Effects of multicumulus convective ensemble on East Asian summer monsoon rainfall simulation

Simulating the Asian summer monsoon is one of the most challenging tasks in climate modeling. In particular, monsoon simulations strongly depend on the cumulus parameterization scheme being used, as attested by many modeling studies. In this paper, the performances of a "unified multicumulus convective ensemble scheme'' ( or unified scheme), Kuo, simplified Arakawa-Schubert, Zhang and McFarlane convective parameterization schemes and moist convective adjustment in simulating the East Asian summer monsoon in a coupled General Circulation Model ( GCM) are assessed. In the unified scheme, the latter four individual parameterizations are run in parallel without any modification. Simple average of outputs from each parameterization is then incorporated to the atmospheric model at every time step. Simulations based on these convection schemes during the boreal summer season are compared with observations, focusing on the formation and seasonal evolution of the Meiyu-Changma-Baiu rainband. It is found that the East Asian summer monsoon evolution in the GCM is sensitive to the convective parameterization being used. While the Kuo scheme is able to capture a reasonably strong subtropical front, its mean onset time is about two to three pentads earlier than the observed. Simulations based on the simplified Arakawa-Schubert scheme give a more realistic onset date. However, the model subtropical rainband tends to be confined over land near the coastal region of East Asia. The unified scheme is able to reproduce a prominent East Asian subtropical rainband. Moreover, the seasonal evolution of the rainband is also well captured in the unified scheme simulations. Further diagnostics are carried out to elucidate the role of various circulation elements in the Meiyu-Changma-Baiu front formation. In the western part of the front over land regions, there is robust precursor signal in incoming shortwave radiation before monsoon onset in observations as well as model simulations. Comparison between performances of different convective parameterizations suggests that sensitivity to low-level vorticity can be crucial for the northward migration of the subtropical front and its extension into the open ocean east of southern Japan.