A sensitivity study of the Kelvin wave and Madden-Julian Oscillation in aquaplanet simulations by the Naval Research Laboratory spectral element atmospheric model

Abstract

The dynamical core of the Naval Research Laboratory (NRL) Spectral Element Atmospheric Model (NSEAM) is coupled with full physics and used to investigate the organization and propagation of equatorial atmospheric waves under the aquaplanet conditions. The sensitivity of the model simulation to the amount of horizontal viscosity, distribution of the vertical levels, and selected details of the precipitation physics is examined and discussed mainly utilizing simulated convective precipitation with the aid of time-longitude plots and the spectral diagrams designed by Wheeler and Kiladis (1999). It is shown that the simulation of the Kelvin wave and Madden-Julian Oscillation depends strongly on the details of the vertical level distribution and the choice of parameters in the convective parameterization. Efforts are made to calibrate the new model to capture the essential interaction between the dynamics and physics of the atmosphere. The speed and spectrum of the eastward propagating Kelvin waves and the signature of the Madden-Julian Oscillation simulated by the new model reveal main features similar to those predicted by the simplified theory and found in limited observations. This study attempts to understand the significant variability found among the aquaplanet simulations by various global atmospheric models and highlights the uncertainties concerning convective processes and their coupling to large-scale wave motion in large-scale models of the atmosphere.