Characterizing Near-Surface Moisture Increase during the Clear-Sky Afternoon-to-Evening Transition Using a Single-Column Model

Abstract The afternoon-to-evening transition is the period when the atmospheric boundary layer transitions from convective to stable conditions. One noticeable feature during this transition period is the rapid increase in water vapor concentration near the surface. However, the mechanism behind the increase in water vapor remains poorly understood. This study investigated the processes contributing to the water vapor increase and the impacts of the land cover and horizontal advection on the water vapor increase using a single-column model for the clear-sky condition. Numerical experiments were conducted on three cases at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site. Evapotranspiration was found to be the main moisture source to the water vapor increase, and that change of the near-surface vertical temperature gradient from negative to positive is the trigger for this increase during the afternoon-to-evening transition. This is because the near-surface turbulence divergence term decreases due to the change in the buoyancy profile caused by vertical temperature gradient change. The impact of horizontal advection on water vapor varies, and it can either lead to an increase or a decrease in water vapor, depending on the spatial horizontal water vapor gradient. This study also found that land cover can influence the timing of the water vapor increase since different land covers may have different Bowen ratios. Water vapor increases earlier under conditions with smaller Bowen ratios compared to those with larger values. Significance Statement This study investigates the processes that lead to the increase in near-surface water vapor during the afternoon-to-evening transition, a well-observed phenomenon. We employ a single-column model to simulate land–atmosphere interactions during this transition. It shows that evapotranspiration is the main contributor to the water vapor increase during the transition and that the change of the near-surface vertical temperature gradient from negative to positive is the trigger for this increase. Furthermore, this study reveals that horizontal advection can either lead to an increase or a decrease in water vapor during the transition, depending on the spatial horizontal water vapor gradient. These findings are consistent with in situ observations, as water vapor increases are typically recorded more frequently in summer than in winter, aligning with the magnitude of evapotranspiration.