Evaluation of the HOMME dynamical core in the aqua-planet configuration of NCAR CAM4: Rainfall

The NCAR-Community Climate System Model version 4 (CCSM4) includes a new dynamical core option based on NCAR's High-Order Method Modeling Environment (HOMME). HOMME is a petascale capable high-order element-based conservative dynamical core developed on the cubed-sphere grid. Initial simulations have been completed in aqua-planet configuration of CAM4, the atmospheric component of CCSM4. We examined the results of this simulation and assessed its fidelity in simulating rainfall, which is one of the most important components of the Earth's climate system. For this we compared the results from two other dynamical cores of CAM4, the finite volume (FV) and Eulerian (EUL). Instantaneous features of rainfall in HOMME are similar to FV and EUL. Similar to EUL and FV, HOMME simulates a single peak Inter Tropical Convergence Zone (ITCZ) over the equator. The strength of the ITCZ is found to be almost same in HOMME and EUL but more than that in FV. It is observed that in HOMME and EUL there is higher surface evaporation, which supplies more amount of moisture into the deep tropics and gives more rainfall over the ITCZ. The altitude of maximum precipitation is found to be at almost the same level in all the three dynamical cores. The eastward propagation of rainfall bands is organized in FV and HOMME, and more prominent than in EUL. The phase speed of the eastward propagation in HOMME is found to be higher than that in FV. Our results show that, in general, the rainfall simulated by HOMME falls in a regime between that of FV and EUL. Hence, we conclude that the key aspects of rainfall simulation with HOMME falls in an acceptable range, as compared to the existing dynamical cores used in the model.