Development of a bulk mass flux convection scheme for climate model and its impact on simulation of the Madden-Julian oscillation

In this study, a bulk mass flux convection scheme is developed for climate model. The development of new convection scheme aims to resolve the common problems of current convection schemes and climate models. They are i) lack of sensitivity of cloud top and strength of convective activity on environmental humidity, and ii) poor simulations of the Madden-Julian oscillation.

New features of developed convection scheme compared to old one (simplified Relaxed Arakawa-Schubert scheme; Numaguti, 1995) include, 1) triggering mechanism based on parcel vertical velocity and liquid water, 2) environmental moisture dependence of entrainment rate, 3) detrainment rate formulation using critical fraction, 4) cloud microphysics adopted from McRAS scheme (Yogesh and Sud, 1999). New convection scheme replaces old convection (simplified relaxed Arakawa-Schubert scheme) and large-scale condensation.

Developed convection scheme is verified in single column model (SCM) mode with the aid of intensive observation from TOGA-COARE (Tropical Ocean Global Atmospheres/Coupled Ocean Atmosphere Response Experiment), and cloud resolving model simulations. SCM results with idealized and realistic configurations show that new convection scheme is able to represent moisture sensitivity more realistically compared to old scheme.

The scheme is implemented into Seoul National University atmospheric general circulation model (AGCM). New scheme also improves dependency of convection on environmental moisture when implemented into AGCM, as in SCM. Further, AGCM with new convection scheme produces more coherent and organized eastward propagating intraseasonal variability over the Tropics compared to that in AGCM with old scheme. The results support several theoretical works with simple models in which the dependency of convection on environmental moisture is emphasized in MJO dynamics.