Large-eddy simulation of a residual layer: Low-level jet, convective rolls, and Kelvin-Helmholtz instability

Diurnal variations of an atmospheric boundary layer from 0900 LST on day 33 to 0600 LST on day 34 of the Wangara experiment are studied using a large-eddy simulation (LES) model that includes longwave radiation and baroclinicity. The present study directs its particular attention to phenomena in a residual layer (RL). As the surface heat flux decreases, an inertial oscillation is initiated and is accompanied by a low-level jet (LLJ) at a height of approximately 200 m. The maximum wind speed of the LLJ exceeds 12ms^-1 at 0300 LST on day 34. After 2100 LST on day 33, the horizontal advection due to the LLJ under a large-scale horizontal gradient of temperature destabilizes the RL and consequently induces horizontal convective rolls, parallel to a vertical wind shear (VWS) vector, between heights of 400 and 1400 m. The VWS in the layer between the bottom of the convective rolls and the gradually growing LLJ maximum is intensified after midnight, and the gradient Richardson number falls below its critical value of 0.25 at a height of 400m at 0130 LST on day 34. An empirical orthogonal function analysis demonstrates that Kelvin-Helmholtz (KH) vortices appear below the convective rolls and are coupled with them. This study suggests that horizontal convective rolls can occur in an RL because an LLJ often advects warmer air to the lower layer according to a large-scale gradient of temperature and that the rolls may coexist withKH vortices in a stable boundary layer because the LLJ gradually increases a VWS.