Observations and simulations of gravity waves in the lower stratosphere

In the lower stratosphere, tropical gravity waves play an important role in the Quasi Biennial Oscillation (QBO). MLS (Microwave Limb Sounder) and GPS occultation can both measure gravity waves at these altitudes, but they have different sensitivities to horizontal and vertical wavelengths. MLS measures waves of short (~200 km) horizontal and long (>7 km) vertical wavelengths, whereas GPS occultation is more sensitive to waves of long (>200 km) horizontal and short (>1 km) vertical wavelengths. By applying an appropriate band-pass filter to the GPS/CHAMP data, we find the gravity wave morphology at low latitudes agrees well between the recent GPS and Aura MLS observations. Both instruments observe significant wave activity enhancements in the lower stratosphere with continuous excitation across all tropical longitudes. This agreement suggests that the tropical excitation is associated with a broad wave spectrum. The new observations also reveal an interesting detail, showing double peaks in the latitudinal variation of wave activity associated with long (5-10 km) vertical wavelengths in both MLS and GPS data. Possible QBO influences, as well as effects due to the instrument observational filters, will be investigated with long-term satellite data (e.g. UARS MLS, AMSU-A). Moreover, we will use the NRL NOGAPS-ALPHA model to study subtropical jet instability, one likely mechanism exciting waves of long vertical wavelengths, and compare these wave results to those generated by deep convection as resolved in a high-resolution cloud model.