Vertical Modes and Effective Stability of Quasi-2-Day Waves

Abstract

Abstract A quasi-2-day wave is known as a convectively coupled westward inertia–gravity (WIG) wave with a shallower equivalent depth (or slower phase speed) than the dry counterpart. This study investigates the relationship between the phase speed of quasi-2-day waves and effective static stability in terms of a vertical mode perspective. By using WIG filters with different equivalent depths, different phases of the 2-day wave are identified by filtering brightness temperature data obtained from geostationary satellites. The composite time series and the vertical modes in the tropical atmosphere are calculated from reanalysis data. The large-scale dynamical fields of the composite WIG waves are explained by the superposition of the first four baroclinic modes. Phase speed of the moist vertical mode is computed by applying the Radon transform to the mode transform coefficient. Different vertical modes share a common phase speed, which is slower than its dry counterpart, implying that the wave is not dispersive. To address the question of what slows the vertical modes, the effective static stability is evaluated by defining the degree of cancellation between diabatic heating and adiabatic cooling due to the ascent. This cancellation is confirmed to be almost complete for the first baroclinic mode as expected theoretically. The effective static stability is found to be higher for a higher vertical mode, but this change over different vertical modes is not as rapid as predicted from nondispersiveness. Possible reasons for this disagreement are discussed herein.