Satellite Observations of Atmospheric Gravity Waves

A new methodology of gravity wave observations has been developed for the HIgh Resolution Dynamics Limb Sounder (HIRDLS). Individual vertical profiles of gravity-wave temperature perturbations that were determined by subtraction of a dynamic 31 day background field and a 1000 km along-track temperature filter were Fourier transformed to estimate the gravity-wave temperature amplitudes and vertical wavelengths (~2 – 16 km) in the stratosphere. Gravity wave activity is highly variable with season and can be highly orographically dependent, especially in the winter extratropics. Investigations of episodes of enhanced gravity waves over the southern Andes, the Cascade Range and the Rockies in the winter months of 2006 indicate that orographic gravity waves propagate downwind from the mountains. By way of contrast, observations of gravity waves around the Himalayas show a strong relationship with the cyclones in that region. HIRDLS observations over the southern Andes during July-September 2006 were compared to the orographic gravity-wave parameterization scheme in the UK Met Office Unified Model®. The results indicate that the observed waves are likely to be orographically excited. The observed wave activity extends large distances (a few thousand kilometres) downwind of the mountains and over the ocean. This downstream wave activity is not represented by the parameterization scheme similar to many schemes, which assume that the waves propagate vertically above the mountains only. Gravity waves over the tropics and tropical South America were compared with the AVHRR Outgoing Longwave Radiation (OLR), TRMM convective rainfall and ECMWF winds for convective sources. The comparisons show that the peak gravity wave temperature amplitudes correspond closely to the OLR ≤ 200 W/m ², in good agreement with the mesoscale cyclones and are above the updrifts, which indicate deep convective generation of the gravity waves. These waves show vertical propagation with higher-frequency and ~ 7.5 km vertical wavelengths in the lower stratosphere.