On the influence of synoptic development on the production of high level turbulence

This paper considers free shear layer turbulence in the atmosphere (clear air turbulence) as a mechanism for relieving the formation of discontinuities of wind and temperature in the vertical plane by shearing and stretching deformation. An expression for the rate of change of the logarithm of the gradient Richardson number following the air motion is derived and the assumption is made that turbulence counteracts the dynamical processes which are reducing the Richardson number, maintaining it at a limiting value. The rate of working required to do this is equated to the turbulent energy dissipation rate and in the model used is given by Φ (ΔV)2/24 (where the logarithmic rate of reduction of Ri which would take place in the absence of turbulence, and ΔV is the velocity difference across the turbulent layer). Two preliminary tests of the theory as a forecasting tool using the Bushby-Timpson 10-level numerical model show that the dynamical processes changing Ri are largest in areas where clear air turbulence might be expected from synoptic experience.