Assessment of tropical intraseasonal variability in versions 2 and 3 of the GFDL atmosphere model

The depiction of tropical intraseasonal variability will be compared between versions 2 and 3 of the Geophysical Fluid Dynamics Laboratory Atmosphere Model (AM). Relative to AM2, AM3 utilizes a new treatment of deep and shallow cumulus convection and mesoscale cloud effects. The cumulus parameterization used in AM3 is a mass flux-based scheme but also, unlike many other general circulation models including AM2, incorporates convective-scale vertical velocities that play a key role in cumulus microphysical processes. The AM3 convection scheme allows water vapor and condensate generated within deep cumulus plumes to be transported directly into adjacent mesoscale cloud systems, which can strongly impact larger-scale moisture and radiation fields. We will examine several multi-year simulations of AM2 and AM3 forced by prescribed sea-surface temperatures, including versions of AM3 in which model convective parameters are varied. A discussion of differences in the mean state and intraseasonal variability among model versions will be provided, and factors contributing to these differences will be reviewed. It is shown that notable improvements in AM3 tropical intraseasonal variability can be engendered by modifying convective parameters.