Effects of Low‐Level Jets on Near‐Surface Turbulence and Wind Direction Changes in the Nocturnal Boundary Layer
Bai Yang, Dennis Finn, J. D. Rich, Zhongming Gao, Heping Liu
Abstract
Abstract 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 (∆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 ∆WD values. R b can be a useful parameter in assessing turbulence strength and ∆WD as well. The dependence of ∆WD on R b appears to be well defined under weakly stable conditions (0.0 < R b ≤ 0.25) and ∆WD is generally confined to small values. However, the relationship between ΔWD and R b breaks when R b increases, especially R b > 1.0 (very stable conditions), under which ΔWD varies across a very wide range and the potential for large ΔWD increases greatly. Our findings have provided important implications to the plume dispersion in the nocturnal boundary layers.