AGGREGATED CONVECTION AND THE REGULATION OF TROPICAL CLIMATE

Moist convection in the Earth’s atmosphere is mostly composed of relatively small convective clouds that are typically a few kilometers in horizontal dimension (Byers and Braham, 1948, Malkus, 1954). These often merge into bigger clusters of ~10 km in horizontal dimension, such as air-mass showers. More rarely, under special circumstances, moist convection is organized on even larger scales; this includes squall lines (e.g. Houze, 1977), mesoscale convective complexes, (e.g. Maddox, 1980), and tropical cyclones. Convection is a response of the atmosphere to destabilization by column processes such as surface enthalpy fluxes and radiative cooling, and also by large-scale circulation of atmosphere and associated enthalpy transport. On long space and time scales, convection is often viewed as being in quasi-equilibrium with the forcing, that is, convective clouds stabilize the atmosphere at exactly the rate it is destabilized by large-scale processes (Arakawa and Schubert, 1974, Emanuel et al., 1994). In the simplest form of such equilibrium, the so-called radiative-convective equilibrium (RCE), the effects of large-scale circulation on convection are ignored, and only radiation, convection, and surface enthalpy fluxes are allowed to transfer energy. It is understood that the RCE is an idealization of nature; however, despite its comparative simplicity, it is a reasonable framework for understanding convective equilibrium states. The RCE states were first simulated using one-dimensional models with parameterized convection (e.g. Rennó et al., 1994), and, consequently, the states that require spatial clusters of clouds have not been modeled. In the last two decades, advances in computer power have made it possible to use cloud-system-resolving models (CRMs) to simulate RCE in domains large enough to contain many convective clouds (e. g. Held et al., 1993, Islam et al., 1993, Robe and Emanuel, 1996, Bretherton and Khairoutdinov, 2004, Bretherton et al., 2005, Nolan et al., 2007). When relatively small three-dimensional domains with sizes in the order of a hundred kilometers are used, the simulated convective cloud fields appear nearly random in space and in time (Islam et al., 1993). When much bigger domains are employed and certain conditions are met, the convection tends to self-aggregate into a single clump (Bretherton and Khairoutdinov, 2004; Bretherton et al., 2005), which, if subjected to sufficient Coriolis force, can develop into a tropical cyclone (Nolan et al., 2007). Several conditions have been identified that seem to be necessary for self-aggregation to occur (Held et al. 1993; Bretherton et al., 2005; Nolan et al. 2007). Among them are small vertical shear of the horizontal wind, interaction of radiation with clouds and/or water vapor, and effect of convectively enhanced surface winds on surface fluxes. Although Nolan et al. (2007) demonstrated that reducing the SST could increase the time to spontaneous tropical cyclogenesis in RCE, there has been basically no systematic study of the dependence of self-aggregation on the sea-surface temperature (SST). In this extended abstract, we report on the results of the first such study, using both a simplified model and a CRM to simulate self-aggregation with different SST. A strong nonlinear dependence of self-aggregation on SST leads us to hypothesize that tropical convection can be in the state of a self-organized criticality (SOC; e.g. Bak et al., 1987), which is analogous to the critical point in phase transition. There have already been studies suggesting that observed tropical convection may indeed exhibit SOC-like behavior (Peters and Neelin, 2006; Muller et al., 2009). Here, we formulate an SOC hypothesis for tropical convection and present results supporting the SOC hypothesis in an idealized RCE setting.