High-Resolution Simulation of Mean Convection and Its Intraseasonal Variability over the Tropics in the MRI/JMA 20-km Mesh AGCM

Abstract

Tropical mean convection and its organization on different spatiotemporal scales in a simulation using the Meteorological Research Institute/Japan Meteorological Agency (MRI/JMA) global atmospheric general circulation model (AGCM) at 20-km resolution (TL959L60) has been investigated. Comparison with two lower resolution simulations of 120 km (TL159L40) and 180 km (TL95L40) shows that convection, climatological fields, and moist stability over the Indian Ocean and South Pacific convergence zone are better represented in TL959L60 than in lower resolution simulations. However, the simulated three-dimensional structure of the Walker and Hadley cells, and the vertical structure of convective heating, do not show marked improvement relative to lower resolution simulations. The amplitude and phase speed of convectively coupled equatorial waves show that, although the Kelvin waves, the observed hierarchical structure of cloud clusters associated with the Madden?Julian oscillation (MJO) convection, and the wave interrelationship among MJO, Kelvin, and equatorial Rossby waves are marginally improved in TL959L60, increased resolution does not impact the simulation of the MJO itself. In the model, the MJO appears as a standing oscillation with much weaker variance compared to observations. Power spectra and a composite MJO life cycle constructed based on extended empirical orthogonal functions reveal that the amplitude, structure, and propagation characteristics of the MJO remain deficient at all resolutions. Thus, while the very high resolution yields improvements in some aspects of tropical mean convection, these appear to be less important compared to the basic deficiency of the parameterized convection in capturing tropical convection and its spatiotemporal variability.