of the SCBL. The entrainment energy from the deep RL to the CBL is the key energy supply for the continuous development of the CBL. The sum of the entrainment energy and surface sensible heat energy coincides with the energy absorbed by the development of the SCBL. The reason for the occurrence of an SCBL in arid areas is not only the strong sensible heating in summer but also the persistent clear skies in such areas. In each continuous clear sky period, the positive feedback mechanism between the CBL and the RL will become operational. Under this mechanism, the daily maximum thickness of the CBL and the thickness of the RL will increase continuously. The thickness of the SCBL is generally over 3 km, although depths of over 5 km can develop through a cyclic growth mechanism during periods of strong surface heating. Otherwise, the thickness of the CBL can only reach 2–3 km in summer, and it is unlikely that an SCBL will develop. Strong sensible heating is the key trigger of the positive feedback cycle growth mechanism between the CBL and the RL, which explains why the SCBL phenomenon can only occur in dry areas, with intense surface heating in summer.