William M. Frank
Penn State University, University Park, PA
Recent research has revealed that even moderate changes in the magnitude or vertical shear of the mean winds can alter the storm in ways that could potentially affect the structure and intensity of the cyclone, as well as its path (e.g. Shapiro, 1992; Jones, 1995; DeMaria, 1996; Bender, 1997). The current paper examines the basic question of how the mean zonal winds and the vertical shear of these winds affect storm structure. The major emphasis is upon the manner in which the zonal winds and boundary layer processes modulate the vertical velocity field within the storm. It is hypothesized that these processes dynamically force regions of ascent and descent within the cyclone, and that they thereby organize and control the amount and distribution of latent heat release. This process should have significant effects upon the structure and intensity of the system.
This paper presents results of a series of numerical simulations designed to study the effects of imposed external circulation features upon the structure of tropical-cyclone-like vortices. It examines the first-order effects of mean zonal flow, including vertical shear, and of surface friction upon the structure of an initially balanced baroclinic vortex that resembles a tropical depression.
The simulations described below describe the behavior of an initial tropical-depression-like vortex in different environmental flows. The initial vortex resembles a strong tropical depression that is only slightly below the intensity threshold of 17 m/s winds used to classify the system as a tropical storm in the Atlantic basin. It is based on data from a composite analysis of a pre-storm tropical depression by McBride and Zehr (1981). It is a warm-core system with maximum winds of 15 m/s at a radius of 135 km, and there is a weak upper-level anticyclone.
The simulations are performed with MM5, the nonhydrostatic version of the Penn State / NCAR Mesoscale Model. This model incorporates multiple nested grids and includes both explicit moist physical processes and a variety of cumulus parameterization and boundary layer schemes. It solves the nonlinear, primitive equations using Cartesian coordinates in the horizontal and a terrain-following sigma coordinate in the vertical. The model has been adapted for use with idealized initial conditions to permit careful control of initial value experiments.
All simulations are performed on an f-plane valid at 15 N. There are 20 vertical levels with 7 levels below 850 hPa. The domain is square and is 5,400 km on each side. The coarse mesh is 45 km, and there is a nested fine mesh region that is 1,800 km on a side with 15 km resolution. Each mesh consists of 121 by 121 points. The high-resolution boundary layer scheme is used. The lower boundary is an ocean surface at a constant temperature of 27 C. For the moist simulations, a cumulus parameterization is used (Betts and Miller, 1986). The explicit moisture scheme is also activated for the moist runs to deal with any grid-scale condensate.
Dry simulations with shear produced a wave number one asymmetry in the vertical motion field through most of the troposphere, with the maximum uplifting in the quadrant downshear right of the storm center, with matching descent in the upshear-left quadrant. The mechanism responsible was the isentropic flow along tilted isentropes described by Jones (1995). This pattern was stronger and deeper than that forced by frictional convergence in the boundary layer (e.g. - Shapiro, 1983), but both were significant.
In the moist simulations, the initial tropical depression quickly begins to intensify. Most runs achieve hurricane intensity within 36h, and they usually reach an approximately steady state between 36-48h. In all of the moist cases, there is a pronounced asymmetry in the distribution of convection and associated variables. The convection is consistently strongest on the left side of the storm looking downshear. There is some movement of the region of maximum convection from time to time in each run, but the overall pattern is persistent suggesting that it is being controlled by dynamical forcing processes rather than random chance. However, the dry runs suggested forced deep uplifting in the right front, rather than left front, quadrant.
The mechanisms responsible for the observed distribution are under investigation and will be discussed at the conference. Part of the difference results from the development of expicit rain after the first 24h. The parameterized convection occurs in a local vertical column where it is forced, while the rainfall from the exlicit moisture scheme develops from air that is advecting cyclonically around the storm as it rises. Since maximum vertical velocities are never more than a few m/s, the rising saturated air is advected downstream with increasing height. Both types of rainfall development occur in real hurricanes, but the results here do indicate the sensitivity of such simulations to the type of cumulus parameterization used. Future simulations will explore this topic with much finer resolution and fully explicit resolution of the convection rather than the current hybrid approach with both parameterized and explicit representation of moist processes.
Dry simulations of idealized tropical cyclones indicate that there are dynamical mechanisms that favor persistent asymmetrical patterns of vertical velocity. These patterns suggest that the effects of environmental parameters such as vertical shear and deep layer mean flow could affect the organization of latent heat release and ultimately the storm intensity and structure. Simulations performed with a moist version of the model indicate that this broad hypothesis is correct, but the relationships between the convection and the large-scale flow are different from those of the dry runs. The nature of these differences is being explored, and simulations using finer mesh grids and explicit microphysics are being performed to isolate the complex processes occurring in the full-physics version of the model.
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