Diurnal Variation of Water Vapor Mixing between the Atmospheric Boundary Layer and Free Atmosphere over Changwu, the Loess Plateau in China

Abstract

We investigated the diurnal variation of water vapor mixing between the atmospheric boundary layer (ABL) and the free atmosphere over the Loess Plateau in China. Water vapor and wind velocity in the troposphere were observed using a ground-based microwave radiometer and a wind profiler radar in 2005 and 2006. On sunny days in early summer, a strong vertical wind was generated in the afternoon followed by ABL development. Strong convection was enhanced when active cumulus convection developed in the afternoon. In such cases, water vapor decreased in the lower atmosphere from the early morning until late afternoon, while water vapor increased in the upper atmosphere. This finding suggests that water vapor was exchanged diurnally between the ABL and the free atmosphere. The strong convection in the ABL, which was developed by sensible heat from the land surface, played critical roles with link to cumulus convection in such vertical mixing of water vapor. Influences of other processes such as a local circulation and advection of cloud systems were also discussed. IntroductionWater plays an important role in the Earth's climate by transporting energy over long distances, forming clouds and rainfall, and thereby influencing biological and ecological activities worldwide. Water is also crucial for human lives and activities. It is especially critical in arid and semi-arid regions where water resources are limited.In East Asia, vast areas of such severe climate regimes are distributed across northern China. Located in the middle part of the Yellow River basin, the Loess Plateau significantly affects the water cycle in the Yellow River basin (Zhang and Liu 2005).The Loess Plateau experiences dry conditions in most periods of the year. Over the dry surface exposed to intense solar radiation, the atmospheric boundary layer (ABL) can develop to high altitudes (Gamo 1996). A strong vertical wind, which can reach the free atmosphere, develops in the ABL over the plateau in early summer . Over the Tibetan Plateau, dry convection in the ABL and cumulus convection have been shown to play significant roles in heating the upper troposphere in spring (Taniguchi and Koike 2007). Thus, the developments of the ABL and clouds are critical factors in atmospheric dynamics and water cycles.To clarify the role of cloud activity, it is necessary to investigate the temporal variation of the atmospheric thermodynamic state. Recent progress in collecting in situ observation variables has made it possible to measure precipitable water vapor (PWV) at a high time resolution (e.g., Takagi et al. 2000). Passive microwave radiometry, which was adopted in this study, is an efficient method to retrieve continuous profiles of atmospheric humidity and temperature (Lohnert et al. 2007). In this study, data were also obtained using a wind profiler radar to assess the kinematic structures of the ABL and the free atmosphere.The aims of this study were 1) to clarify the diurnal variation of the atmospheric water vapor over the Loess Plateau in early ...

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