Simulated Diurnal Cycles and Seasonal Variability of Low-Level Jets in the Boundary Layer over Complex Terrain on the Coast of Southeast China

The thermal and dynamical causes of boundary layer low-level jets (LLJs) over the southeast coast of China (a very complex terrain) are explored using a very high resolution diagnostic model California Meteorological (CALMET) along with the Advanced Research Weather Research and Forecasting (WRF-ARW) model, combined with observations. Both diurnal and seasonal variations in LLJs are investigated using simulations during two observational periods and four months in 2011. Two configurations, with different vertical and horizontal resolutions, are compared. The results show that the use of higher vertical and horizontal resolutions (including land cover/use) in the diagnostic model CALMET leads to large improvements in simulating boundary layer LLJs over complex terrain as compared with using lower vertical and horizontal resolutions in both WRF-ARW and CALMET models. The simulations using the diagnostic model CALMET better reproduced the observations, in that both LLJ events occur in the night (nocturnal LLJs) and in the afternoon (afternoon LLJs) are noticed. Compared to the nocturnal LLJs, the afternoon LLJs have larger wind speeds and occur at lower heights. The afternoon LLJ characteristics are closely associated with local thermodynamic circulations including the mountain-valley breeze and land-ocean breeze, which are regulated by the thermal contrasts between the ocean and mountains, and diurnal cycle of boundary layer friction. A comprehensive analysis of an afternoon LLJ and nocturnal LLJ indicates that there are large differences in wind field, vertical motions, and water vapor distributions between them. The local thermodynamic circulations also strongly affect vertical motions; even under a relatively stable atmosphere, the vertical motions during LLJs are stronger than the monthly average. Afternoon LLJ events, associated with the southeasterly land-ocean breeze in summer, occur more frequently than in winter; in contrast, nocturnal LLJ events occur in summer less frequently than in winter. The seasonal variation analysis shows that land-ocean breezes have significant effects on the wind speed, wind direction, and heights of the afternoon LLJs; local thermal contrast forcing is believed to be the main factor that affects the LLJs during the daytime especially in warm seasons.