An unbalanced flow/gravity wave trigger mechanism for clear air turbulence

McCann (2001, Natl. Wea. Digest) outlined an ingredients-based clear air turbulence (CAT) forecast technique and presented a "recipe" for combining the ingredients: Gravity waves locally modify the environmental Richardson number which can trigger CAT. The production of gravity wave enhanced turbulent kinetic energy (TKE) may be estimated but requires knowledge of the wave amplitudes and phase velocities which, for the most part, are unknown. The kinematic relationship between divergence and vertical velocity shows that the time rate of divergence change causes vertical accelerations. Thus the divergence tendency may be used as a proxy for the occurrence of gravity waves.

Terms from the full divergence tendency equation were diagnosed from one-hour RUC2 model forecasts to estimate the non-dimensional wave amplitude needed to estimate the gravity wave enhanced TKE production. The estimated TKE production was verified for its skill on a database of 7584 turbulence pilot reports. The results suggest that only a negative sum of the inertial terms and a positive sum of the forcing terms have diagnostic value. These results are combined into a single diagnostic algorithm (relevant divergence tendency terms ® gravity wave occurrence/amplitude/phase velocity ® TKE production) to provide CAT forecast guidance.

For moderate turbulence the guidance is slightly better than traditional forecast techniques. The bias indicates significant underforecasting which indicates that the divergence equation terms are not the only cause of gravity waves. For severe turbulence the guidance shows comparable skill to moderate turbulence and is somewhat better than traditional techniques.