John A. Knox, University of Georgia, Athens, GA; Gary P. Ellrod and Kenneth Pryor, Center for Satellite Applications and Research, NOAA/NESDIS, Camp Springs, MD; Donald W. McCann, McCann Aviation Weather Research, Inc., Kansas City, MO; Paul D. Williams, University of Reading, Reading, UK.
In this presentation we explore issues of spontaneous imbalance from the perspective of aviation observations and numerical forecasts of clear air turbulence (CAT). We also examine the subject of CAT forecasting in light of recent theoretical and laboratory work on the Lighthill-Ford theory of spontaneous adjustment.
Clear air turbulence is a phenomenon that would seem to be directly related in many instances to aircraft travel in (or near) regions of spontaneous imbalance. However, CAT forecast techniques have often been developed more from rule-of-thumb or statistical bases rather than from first principles of imbalance. The well-known Richardson number approach does relate directly to Kelvin-Helmholtz instability, but this is only one of several mechanisms that can lead to CAT. Among the shortcomings of conventional CAT forecasting approaches is that they are unable to account for CAT caused by gravity waves induced by unbalanced flow within the jet stream, which are likely to be closely related to spontaneous imbalance.
To connect theory with forecasting practice more firmly, we are developing CAT diagnostics based on principles inherent in the dynamics of adjustment to balance. Results from the NAM and RUC2 will be presented for one such diagnostic, the D-DVSI (Divergence-Deformation Vertical wind Shear Index), which incorporates the "divergence trend" into Ellrod and Knapp's Turbulence Index (TI). The utility of the D-DVSI, particularly in regions of anticyclonic flow, will be highlighted.
In addition, initial work on an innovative diagnostic based on the Lighthill-Ford theory of spontaneous adjustment to balance will also be examined. This work links experimental fluid mechanics work with rigorous dynamical theory to develop an operationally usable CAT diagnostic that is more directly linked to spontaneous imbalance processes than any other forecast method. Results using modified forms of this theory will be presented and discussed.