Sensitivities of the Madden–Julian oscillation forecasts to configurations of physics in the ECMWF global model

Abstract

Abstract. The sensitivities of the Madden–Julian oscillation (MJO) forecasts to various different configurations of the\nparameterized physics are examined with the global model of ECMWF s Integrated\nForecasting System (IFS). The motivation for the study was to simulate\nthe MJO as a nonlinear free wave under active interactions with\nhigher-latitude Rossby waves. To emulate free dynamics in the IFS, various\nmomentum-dissipation terms (“friction”) as well as diabatic heating were\nselectively turned off over the tropics for the range of the latitudes from\n20 ∘ \u2009S to 20 ∘ \u2009N. The reduction of friction sometimes improves the MJO forecasts,\nalthough without any systematic tendency. Contrary to the original motivation,\nemulating free dynamics with an operational forecast model turned out to be\nrather difficult, because forecast performance sensitively depends on the\nspecific type of friction turned off. The result suggests the need for\ntheoretical investigations that much more closely follow the actual\nformulations of model physics: a naive approach with a dichotomy of with or\nwithout friction simply fails to elucidate the rich behaviour of complex\noperational models. The paper further exposes the importance of physical\nprocesses other than convection for simulating the MJO in global forecast\nmodels.