The effects of precipitation schemes and horizontal resolution on the major rainband in typhoon Flo (1990) predicted by the MRI mesoscale nonhydrostatic model

Abstract

¶This paper describes a numerical study of the major spiral rainband in typhoon Flo (1990) using the Meteorological Research Institute Mesoscale Nonhydrostatic Model (MRI-NHM). The effects of precipitation schemes and horizontal resolution on the representation of the simulated rainband are discussed. Dynamic and thermodynamic structures of the simulated major rainband to the north of the storm center are well represented in the model with a 5 km horizontal resolution. The structures are consistent with observational results reported for other tropical cyclones. Among the realistic features are: a cold pool and convergence on the inner side of the band; convergence above low-level inflow layers; and the outward slope of the updraft with height. The band is caused by the motion of the storm through its surroundings where horizontal wind has vertical shear. The simulation of the structure and precipitation pattern associated with the major rainband depends on the precipitation scheme rather than the horizontal resolution. The band appears more realistic when using explicit cloud microphysics as a precipitation scheme, rather than moist convective adjustment. This result is attributable to the difference in scheme triggering. In the simulation with moist convective adjustment, the elimination of vertical instability in low-level atmosphere is excessive, suppressing band formation. The overall structure of the band is also more realistic in the simulation using explicit cloud microphysics, because a cold pool exists in the lower layers and the vertical axis of upward flow tilts outward. This result suggests that prediction will partly depend on variables associated with cloud microphysics, such as the mixing ratio of cloud water. The horizontal grid distance, which varied between 5 and 20 km, quantitatively influenced the rainfall amount, although the large-scale band structure remained unchanged. The rainfall amount increased as the grid interval was reduced from 20 to 10-km but decreased as the interval was further reduced from 10 to 5 km.