Scale Interactions in the Tropics from a Simple Multi-Cloud Model

Our lack of a complete understanding of organized tropical convection, especially of the interaction between the moisture convection and equatorial waves remains an impediment in the numerical simulation and forecasting of large-scale organization, such as the Madden-Julian Oscillation (MJO).The aim of this project is to understand interactions across spatial scales in the tropics in the context of a simplified framework for scale interactions (the IMMD) while a using a simplified framework to describe the basic features of moist convection.Using multiple scales asymptotics, Biello and Majda ^[1] derived a multi-scale model of moist tropical dynamics (IMMD^[1]), which separates three regimes: the planetary scale climatology, the synoptic scale waves, and the planetary scale anomalies regime. The scales and strength of the observed MJO would categorize it in the regime of planetary scale anomalies - which themselves are forced from non-linear upscale fluxes (like Reynolds stresses) from the synoptic scales waves.In order to close this model and determine whether it provides a self-consistent theory of the MJO. A model for diabatic heating due to moist convection must be implemented along with the IMMD.The multi-cloud parameterization is a model proposed by Khouider and Majda ^[2] to describe the three basic cloud types (congestus, deep and stratiform) that are most responsible for tropical diabatic heating. We implement a simplified version of the multi-cloud model that is based on results derived from large eddy simulations of convection ^[3].We present this simplified multi-cloud model and show the sensitivity of the results to of the model parameters. We show results of numerical experiments beginning with a variety of convective forcing states. Preliminary results on upscale fluxes, from synoptic scales to planetary scale anomalies, will be presented.

[1] Biello J A, Majda A J. Intraseasonal multi-scale moist dynamics of the tropical atmosphere[J]. Communications in Mathematical Sciences, 2010, 8(2): 519-540.

[2] Khouider B, Majda A J. A simple multicloud parameterization for convectively coupled tropical waves. Part I: Linear analysis[J]. Journal of the atmospheric sciences, 2006, 63(4): 1308-1323.

[3] Dorrestijn J, Crommelin D T, Biello J A, et al. A data-driven multi-cloud model for stochastic parametrization of deep convection[J]. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 2013, 371(1991): 20120374.