The Generation of Turbulence below Mid-level Cloud Bases

In the author's experience as a forecaster, commercial aircraft sometimes report turbulence below mid-level clouds that extend above upper frontal zones. Turbulence caused by Kelvin–Helmholtz instability occurs in upper frontal zones with strong vertical shear of horizontal winds. However, the turbulence seems to occur not only in the cloud bases (where upper frontal zones are) but also below the cloud bases where the vertical shear is not strong. Because those clouds are usually accompanied by precipitation that does not reach the ground, cooling by the evaporation or sublimation seems to contribute to the generation of turbulence.

In order to investigate the mechanisms generating turbulence below mid-level cloud bases, we performed high-resolution three-dimensional numerical simulations with idealized initial conditions. The numerical simulations showed that the following sequence of events led to turbulence. Falling snow sublimated below cloud bases and cooled the air, which created absolute instability. This generated Rayleigh–Bénard convection cells. The vertical motion caused turbulence. The horizontal scale of the convection was about 800–1000 m, and the variations of vertical wind velocity were up to about 7 m/s. The cloud base was accompanied by a virga-like distribution of snow.