The influence of turbulence memory on idealized tornado simulations

Surface friction contributes to tornado formation and maintenance by enhancing the convergence of angular momentum. The traditional lower boundary condition in atmospheric models typically assumes an instant equilibrium between the unresolved stress and the resolved shear. This assumption ignores the physics that turbulent motions are generated and dissipated at finite rates—in effect, turbulence has a memory through its lifetime. In this work, a modified lower boundary condition is proposed to account for the effect of turbulence memory. Specifically, when an air parcel moves along a curved trajectory, a normal surface-shear-stress component arises owing to turbulence memory. In the accompanying large-eddy simulation (LES) of idealized tornadoes, the normal surface-shear-stress component is a source of additional dynamic instability, which provides an extra pathway for the development of turbulent motions. The influence of turbulence memory on the intensity of quasi-steady-state tornadoes remains negligible as long as assumptions employed by the modified lower boundary condition hold over a relatively large fraction of the flow region of interest. However, tornadoes in a transient state may be especially sensitive to turbulence memory.