A Representation of Cumulus-scale Effects in a Mesoscale-Convection-Resolving Model for Tropical Cyclones

[摘要] A new mesoscale-convection-resolving model (MCRM) for tropical cyclones (TCs) is proposed along a line of Yamasaki’s (1986) model in which mesoscale organized convection (MC) is resolved by the grid, but cumulus-scale convection (CC) is treated as the subgrid-scale. The most significant difference from Yamasaki’s model is that Kuo’s (1965) formulation is used to represent the CC-effects. Some important modifications from Kuo are made: (1) it is assumed that part of moisture converged in an air column is used for CC and that the ratio depends on low-level convergence and the degree of latent instability; (2) the ratio of moisture consumed for heating to moisture redistributed by CC is assumed to be significantly larger than that given by Kuo; (3) turbulent and dynamic entrainment and detrainment are taken into account so that the vertical profile of heating, which is crucial to MC, may be effectively controlled; (4) cloud water and rainwater of the subgrid-scale are treated with prognostic equations. Numerical experiments are performed with an axisymmetric model having a horizontal grid size of 10 km. The results are discussed compared with those from a 1 km grid model that can resolve CC. It is shown that the new model can simulate important features of MC and a TC, provided that the values of model parameters are specified properly. The CC-effects are made clear by comparison with a case that does not include them. It is shown that CC has two different effects, depending on the low-level relative humidity. One is to give realistic growth of MC through the upward transports of heat and moisture under a very humid condition, and the other is to enhance the formation and growth of MC when the air is not very humid. This study suggests that though Kuo’s parameterization was not proposed as that representing the CC-effects, a reasonable model can be developed when Kuo’s formulation is adopted in the framework of the MCRM. Important aspects of the model that contribute to realistic simulation of MC and a TC are discussed in comparison with those of Yamasaki’s model.

[学科分类] 大气科学