Cloud Radiative Feedbacks in GCMs : A Challenge for the Simulation of Tropical Climate Variability and Sensitivity

The simulation of cloud radiative effects by general circulation models (GCMs) has long been recognized as a challenging issue for climate modeling. Here, we discuss two areas where biases in the simulation of cloud-radiation interactions is likely to be critical. First we show that cloud-radiative feedbacks associated with deep convective clouds may play a key role in the difficulties experienced by GCMs in simulating the intraseasonal variability of the equatorial atmosphere. Second, we show that in climate change simulations, the radiative response of marine boundary-layer clouds to global warming dominates, currently, the uncertainty in tropical cloud feedbacks. This calls for a better representation of clouds in large-scale models, and in particular for a better coupling of cloud parameterizations to convective, turbulent and radiative processes. 1 Cloud radiative feedbacks and tropical intraseasonal variability A recent study shows that current state-of-the art coupled ocean-atmosphere GCMs still have significant problems and display a wide range of skill in simulating the tropical intraseasonal variability (Lin et al. 2006). In particular, there appears to be a lack of highly coherent eastward propagation of the Madden-Julian Oscillation (MJO) in many models. In addition, the phase speeds of convectively coupled equatorial waves are generally too fast, whish suggests that these models may not have a large enough reduction in their effective static stability by diabatic heating (Lin et al. 2006). Radiative processes contribute to the diabatic heating of the atmosphere, and observational studies have revealed large variations of the tropospheric radiative cooling in regions of a strong intraseasonal climate variability, such as the Indian and the western Pacific oceans (e. g. Mehta and Smith 1997, Johnson and Ciesielski 2000, Lin and Mapes 2004). These variations are primarily related to the presence of deep convective clouds and to their interaction with longwave radiation. Given the difficulties of GCMs in simulating clouds and the radiative effects of clouds, the question arises whether the simulation of cloud radiative processes and feedbacks may explain part of the problems revealed by Lin et al. (2006). To answer this question, one has first to understand the role that cloud radiative processes play in the natural variability of the tropical atmosphere, and in the intraseasonal variability in particular. We have investigated the influence of feedbacks between between moisture (including clouds), radiation and convection on the large-scale organization of the equatorial atmosphere by using two models of different complexity. First, we used the simple two-layer linear model of the tropical atmosphere proposed by Emanuel (1987) and improved by Yano and Emanuel (1991) and Emanuel (1993), in which we have added a representation of radiative processes (Bony and Emanuel 2005). Then, we used an aquaplanet general circulation model (Zurovac-Jevtic et al. 2006) including parameterizations of clouds and convection that have been carefully evaluated against TOGA-COARE data (Emanuel and Zivkovic-Rothmann 1999, Bony and Emanuel 2001). ECMWF Workshop on Parametrization of clouds in large-scale models, 13-15 November 2006 9 S. BONY: TROPICAL CLOUD-CLIMATE FEEDBACKS 1 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 wavenumber k α (r ad −m o is tu re f ee d b ac k) GROWTH RATE [ day ] 0 0 0 0.0125 0.0125 0.025 0.025 0 . 02 5 0.05 0.05 0 . 05 0.1 0.1 0.1