Modeling the hurricane dynamics
with explicit moist convection
Principal Investigator:
K. V.
Ooyama
Objective:
Comprehensive understanding of hurricane dynamics with a numerical model
that explicitly includes the cloud-scale moist convection interacting
with the hurricane environment.
Rationale:
Moist convection is an exquisite yet powerful participant in creation of
weather systems including tropical cyclones. While the cyclone's own
circulation creates the mechanism for organizing and supporting the
convection in the eye wall and rain bands, the large-scale environment
also exerts controlling influences on the convection, especially in the
initial stage of a cyclone and, often, at critical moments of
intensification. In order to understand the multiscale dynamics of
cyclones in a realistic environment, a numerical model with explicit
moist convection is to be constructed on the fundamental principles of
dynamics and thermodynamics.
Method:
The model utilizes a versatile and accurate spectral method based on a
cubic-spline representation of spatial fields (in all directions). In
order to allow a wide range of scale interactions, the model is
configured on multiply-nested domains of outwardly decreasing
resolution, with noise-free, two-way interfaces. The semi-implicit
method provides an efficient time integration for the nested spectral
model.
With this numerical method, the model competently predicts the mass,
momentum and energy in direct application of conservation principles.
The formulation of moist process follows classical thermodynamics
(avoiding the so-called meteorological approximations), with an
extension for parameterized microphysics. The dynamic equations
correctly account for the momentum of precipitation.
The atmospheric boundary layer is also parameterized in the model,
following a traditional line of approach. On the other hand, the model
does not postulate any internal diffusion or dissipation, since it
generates no computational noise that needs to be suppressed by such
means. If physically required, however, those terms can be added to the
model.
Accomplishment:
A model has been constructed to test the theory and numerical
procedures, in the vertical two-dimensional space. The model can run in
either of two modes, slab-symmetric or axisymmetric. The tests
performed in the former mode are: the growth of a single-cell cloud, the
generation of a multicell squall line in a sheared environment, and the
diurnal cycle of convection over a heated peninsula. In the latter
mode, experiments on the growth of an axisymmetric hurricane are being
conducted at the Dept. of Atmospheric Science, Colorado State
University, in collaboration with HRD.
The results favorably compare with those of similar studies in the
literature. Our results, however, give a clear insight into the
resolution-dependent interplay between dynamics and precipitation. A
few highlights from the test runs are presented below.
The nondispersive and noise-free propagation of waves over the nested
domains is demonstrated in Fig.1 for the Lamb
wave (left
column) and the first internal mode of gravity wave (right column).
The mature stage of a single cloud with a spreading canopy is shown in
Fig.2 at 31 and 33 min after its initiation
as a warm bubble. The
growth of two mushroom-shaped protrusions at the top and mamma-like hanging
protrusions near the edge of the canopy are associated with intense
eddies shown by wind arrows. An enlarged view of the upper right portion
of the cloud is in Fig.3.
In a sheared environment, the same initial cloud spawns many daughter
cells, and a forward-pushing wedge of cold air is eventually (in 3 or 4
hours) formed under the raining clouds, resulting in a self-perpetuating
squall line. The well-organized structure of this multicell system is
shown in Fig.4, together with the profiles of
u-velocity and entropy at a few key points.
The theoretical foundation, as well as the numerical method, is
extendable to three spatial dimensions, and the construction of a 3-D
model is contemplated. The current 2-D model code can be made available
to any interested party for further applications and development.
Key references:
DeMaria, M., S. D. Aberson, K. V. Ooyama and S. J. Lord,
1992: A nested spectral model for hurricane track forecasting. Mon.
Wea. Rev., 120, 1628-1643.
Ooyama, K. V., 1990: A thermodynamic foundation for
modeling the moist atmosphere. J. Atmos. Sci.., 47,
2580-2593.
Ooyama, K. V., 2000: A dynamic and thermodynamic foundation
for modeling the moist atmosphere with parameterized microphysics.
[submitted to J. Atmos. Sci.]
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Last modified: 01/27/00