Numerical simulation of roll vortices in the convective boundary layer

Roll vortices, which often appear when cold air outbreaks over warm ocean surfaces, are an important system for energy and substance exchange between the land surface and atmosphere. Numerical simulations were carried out in the study to simulate roll vortices in the convective boundary layer (CBL). The results indicate, that with proper atmospheric conditions, such as thermal instability in the CBL, stable stratification in the overlying layer and suitable wind shear, and a temperature jump between the two layers in a two-layer atmosphere, convective bands appear after adding initial pulses in the atmosphere. The simulated flow and temperature fields presented convective bands in the horizontal and roll vortices in the crosswind section. The structure of the roll vortices were similar to those observed in the cloud streets, as well as those from analytical solutions. Simulations also showed the influence of depth and instability strength of the CBL, as well as the stratification above the top of the CBL, on the orientation spacing and strength of the roll vortices. The fluxes caused by the convective rolls were also investigated, and should perhaps be taken into account when explaining the surface energy closure gap in the CBL.