Characteristics of gravity waves resolved by ECMWF

Abstract

Global model data from the European Centre for Medium-Range Weather Forecasts (ECMWF) are analyzed for resolved gravity waves (GWs). Based on fitted 3-D wave vectors of individual waves and using the ECMWF global scale background fields, backward ray tracing from 25 km altitude is performed. Different sources such as orography, convection and winter storms are identified. It is found that due to oblique propagation waves spread widely from narrow source regions. Gravity waves which originate from regions of strong convection are frequently excited around the tropopause and have in the ECMWF model low phase and group velocities as well as very long horizontal wavelengths compared to other models and to measurements. While the total amount of momentum flux for convective GWs changes little over season, GWs generated by storms and mountain waves show large day-to-day variability, which has a strong influence also on total hemispheric fluxes; from one day to the next the total hemispheric flux may increase by a factor of 3. Implications of these results for using the ECMWF model in predicting, analyzing and interpreting global GW distributions as well as implications for seamless climate prediction are discussed.