An Analytic Vortex Initialization Technique for Idealized Tropical Cyclone Studies in AGCMs

Abstract

Abstract The paper discusses the design of idealized tropical cyclone experiments in atmospheric general circulation models (AGCMs). The evolution of an initially weak, warm-core vortex is investigated over a 10-day period with varying initial conditions that include variations of the maximum wind speed and radius of maximum wind. The initialization of the vortex is built upon prescribed 3D moisture, pressure, temperature, and velocity fields that are embedded into tropical environmental conditions. The initial fields are in exact hydrostatic and gradient-wind balance in an axisymmetric form. The formulation is then generalized to provide analytic initial conditions for an approximately balanced vortex in AGCMs with height-based vertical coordinates. An extension for global models with pressure-based vertical coordinates is presented. The analytic initialization technique can easily be implemented on any AGCM computational grid. The characteristics of the idealized tropical cyclone experiments are illustrated in high-resolution model simulations with the Community Atmosphere Model version 3.1 (CAM 3.1) developed at the National Center for Atmospheric Research. The finite-volume dynamical core in CAM 3.1 with 26 vertical levels is used, and utilizes an aquaplanet configuration with constant sea surface temperatures of 29°C. The impact of varying initial conditions and horizontal resolutions on the evolution of the tropical cyclone–like vortex is investigated. Identical physical parameterizations with a constant parameter set are used at all horizontal resolutions. The sensitivity studies reveal that the initial wind speed and radius of maximum wind need to lie above a threshold to support the intensification of the analytic initial vortex at horizontal grid spacings of 0.5° and 0.25° (or 55 and 28 km in the equatorial regions). The thresholds lie between 15 and 20 m s−1 with a radius of maximum wind of about 200–250 km. In addition, a convergence study with the grid spacings 1.0°, 0.5°, 0.25°, and 0.125° (or 111, 55, 28, and 14 km) shows that the cyclone gets more intense and compact with increasing horizontal resolution. The 0.5°, 0.25°, and 0.125° simulations exhibit many tropical cyclone–like characteristics such as a warm-core, low-level wind maxima, a slanted eyewall-like vertical structure and a relatively calm eye. The 0.125° simulation even starts to resolve spiral rainbands and reaches maximum wind speeds of about 72–83 m s−1 at low levels. These wind speeds are equivalent to a category-5 tropical cyclone on the Saffir–Simpson hurricane scale. It is suggested that the vortex initialization technique can be used as an idealized tool to study the impact of varying resolutions, physical parameterizations, and numerical schemes on the simulation and representation of tropical cyclone–like vortices in global atmospheric models.