The spatial area covered by a tropical storm at a given time is specified in
the PBL model to correspond to a set of nodes on a numerical grid. Wind
velocities and atmospheric pressure values are computed at each node in the
grid. Whereas some models employ a fixed grid system to simulate a tropical
storm (i.e., stationary grid with a moving storm), the PBL model simulates a
typhoon as a stationary storm with a moving grid. Forward motion of the storm
is calculated as the vector sum of the forward and rotational velocity vector
components. The numerical grid is moved with the storm at the calculated
forward velocity at each time-step so that the grid center always coincides with
the storm center.
The distribution of wind speed and radial change in wind speed varies
spatially within a tropical storm such that higher spatial resolution of the wind
field is required in the central region of the storm, whereas coarser resolution
suffices on the outer areas. To provide spatially-graded resolution of the wind
field, a nested gridding technique is applied consisting of five layers or subgrids.
The grid nesting is applied such that all subgrids contain the same number of
nodes. However, the spatial coverage and resolution differs and is successively
graded. Each subgrid is composed of 21 by 21 nodes in the x- and y-directions,
respectively. The centers of all subgrids lie on node (11,11), defined at the eye of
the tropical storm. For this study, the subgrid with the finest resolution had an
incremental distance of 5 km (3.1 miles) between nodes and covered an area of
10,000 sq km (3,861 square miles). Incremental distances for the remaining
subgrids were 10, 20, 40, and 80 km (6.2, 12.4, 24.9, and 49.7 miles) and their
areas of coverage were 40,000, 160,000, 640,000, and 2,560,000 sq km (15,444,
61,776, 247,104, and 988,428 square miles), respectively.
For each snapshot, the equations of motion are first solved for the subgrid
covering the greatest area. Computed wind velocities are then applied as
boundary conditions on the second-largest grid, and the equations are solved
again. This procedure is followed for the remaining grids where wind fields are
computed on successively smaller grids. Thus, the nested grid technique pro-
vides wind field information over a wide spatial area while sufficient grid
resolution is provided to accurately compute winds in the vicinity of the tropical
storm eye.
After all snapshots have been processed, hourly wind and atmospheric pres-
sure fields are interpolated using a nonlinear blending algorithm which produces
a smooth transition from one snapshot to the next. Hourly wind and pressure
fields are then interpolated from the PBL grid onto the hydrodynamic or wave
model grid and subsequently stored for use by those models. Wind velocities
produced by the PBL model represent an averaging time of 30-60 min, which is
appropriate for wave and storm surge modeling (Thompson and Cardone 1996).
Storm Surge Model
The ADvanced CIRCulation (ADCIRC) numerical model was applied for
simulation of the long-wave hydrodynamic processes in the study area. The
model calculates a two-dimensional, depth-integrated finite-element solution of
12
Chapter 3
Modeling Approach
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