Characteristics of Tropical Convective Gravity Waves Resolved by ERA5 Reanalysis

Abstract

Abstract The ERA5 reanalysis with hourly time steps and ∼30 km horizontal resolution resolves a substantially larger fraction of the gravity wave spectrum than its predecessors. Based on a representation of the two-sided zonal wavenumber–frequency spectrum, we show evidence of gravity wave signatures in a suite of atmospheric fields. Cross-spectrum analysis reveals (i) a substantial upward flux of geopotential for both eastward- and westward-propagating waves, (ii) an upward flux of westerly momentum in eastward-propagating waves and easterly momentum in westward-propagating waves, and (iii) anticyclonic rotation of the wind vector with time—all characteristics of vertically propagating gravity and inertio-gravity waves. Two-sided meridional wavenumber–frequency spectra, which are computed along individual meridians and then zonally averaged, exhibit characteristics similar to the spectra computed on latitude circles, indicating that these waves propagate in all directions. The three-dimensional structure of these waves is also documented in composites of the temperature field relative to grid-resolved, wave-induced downwelling events at individual reference grid points along the equator. It is shown that the waves radiate outward and upward relative to the respective reference grid points, and their amplitude decreases rapidly with time. Within the broad continuum of gravity wave phase speeds there are preferred values around ±49 and ±23 m s−1, the former associated with the first baroclinic mode in which the vertical velocity perturbations are of the same sign throughout the depth of the troposphere, and the latter with the second mode in which they are of opposing polarity in the lower and upper troposphere.