The Impact of Cumulus Suppression on the Baiu Front Simulated by an AGCM

Abstract

This paper discusses the effect of cumulus suppression (CS), applied in the Arakawa-Schubert (AS) cumulus parameterization, on the Baiu front simulated with an atmospheric general circulation model (AGCM; T106L56: horizontal resolution ~1.1°, 56 vertical layers). As an additional condition of the AS cumulus parameterization, CS permitted cumulus convection only when the environmental relative humidity averaged in the modeled cumulus cloud of the AS scheme exceeded 80%. To evaluate the effect of CS, the results of simulations with CS (CS run) are compared with those without CS (NOCS run). Detailed analysis was carried out for June, because the Baiu front in the real atmosphere is usually sustained quasi-steadily during June. Whereas the distribution and amount of 5-year June mean Baiu precipitation in the CS run was generally similar to that in the NOCS run, the standard deviation of daily precipitation of 5-year June in the CS run was significantly larger than that in the NOCS runs. The CS condition increased heavy rainfall and temporal variation of Baiu precipitation. The CS run also suppressed precipitation in the southern area of the Baiu front, as seen in observations. Examination of the Baiu front during intense-rainfall periods in the model showed that characteristics of the Baiu front and frontal disturbances were reasonably simulated, in some years, by both the CS and NOCS runs, to a certain extent. However, the CS condition improved the occurrence of intense rainfall events at the Baiu front, and the reproduction of the Baiu frontal disturbances. Both of these features became closer to actual observed features.