Organization of tropical convection in a GCM with varying vertical resolution: Implications for the simulation of the MJO

Abstract

Experiments using a GCM with 2 different vertical resolutions show differences in the amount of variability of the upper tropospheric zonal wind component associated with the Madden-Julian Oscillation (MJO). The GCM with lower vertical resolution shows very little variability in this quantity whereas, when the resolution is doubled in the free troposphere, the GCM produces variability which is of the same strength as observations. However, the propagation of an enhanced convective region from the Indian Ocean to the west Pacific is not well represented in either simulation of this atmosphere-only GCM.

An aqua-planet version of the same GCM is used to study the behaviour of tropical convection when the vertical resolution is doubled. The spectrum of tropical clouds is found to change from a bimodal distribution dominated by shallow trade-wind cumulus and deep cumulonimbus clouds to a more trimodal distribution with the appearance of cumulus congestus clouds with tops around the freezing level. Associated with the periods when these congestus clouds are the dominant cloud type are periods when convection acts as a moisture source to the mid troposphere. The reasons for the development of these clouds in the high vertical resolution version of the GCM is investigated and the implications for the simulation of the MJO will be discussed.