including a significant wave contribution, are extracted for each storm. The EST
analysis is applied to calculate water levels for various return periods.
Wind and Atmospheric Pressure Field Model
The PBL numerical model was used for simulation of typhoon-generated
wind and atmospheric pressure fields. The model applies vertically averaged
primitive equations of motion for predicting tropical storm wind velocities. The
model includes parameterization of momentum, heat, and moisture fluxes
together with surface drag and roughness formulations. Through hindcast
applications, Cardone, Greenwood, and Greenwood (1992) found that the PBL
model calculates accurate surface wind speeds and directions as compared to
measurements collected in tropical storms over open water.
The PBL model requires a set of storm parameter snapshots for input. The
snapshots consist of meteorological storm parameters that define the storm at
various stages in its development or at particular times during its life. These
parameters include: latitude and longitude of the storm's eye; track direction
and forward speed measured at the eye; radius to maximum winds; central and
peripheral atmospheric pressures; and an estimate of the geostrophic wind speed
and direction. Also, the direction and speed of steering currents can be provided
for representing asymmetric storms.
Storm tracks and maximum sustained 1-min mean surface winds were
obtained from the JTWC database described in Chapter 2. Information contained
in this database is provided at 0000-, 0600-, 1200-, and 1800-hr Greenwich
Mean Time (GMT). The JTWC storm files were preprocessed to put them into
the required snapshot format and to estimate other necessary parameters. Central
pressure was calculated from maximum sustained 1-min mean surface wind
speed using the relationship developed by Atkinson and Holliday (1977), based
on data from Guam
W = 6.7( Pa - Pc )0.644
(1)
where W is the maximum sustained 1-min mean surface wind speed, Pa is the
ambient pressure, and Pc is the central pressure. Ambient pressure is taken to be
1010 mb, in accordance with Atkinson and Holliday's (1977) recommendation
for the western North Pacific area.
Radius to maximum winds (RMW) is approximated by application of rela-
tionships developed in a generalized numerical model study of storm character-
istics (Jelesnianski and Taylor 1973). The RMW is based on W and the central
pressure deficit, Pa - Pc . Track directions and forward speeds required by the
PBL model are approximated by cubic spline interpolation at hourly intervals
from 6-hr coordinate positions provided in the database. Geostrophic wind
speeds were specified as 6 m/s.
11
Chapter 3
Modeling Approach
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