A consecutive development of MJO events in the 2018-2019 winter season reproduced by a three-month SST-forced experiment with NICAM

Simulation of the Madden-Julian Oscillation (MJO) has been notoriously difficult in atmospheric models. This is partly due to the fact that the reproducibility of the MJO is highly sensitive to parameters that are difficult to fix from observation or theory, and require empirical tuning based on model behaviors. Moreover, model settings fine-tuned for MJO simulations are not necessarily compatible with longer simulations due to biases in the long-term mean.

To address this problem, we first conducted 46-days parameter sweep experiments on a convection-permitting model, NICAM (Nonhydrostatic ICosahedral Atmospheric Model) at 14 km horizontal resolution. Combinations of different microphysics and surface flux parameters were tested with the target of improving the reproducibility of an MJO event in December 2018. Sensitivities of the simulated mean atmospheric states to the tested parameter settings were also examined. From the tested parameter settings, we selected three sets of parameters which were able to reproduce the eastward movement of the targeted MJO event without severe bias in the mean precipitation pattern for a longer three-months simulations to investigate the potential of NICAM to consecutively reproduce a second MJO event within an integration period.

The three-months simulations were integrated for 90 days from November 1, 2018, which includes three MJO events. The simulations were conducted at 14 km and 7 km resolutions. No-air sea interaction was included in the simulations, and sea surface temperatures were given from the observation. We show that in some of the three-months simulations, NICAM was able to reproduce not only the first MJO event that initiated in early November 2018 but also the second MJO event during the integration period which initiated in early December 2018. This implies that there is potential for convection-permitting models to reproduce MJO events not only as an solution to an initial value problem but also as an internal variability of the atmosphere reproduced by the model.