The Sensitivity of Idealized Hurricane Structure and Development to the Distribution of Vertical Levels in MM5

Abstract

In the course of studying the development of hurricanes using the fifth-generation Pennsylvania State University?National Center for Atmospheric Research Mesoscale Model (MM5), a relationship between storm intensity and the distribution of vertical levels became apparent, even when the same total number of sigma levels was used. A specific case of an idealized hurricane, on an f plane, in a quiescent environment, with constant and uniform SST of 28°C, was used to study the sensitivity of hurricane structure and evolution to the distribution of sigma levels. The distribution of vertical levels in the inflow, outflow, and middle layers of the atmosphere clearly affects the intensity, size, and structure of the storms, causing certain physical processes to be under- or overresolved. A well-resolved outflow layer is found to be necessary for proper storm intensification, while a well-resolved inflow layer does not necessarily correspond to an intense storm. In fact, when a well-resolved inflow layer is coupled with a poorly resolved outflow layer, a particularly weak storm evolves. When too few levels are assigned to the upper layer, the storm?s outflow is restricted, causing the eyewall column to become statically stable until surface fluxes can replenish low-level equivalent potential temperature content. Convection in the eyewall and compensating subsidence in the eye occur at a moderate rate and weak storms evolve. However, too few levels in the planetary boundary layer (PBL) can cause a storm to overintensify because of overestimated surface fluxes. When such a PBL is coupled with a poorly resolved outflow, the excessive surface fluxes can compensate for the stifled secondary circulation. Hence, this storm may develop to an expected intensity, but for the wrong reasons. Better guidelines for vertical-level distribution in numerical models, perhaps developed from observations of real-case hurricanes, are required.