The Ellrod Index results from an objective technique for forecasting clear-air-turbulence (CAT). The index is calculated based on the product of horizontal deformation and vertical wind shear derived from numerical model forecast winds aloft.
The deformation predictors are given by:
Shearing deformation = DSH = (dv/dx + du/dy)
Stretching deformation = DST = (du/dx - dv/dy)
(where u and v are the horizontal wind components and d represents "del").
Total deformation = DEF = (DSH2 + DST2 )1/2
Convergence = CVG = -(du/dx + dv/dy)
Vertical wind shear = VWS = (delta V / delta Z)
Resulting index is computed over specified layers and is given by:
Ellrod Index = VWS x (DEF + CVG)
Reports of CAT are subjective in nature and, therefore, uncalibrated since they depend upon the pilot's perception of the degree of turbulence, which in turn depends upon the weight, speed, and aerodynamic characteristics of the aircraft being flown.
|Turbulence Intensity||Index Threshold|
*Threshold values are also subjective in nature and are dependent on model differences (see Ellrod and Knapp, 1992: An Objective Clear-Air Turbulence Forecasting Technique: Verification and Operational Use; Weather and Forecasting, Vol 7, pp 150-165).