S.P. Alexander and T. Tsuda
Abstract Gravity (buoyancy) waves are mainly generated in the lower atmosphere and propagate upwards, transporting energy and momentum. They must be characterized and parameterized in models because their cumulative effect on the atmosphere is important. GPS Radio Occultation (RO) satellites are able to measure the temporal and spatial evolution of gravity waves with global coverage. COSMIC enables their study at a much higher temporal and spatial resolution than was previously possible, while CHAMP provides a multi-year dataset. New CHAMP and COSMIC gravity wave results are presented here and discussed in both a global and regional scale context.
The study of atmospheric waves using Global Positioning System (GPS) Radio Occultation (RO) has expanded markedly since the seminal paper of Tsuda et al., (2000), where data from the GPS/MET satellite were used to construct seasonal maps of stratospheric gravity wave potential energy (Ep). These showed that large Ep occurred above deep convection, especially over the Indonesian Maritime Continent region.
The launch in 2000 of the CHAllenging Minisatellite Payload (CHAMP) mission and its continuing operation has allowed the construction of multi-year gravity wave global and regional climatologies (Ratnam et al. 2004; Randel and Wu 2005; de la Torre et al. 2006; Baumgaertner and McDonald 2007; Hei et al. 2008). Results from the tropical regions have been favorably compared with ground-based instrumentation (Tsuda et al. 2004, 2006) allowing an extension of local scale results to a regional scale understanding of wave energy. CHAMP and SAC-C (Satelite de Aplicaciones Científicas-C) data have also been used to study the generation of mountain waves by the Andes, and subsequent propagation upwards (de la Torre et al. 2004,
Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Kyoto, Japan e-mail: [email protected]
de la Torre and Alexander 2005). Stratospheric gravity wave activity measured by GPS-RO above the Andes has shown clear correlations with ionospheric fluctuations (Hocke et al. 2002), indicating the important role that these waves play in coupling the entire atmospheric system. Results obtained from GPS-RO have been compared with model data. For example, large stratospheric Ep noticed by Tsuda et al. (2000) above the Bay of Guinea (10°S) away from any convective or topographic wave source were investigated using high-resolution GCM data (Kawatani et al. 2003), where it was found to be due to waves generated by a region of moist heating near 10°N crossing the equator and converging near 10°S.
Gravity wave energy is the sum of kinetic energy Ek and potential energy Ep terms, with the former only calculable with knowledge of the wind velocities. However, the temperature and wind velocities are coupled to each other via the wave polarization equations. When the intrinsic frequency is significantly greater than the inertial frequency and much less than the squared Brunt-Vaisala frequency N2, linear gravity wave theory predicts that the ratio of Ek to Ep is constant and therefore we can consider the total energy of the atmospheric system by studying temperature perturbations alone (Tsuda et al. 2000). It should be noted that the distribution of Ek and Ep are sometimes different in the atmosphere (Sato et al. 1999; Liou et al. 2003; Liou et al. 2006) and so Ep does not always constitute half of the total atmospheric energy.
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