Performance of the Ellrod-Knox and Lighthill-Ford-Knox clear air turbulence algorithms at the Aviation Weather Center

Forecasting clear air turbulence remains a challenge both domestically and worldwide. The Aviation Weather Center (AWC) is currently evaluating the performance of the Ellrod-Knox (EK) and Lighthill-Ford-Knox (LFK) clear air turbulence algorithms as part of the Aviation Weather Center Testbed. This research to operations evaluation is being conducted both on historical and real-time data using NCEP's currently operational modeling systems (i.e. GFS, NAM, RUC, RR) and high resolution next generation models (i.e. 4 km High Resolution Window and the HRRR). Both the EK and LFK are run on these models and then evaluated using traditional verification techniques using pilot reports and EDR data from selected aircraft.

The EK diagnostic is the traditional Ellrod TI1 index based on deformation and vertical shear, plus a 3 hour divergence tendency term. The LFK diagnostic employs deformation and divergence tendencies which are directly related to the spontaneous emission of gravity waves. When the spontaneous gravity wave generation terms are coupled with a measure of atmospheric stability such as the Richardson number or Brunt-Vaisala frequency, the resulting algorithm is an excellent approximation to the generation of turbulent kinetic energy and the disruption of laminar flow.

Results indicate that the EK and LFK improve upon the traditional Ellrod diagnostic of vertical wind shear and deformation in the prediction of clear air turbulence based on the True Skill Statistics score. In addition, EK and LFK skill varies between models due to model formulation which affects the magnitude and location of forcing terms in the EK and LFK.