Low-level-jet scaling of the stable boundary layer over the United States Great Plains

Semi-arid conditions in the Great Plains of the United States produce strong surface cooling on clear nights. Stable boundary layer (SBL) structure and properties are then controlled by the horizontal wind speed U_h at the top of the boundary layer h, which has been shown to correspond to the depth of the low-level jet maximum Z_X most of the time. In agreement with previous studies, three basic regimes have been identified. For strong LLJ speeds, the weakly stable boundary layer (wSBL) has traditional structure for the turbulence quantities (maximum magnitudes at the surface, minimum at the SBL top). As wind speeds decrease and stability (e.g., Ri_B) increases, the moderately stable boundary layer (mSBL) exhibits intermittent bursts of turbulence through the night. Approaching the weak-wind extreme, in the very stable BL (vSBL), the SBL becomes very shallow and turbulent communication between the surface and the atmosphere above the SBL is shut down.

In the strong-wind wSBL, i.e., when LLJ speeds reach 15 m s^-1 at some time during the night, it is observed that Ri_B in the subjet layer is constant with height and equal to ~0.11, the (constant) value of the shear in this layer is somewhat less than 0.1 s^-1, and the stability (lapse rate) is also about constant between the surface layer and Z_X. Using the definition of Ri_J (a bulk Ri based on LLJ properties), we can derive the relationship Z_X = Ri_J ^½ (U_X/N), where U_X is the LLJ speed and N the Brunt-Vaisala frequency. For typical values of N (~0.025 s^-1) this produces a value for the subjet shear of U_X/h = N / Ri ^½ = 0.08, which is close to the observed value. The peak value of σ = (TKE)^½ near the surface has been estimated to be proportional to U_X, with a proportionality factor of 0.05, i.e., σ is 5% of the LLJ speed. One way that σ could be proportional to U_X is if shear increases with U_X, but this is not observed. The other explanation is that the mixing length, or the size of the large eddies, is proportional to the SBL depth h, which is observed to be about proportional to U_X.

Implications for NWP modeling are that for the wSBL and mSBL, accurate determination of U_X is critical. For the vSBL, the model must be able to accommodate shutting down surface-atmosphere interaction via turbulent flux processes. Other implications will be discussed.