Dependence of Hadley circulation on the representation of tropical deep convection

Abstract. Parameterizations of subgrid-scale processes are an important source of bias in climate simulations and uncertainty in climate projections. Both the bias and the uncertainty can be reduced by resolving the most energetic small-scale processes based on the first principles via increasing model resolution, as oppose to relying on empirical or ad hoc (though physically motivated) parametrizations. We made a first step in this direction using a suite of time-slice and fixed-season experiments, specifically designed to deal with the heavy computational costs encountered when using a global 5 km model for climate and climate change simulations. We concentrate on the effect of the representation of deep convection for the Hadley Circulation (HC) and its response to warming.

We find that the time-mean state of HC is sensitive to the representation of deep convection: The HC is significantly weaker and the deep convection is significantly reduced in the case when deep convection is resolved instead of parametrized. Resolving convection leads to less heavy rain and less rain in the ascending branch of the HC – consistent with a weaker HC – but more total precipitation when taking also light rain into account. As a response to a 4 K global warming, the HC becomes broader, deeper, and weaker, no matter whether convection is resolved or parameterized. There is an indication that the weakening of HC is further intensified when deep convection is resolved. As a response to the warming, heavy rain and the rain associated with the HC ascending branch become stronger regardless of convection representation, while the total precipitation is more strongly enhanced when convection is resolved.