Tropical Intraseasonal Variability in Version 3 of the GFDL Atmosphere Model
Abstract
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 clouds. The AM3 cumulus parameterization is a mass flux-based scheme but also, unlike that in AM2, incorporates subgrid-scale vertical velocities; these play a key role in cumulus microphysical processes. The AM3 convection scheme allows multi-phase water substance produced in deep cumuli to be transported directly into mesoscale clouds, which strongly influence large-scale moisture and radiation fields. We examine four AM3 simulations, using a control model and three versions with different modifications to the deep convection scheme. In the control AM3, using a convective closure based on CAPE relaxation, both the MJO and Kelvin waves are weak compared to those in observations. 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 propagation exist among the modified AM3 versions. Differences in vertical diabatic heating profiles 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. The more realistic heating structures are associated with an improved depiction of moisture convergence and intraseasonal convective organization in AM3.ag correlations of 850 hPa zonal wind with precipitation at (left column) 90°E and (right column) 150°E. Both fields are bandpass filtered (20-100 days) and averaged between 15°S-15°N. Solid (dashed) contours represent positive (negative) correlations that are shaded dark (light) gray if they exceed the 95% statistical significance level. We use ERAI and TRMM for the observed wind and rainfall fields. In the left panels, index reference longitudes and the 5 m/s phase speed are marked by vertical and slanted thick lines, respectively. Right panels also contain the 10 m/s phase speed line.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A21L..03B
- Keywords:
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- 3337 ATMOSPHERIC PROCESSES / Global climate models;
- 3365 ATMOSPHERIC PROCESSES / Subgrid-scale parameterization;
- 3371 ATMOSPHERIC PROCESSES / Tropical convection;
- 3373 ATMOSPHERIC PROCESSES / Tropical dynamics