Atmospheric turbulence of moderate or greater intensity is commonly encountered by commercial aviation, and is the cause of most weather-related aircraft incidents. In the case of clear-air turbulence (CAT), there are no visual clues of this hazard and pilots are reliant on operational forecasts. One of the main causes of CAT is the breaking of mountain waves at cruising altitudes, which may be responsible for up to 30% of all turbulence encounters over land. Traditionally mountain waves have been sub-grid-scale in global forecast models, but recent developments mean that some NWP models (e.g. the UK Met Office model) are now able to resolve mountain wave activity explicitly, potentially allowing forecasts of mountain wave induced turbulence with greater accuracy and confidence than previously possible.
In this presentation, the prospect of diagnosing CAT from the explicitly resolved mountain wave activity in global NWP forecasts is investigated. Rare research aircraft observations of mountain wave absorption at a wave-induced critical level and associated turbulence are presented from the Antarctic Peninsula. The wave response and turbulence is shown to be generally well represented by regional configurations of the Met Office Unified Model when run at high resolution (1.5 km grid spacing). This result is corroborated in further model validation provided by commercial aircraft observations over Greenland. Whilst the success of the high resolution model in diagnosing this turbulence is reassuring, in the global model the fine-scale wave overturning regions are less well resolved, even with the recent improvements. Using the model’s native turbulence parameterization to diagnose CAT is therefore likely to underestimate the turbulent kinetic energy. Accordingly, the ability of modified TKE diagnostics to identify regions which are close to being unstable is investigated. Case study analysis suggests that this diagnostic can be effective at identifying mountain wave induced turbulence over Greenland.
*email: andy.elvidge@metoffice.gov.uk
*Preference: Oral