Effects of Low-Level Jets on Near-Surface Turbulence and Wind Direction Changes in the Nocturnal Boundary Layer

In this study we examined a data set of nearly two-year collection and investigated the effects of low-level jets (LLJ) on near-surface turbulence, especially wind direction changes, in the nocturnal boundary layer. Typically, nocturnal boundary layer is thermally stratified and stable. When wind profiles exhibit low gradient (in the absence of LLJ), it is characterized by very weak turbulence and very large, abrupt, but intermittent wind direction changes ( increment WD) in the layers near the surface. In contrast, presence of LLJs can cause dramatic changes through inducing wind velocity shears, enhancing vertical mixing, and weakening the thermal stratification underneath. Ultimately, bulk Richardson number (R-b) is reduced and weakly stable conditions prevail, leading to active turbulence, close coupling across the layers between the LLJ height and ground surface, relatively large vertical momentum and sensible heat fluxes, and suppressed increment WD values. R-b can be a useful parameter in assessing turbulence strength and increment WD as well. The dependence of increment WD on R-b appears to be well defined under weakly stable conditions (0.0 R-b = 0.25) and increment WD is generally confined to small values. However, the relationship between ?WD and R-b breaks when R-b increases, especially R-b > 1.0 (very stable conditions), under which ?WD varies across a very wide range and the potential for large ?WD increases greatly. Our findings have provided important implications to the plume dispersion in the nocturnal boundary layers.