Abstract

Spontaneous gravity wave radiation from nearly balanced rotational flows is investigated in shallow water system on a rotating sphere. Gravity waves are very important for their role on global material circulation in the middle atmosphere. It is suggested that gravity waves are radiated from strong rotational flows, such as polar night jet, sub-tropical jet, and typhoon in recent observational studies. This radiation process is called as a spontaneous gravity wave radiation, since gravity waves are spontaneously radiated from initial balanced rotational flows. Although there are several numerical studies, this process has not been fully understood. In the present study, we use the most simplified system of shallow water that includes both gravity waves and rotational flows. In addition, the spectral-like three point combined compact difference (sp-CCD) scheme is used, which has high accuracy as well as the spherical harmonics model. This model allows us to estimate gravity wave amplitude with high accuracy. We investigate dependencies of positions of the jet on spontaneous gravity wave radiation. To discuss on the conditions of gravity wave radiation and propagation, we use the analogy with the theory of the aero-acoustic sound wave radiation (Lighthill theory). We also show results of two layer shallow water system in which small internal gravity waves are spontaneously radiated from baloclinic instability. Finally, we will mention recent theoretical study of spontaneous gravity wave radiation from simple vortex pairs.