Convectively Coupled Equatorial Waves: Analysis of Clouds and Temperature in the Wavenumber–Frequency Domain

Abstract

A wavenumber-frequency spectrum analysis is performed for all longitudes in the domain 15ЊS-15ЊN using a long (ϳ18 years) twice-daily record of satellite-observed outgoing longwave radiation (OLR), a good proxy for deep tropical convection. The broad nature of the spectrum is red in both zonal wavenumber and frequency. By removing an estimated background spectrum, numerous statistically significant spectral peaks are isolated. Some of the peaks correspond quite well to the dispersion relations of the equatorially trapped wave modes of shallow water theory with implied equivalent depths in the range of 12-50 m. Cross-spectrum analysis with the satellite-based microwave sounding unit deep-layer temperature data shows that these spectral peaks in the OLR are ''coupled'' with this dynamical field. The equivalent depths of the convectively coupled waves are shallower than those typical of equatorial waves uncoupled with convection. Such a small equivalent depth is thought to be a result of the interaction between convection and the dynamics. The convectively coupled equatorial waves identified correspond to the Kelvin, n ϭ 1 equatorial Rossby, mixed Rossby-gravity, n ϭ 0 eastward inertiogravity, n ϭ 1 westward inertio-gravity (WIG), and n ϭ 2 WIG waves. Additionally, the Madden-Julian oscillation and tropical depression-type disturbances are present in the OLR spectra. These latter two features are unlike the convectively coupled equatorial waves due to their location away from the equatorial wave dispersion curves in the wavenumber-frequency domain. Extraction of the different convectively coupled disturbances in the time-longitude domain is performed by filtering the OLR dataset for very specific zonal wavenumbers and frequencies. The geographical distribution of the variance of these filtered data gives further evidence that some of the spectral peaks correspond to particular equatorial wave modes. The results have implications for the cumulus parameterization problem, for the excitation of equatorial waves in the lower stratosphere, and for extended-range forecasting in the Tropics.

The diurnal cycle of precipitation over the Maritime Continent in a high‐resolution atmospheric model

“…Such 'convective coupling' has been observed between convection and largescale equatorial waves in the tropics (Wheeler and Kiladis, 1999) and may be operating on the smaller scales here. Latent heat release from condensation and precipitation in the ascending part of the wave reduces the effective static stability, slowing the wave speed.…”

Analysis of vertically propagating convectively coupled equatorial waves using observations and a non‐hydrostatic Boussinesq model on the equatorial beta‐plane

“…Liebmann and Hendon, 1990;Wheeler and Kiladis, 1999;Roundy and Frank, 2004;Yang et al, 2007aYang et al, , 2007bYang et al, , 2007c, substantially different values of N and l also yield reasonable dispersion solutions. Comparison of this dispersion relation with the shallow-water model of Matsuno (1966) reveals that the shallow-water model equivalent depth…”

“…This dispersion is a consequence of relaxation of the hydrostatic approximation, and it is relevant only at a high zonal wave number. Wheeler and Kiladis (1999) and many others have demonstrated that observed convectively coupled Kelvin waves exhibit some dispersion especially between wave numbers 5 and 10, but Eq. (24) demonstrates that relaxing the hydrostatic approximation cannot explain this observed behaviour.…”

Modelling convective processes during the suppressed phase of a Madden–Julian oscillation: Comparing single‐column models with cloud‐resolving models

“…During the active phase of the MJO, strong winds along with high humidity throughout the troposphere support the development of deep convective cloud systems that can persist for several days. Capturing the differences in these convective processes between the active and suppressed phases of the MJO-in particular, their vertical heating profiles-is key in order for GCMs to simulate tropical variability, in which the MJO plays a major role alongside other convectively coupled equatorial waves (Wheeler and Kiladis, 1999).…”