A Modeling Study of a Low-Level Jet Along the Yun-Gui Plateau in South China

In this study, the three-dimensional structures and diurnal evolution of a typical low-level jet (LLJ) with a maximum speed of 24 m s^-1 on the eastern foothills of the Yun-Gui Plateau in South China are examined with both the large-scale analysis and a high-resolution model simulation. The effects of surface heating, topography, and latent heat release on the development of the LLJ are also studied.

Results show that a western Pacific subtropical high and a low-pressure system on the respective southeast and northwest side of the LLJ provide a favorable large-scale mean pressure pattern for the LLJ development. The LLJ reaches its peak intensity at 850 hPa near 0200 LST with wind directions veering from southerly before sunset to southwesterly at midnight. A hodograph at the LLJ core shows a complete diurnal cycle of the horizontal wind with a radius of 5.5 m s^-1. An LLJ coordinates system is proposed to better quantify the relative contributions of various parameters to the formation of the LLJ. It is found that the along-LLJ geostrophic component determines the distribution and 60% of the intensity of LLJ, whereas the ageostrophic component contributes to the clockwise rotation, thus leading to the formation and weakening of the LLJ during night- and daytime, respectively. Numerical sensitivity experiments confirm the surface heating as the key factor to determine the formation of the nocturnal LLJ. The reversal of temperature gradients between the Yun-Gui Plateau and the plains region to the east, and the downstream condensational heating along the Meiyu front play secondary roles in the LLJ formation.