Idealized numerical simulations are carried out to examine the role of shallow convection on tropical-cyclone intensification. The calculations use the Pennsylvania State University/National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5), which is a nonhydrostatic, three-dimensional model that includes both parameterized and explicitly resolved moist processes. Shallow convection is modelled using the scheme based on the method proposed by Arakawa and Schubert (1974), but modified by Grell (1994). We examine here the evolution of an initially axisymmetric vortex in a quiescent environment on an f-plane in cases with and without a representation of shallow convection. The inclusion of shallow convection delays the onset of rapid intensification by about 6 h and the vortex intensity is reduced by about 10-15 m s1 shortly after the period of rapid intensification and by about 5-10 m s1 during the later mature stage. The principal effects of shallow convection are to warm and dry the boundary layer, and to moisten and cool the lower troposphere, thereby diminishing the boundary layer equivalent potential temperature (?e) and increasing that aloft. A consequence is to reduce the instability to deep convection. With shallow convection included, the maximum value of ?e at the lowest sigma level is about 2-4 K less than in the calculation without during the period of rapid intensification and about 1-2 K less during the mature stage. Outside the central core region, the relative humidity is about 1-2 % less, and inside the core region the area of saturation air is smaller. Shallow convection also reduces the amount of grid-scale precipitation, especially during the period of rapid intensification. Some calculations have been carried out with shallow convection included, but with the vertical diffusion coefficients reduced to test the effects of vertical diffusion in comparison with shallow convection. These calculations indicate little sensitivity to the vertical diffusion for the range of valus examined.

References

Arakawa, A., and W. H. Schubert, 1974: Interaction of a cumulus cloud ensemble with the large scale environment. Part I. J. Atmos. Sci., 31, 674-701.

Grell, G. A., J. Dudhia, and D. R. Stauffer, 1994: A description of the fifth-generation Penn State/NCAR Mesoscale Model (MM5). NCAR Tech. Note NCAR/TN-398 + STR, 138 pp.