Journal of Climate, 26, 426-449.

Tropical intraseasonal variability in version 3 of the GFDL atmosphere model.

James J. Benedict and Eric D. Maloney
Department of Atmospheric Sciences, Colorado State University, Fort Collins, CO.

Adam H. Sobel
Department of Applied Physics and Applied Mathematics, Department of Earth and Environmental Sciences, and Lamont-Doherty Earth Observatory, Columbia University, New York, NY.

Dargan M. W. Frierson
Department of Atmospheric Sciences, University of Washington, Seattle, WA.

Leo J. Donner
Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, NJ.


Tropical intraseasonal variability is examined in version 3 of the Geophysical Fluid Dynamics Laboratory Atmosphere Model (AM3). Compared to its predecessor AM2, AM3 uses a new treatment of deep and shallow cumulus convection and mesoscale cloud effects. The AM3 cumulus parameterization is a mass flux-based scheme but also, unlike the AM2, incorporates subgrid-scale vertical velocities that play a key role in cumulus microphysical processes. The AM3 convection scheme allows multiphase water substance produced in deep cumuli to be transported directly into mesoscale clouds, which strongly impact large-scale moisture and radiation fields.

We examine four AM3 simulations that consist of a control model and three versions with different modifications to the deep convective scheme. The control AM3 using convective closure based on CAPE relaxation lacks sufficient MJO and Kelvin waves. By modifying the convective closure and trigger assumptions to inhibit deep cumuli, AM3 produces reasonable intraseasonal variability but a degraded mean state. MJO-like disturbances in the modified AM3 propagate eastward at roughly the observed speed in the Indian Ocean but up to twice the observed speed in the West Pacific. Distinct differences in intraseasonal convective organization and signal coherence exist among the modified AM3 versions. Differences in the vertical profiles of diabatic heating associated with the MJO are also found. The two AM3 versions with the strongest intraseasonal signals have a more prominent "bottom-heavy" heating profile leading the disturbance center and "top-heavy" heating profile following the disturbance center. The more realistic heating structures are associated with an improved depiction of moisture convergence and intraseasonal convective organization in the AM3.