Effect of Stratiform Heating on the Planetary-Scale Organization of Tropical Convection}

Wednesday, 20 April 2016: 8:30 AM
Miramar 1 & 2 (The Condado Hilton Plaza)
Boualem Khouider, University of Victoria, Victoria, BC, Canada; and Q. Deng, A. J. Majda, and A. M. Ravindran

It is widely recognized that stratiform heating contributes significantly to tropical rainfall and to the dynamics of tropical convective systems by inducing a front-to-rear tilt in the heating profile. Precipitating stratiform anvils that form from deep convection play a central role in the dynamics of tropical mesoscale convective systems; The wide spreading of stratiform-rain-evaporation downdrafts, originating from in the lower troposphere, strengthen the recirculation of subsiding air in the neighborhood of the convection center and trigger cold pools and gravity currents in the boundary layer leading to further lifting. Here, aquaplanet simulations with a warm pool like surface forcing, based on a coarse-resolution GCM , of $\sim$170 km grid mesh, coupled with a stochastic multicloud parameterization, are used to demonstrate the importance of stratiform heating for the organization of convection on planetary and intraseasonal scales. When the model parameters, controlling the heating fraction and decay time scale of the stratiform clouds, are set to produce higher stratiform heating, the model produces low-frequency and planetary-scale MJO-like wave disturbances while parameters associated with lower to moderate stratiform heating yield mainly synoptic-scale convectively coupled Kelvin-like waves. Furthermore, it is shown that when the effect of stratiform downdrafts is reduced in the model, the MJO-scale organization is weakened and a transition to synoptic-scale organization appears despite the use of larger stratiform heating parameters. Rooted from the stratiform instability, it is conjectured here that the strength and extent of stratiform downdrafts are key contributors to the scale selection of convective organizations perhaps with mechanisms that are in essence similar to those of mesoscale convective systems.
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