McCann Aviation Weather Research, Inc. offers four algorithms computable from numerical model forecasts for each of the primary causes of aircraft turbulence. BLTURB is for boundary layer turbulence, ULTURB is for unbalanced flow gravity wave-induced turbulence, MWAVE is for mountain wave-induced turbulence and VVTURB identifies the location and strength of significant convective updrafts.
The Ri method for forecasting turbulence can only determine a yes/no answer. If one assumes that turbulence occurs in the critical eddy size for aircraft to feel, then, over time, the turbulence kinetic energy (TKE) production is equal to the TKE dissipation and is proportional to turbulence intensity. Each of the four turbulence sources has a unique mode for enhancing TKE production which is included in the respective algorithm. Then the TKE production is transformed into eddy dissipation rate, the international standard for reporting turbulence quantity.
Hit the red buttons below for technical descriptions of BLTURB, ULTURB, MWAVE, or VVTURB.
Conceptually, forecasting atmospheric turbulence is straightforward. Like any other fluid, turbulence begins when the Richardson number (Ri), the ratio of atmospheric stability and wind shear squared, falls to less than 0.25. This often occurs in the planetary boundary layer over land on sunny, windy days. Although aircraft only respond to turbulence when the eddies are sized in a certain range, looking for Ri values below 0.25 in the boundary layer is a viable approach to forecasting turbulence near the ground.
Ri values are rarely below 0.25 above the planetary boundary layer and probably exist only in perturbed conditions. Thus methods for forecasting aircraft turbulence above the boundary layer must be able to find the locations and altitudes of the perturbations.
Gravity waves are ubiquitous in the atmosphere and may perturb the stability and wind shear enough to locally lower the environmental Ri to less than 0.25. Two known sources of gravity waves are flow over mountains and unbalanced flow. Convection is another source of atmopsheric turbulence, creating it by draft prodution as well as by generating gravity waves.
Primary causes of turbulence affecting aircraft.
● Boundary layer