Puerto Rico Wave Climate Atlas: Technical Aspects
For the present study we have carried out a 40-year high-resolution
wave climate simulation based on the Simulating Waves Nearshore
(SWAN) spectral wave model, which is described in detail in (Booij
et al., 1999) and (The SWAN Team, 2012). Details of the setup and
validation of the CARICOOS Nearshore Wave Model, a similar model
developed by CARICOOS Co-PI Miguel Canals, may be found in Anselmi
et al. (2012), Canals et al. (2012) and Canals and García (2019).
SWAN is a state of the art spectral wave model which takes into
account the effects of wave shoaling, diffraction and refraction,
nonlinear wave-wave interactions (triads and quadruplets), and
dissipation due to wave breaking and bottom friction. A brief
description of the model setup and surface and boundary forcing data
sources is provided below.
Model mesh and setup
A schematic of the model setup is shown in Figure 1. We use a hybrid
approach combining a coarse structured SWAN grid at 1 kilometer
resolution with an unstructured mesh around Puerto Rico and most of
the USVI. The blue area shows the structured SWAN grid at
1-kilometer resolution. This coarse structured grid obtains boundary
conditions from the regional NOAA NCEP Multigrid WaveWatch III Model
(Chawla et al., 2013) in the form of 2D wave spectra at the
locations shown as white stars. The yellow area shows the footprint
of the unstructured SWAN mesh, which is nested within the structured
SWAN model and which obtains 2D spectral boundary conditions from
the structured SWAN grid at the locations indicated by white
circles. The unstructured mesh was developed using the Oceanmesh 2D
MATLAB package (Roberts et al., 2019). The mesh resolution varies
from 500 meters in deep water to 20 meters very close to the
shoreline. The mesh bathymetry is obtained from the NOAA
Continuously Updated, 1/9th arc-second Digital Elevation Model that
was developed using 2016 and 2018 LIDAR data and single and
multibeam bathymetry data. Figure 2 shows the mesh bathymetry (top)
and spatial resolution (bottom).
The SWAN model resolves wave transformation in spectral space by
solving the spectral energy balance equation which describes the
evolution of the 2D wave spectrum (frequency and direction) at each
computational node. The directional resolution is 5 degrees and the
frequency spectrum is divided into 36 bins with frequencies ranging
from 0.04 Hz to 0.667 Hz, allowing the model to resolve waves with
wave periods between 1.5 and 25 seconds. The SWAN model is run in
third generation mode, which includes model physics such as
quadruplet interactions, exponential wind wave growth and
white-capping (The SWAN Team, 2012). Model physics also include wave
energy dissipation due to bottom friction using the JONSWAP
formulation, triad wave interactions, diffraction and depth-induced
wave breaking (The SWAN Team, 2012).
spectral boundary conditions
To capture far field wave conditions, spectral boundary conditions
in the form of two-dimensional wave spectra are applied at the
structured SWAN model boundaries using output from the NCEP NOAA
NCEP Multigrid WaveWatch III Model Chawla et al. (2013). For the
period between 1979-2009 spectral boundary conditions were obtained
from the NCEP WW3 Phase 2 waves hindcast . This 30 year wave
climatology hindcast was performed using winds from the NCEP Climate
Forecast System Reanalysis and Reforecast (CF10SRR) homogeneous
dataset of hourly high-resolution winds
(http://polar.ncep.noaa.gov/waves/hindcasts/nopp-phase2.php). For
simulations beyond 2009 to present, spectral boundary conditions
were obtained from NCEP’s wave model production hindcast
(http://polar.ncep.noaa.gov/waves/hindcasts/prod-multi_1.php), which
uses wind input from NOAA’s operational Global Forecast System (GFS)
weather model. This spectral forcing has been previously shown to
provide very good results for nearshore SWAN modeling in Puerto Rico
and the US Virgin Islands (e.g. Anselmi et al., 2012; Canals and
García, 2019).
surface boundary conditions
Surface wind forcing was provided to the model using output from the
NCEP Climate Forecast System Reanalysis and Reforecast (CFSRR)
dataset of hourly high-resolution winds at an approximate resolution
of 38 km. It should be noted that the wave model hindcasts described
above use global scale weather models with spatial resolution on the
order of 25 km. Such weather models describe with good accuracy the
wind patterns at regional and basin scales, but do not accurately
describe the small scale structure of intense tropical cyclones. To
take into account the effects of hurricanes and tropical storms into
the hindcast, the 40-year period was partitioned into periods with
and without named storm events in the vicinity of the model domain.
Periods with named storms were identified using the National
Hurricane Center’s HURDAT2 best track hurricane database
(http://www.aoml.noaa.gov/hrd/data_sub/re_anal.html). For these
events and time periods, the wind forcing for the simulation was
obtained using the Generalized Asymmetric Holland Vortex Model,
constructed using the hurricane parameters from the HURDAT2
database. This wind model has been used extensively for hurricane
storm surge and wave simulations. For Hurricanes Irma and María,
HWRF model output produced by NOAA/NHC/NCEP was used as surface
boundary conditions.
The model was run from January 1, 1979 to December 31, 2018 and
separated into month-long runs in hot-start mode. After each
month-long simulation, the next simulation is restarted using a 2D
spectral hot-start file produced at the end of the previous
simulation. The figure below shows an example of the output from the
hybrid SWAN simulations, in this case for March 6, 2018 (Winter
Storm Riley). Colors represent significant wave height in feet and
the arrows represent wave direction (wave direction shown for coarse
structured grid only).
The model has been validated with al available CARICOOS buoy data,
see the
validation section
for more details.