Mount
Hiope Bay/Narragansett Bay Model System
The MHB/NB is an integrated
system characterized by numerous islands, inlets, estuaries
and bays with direct links to point and non-point anthropogenic
stressors. A state-of-the-art coupled physical/ecosystem
model for this region requires (1) grid flexibility to
resolve complex coastline and bathymetry, (2) mass conservation
to accurately simulate water, heat, salt, and nutrient
transports, (3) proper parameterization of vertical mixing
to simulate tidal and wind mixing; (4) modular design
to facilitate selection of the essential model components
needed in process-oriented or management applications,
and (5) ability to use a wide variety of input data, especially
as real-time atmospheric and coastal ocean measurements
become more available. This model system should have a
flexible user interface and also be an “open”
community model, supported by an expanding base of users
that continue to use and improve it.
During the last five year,
we have developed an integrated NB/MHB model system (Fig.
1). The major components of this system include: (1) the
modified fifth-generation community mesoscale atmospheric
model (MM5) (Chen et al., 2005a), (2) the unstructured-grid
Finite-Volume Coastal Ocean circulation Model (FVCOM)
(Chen et al., 2003a; Chen et al., 2004, Zhao et al., 2006;
Chen et al., 2006), (3) a lower trophic-level food web
model with inclusion of DO and benthic flux.
MM5:
We presently use the NCAR/Penn State fifth-generation
mesoscale regional weather model (MM5) (Dudhia et
al., 2003) to compute the surface forcing fields
needed to drive FVCOM. MM5 features non-hydrostatic
dynamics, terrain-following sigma-coordinate, variable
domain and spatial resolution, multiple grid nesting,
4-D data assimilation, and several planetary boundary
layer (PBL) modules to represent turbulent mixing
over the ground and ocean (Grell et al., 1994),
and uses NCEP weather model fields as initial and
boundary conditions with two-way nesting capability.
We have modified MM5 to construct a surface weather
hindcast and forecast system for fishery studies
in the GoM/GB (Chen et al., 2005a). This model (called
GoM-MM5) is configured with a “regional”
domain (covering the Scotian Shelf/GoM/GB/New England
Shelf) and a “local” domain (New England
Shelf/ Mass Bay) with horizontal grid spacing of
9 and 3 km respectively, and 31 sigma levels in
the vertical with finer resolution in the PBL.
The NB/MHB-MM5 was set up as a subgrid of GoM-MM5,
with a horizontal resolution of 1 km covering both
bays and calibrated using observed winds recorded
on 6 NOAA buoys in this region (Fig. 2).
To improve the model-based surface
wind stress and heat flux estimates, GoM-MM5 uses
the COARE 3 bulk algorithm (Fairall et al, 1996,
2003) for the air-sea fluxes, insolation and cloud
cover data from the International Satellite Cloud
Climatology Project for the radiative fluxes, and
all coastal NDBC and C-MAN surface weather data
available in the local domain are incorporated through
4-D data assimilation. |
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NB/MHB-MM5 is presently in operational
use to forecast surface weather conditions over
the eastern NES region, providing a 3-day forecast
of surface winds, heat flux and precipitation/evaporation
for research of the impact of the Brayton Pt. Power
Plant on the MHB ecosystem. With assimilation of
in-situ surface wind and other data collected in
and around NB/MHB, this high-resolution “coastal”
MM5 accurately reproduces the sea breeze which dominates
the surface wind field variability in this region.
We are working on replacing MM5 with the new state-of-the-art
community mesoscale Weather Research Forecast model
(WRF) in 2006, which will allow horizontal resolution
down to 1 km. The new NCEP North American
weather Model (NAM) incorporates the WRF core model
physics and provides a horizontal resolution of
8-12 km. This advance combined with the use
of WRF in our GoM/NB/MHB system should improve the
accuracy of the surface fields used in this project. |
FVCOM:
FVCOM is a prognostic, unstructured-grid, finite-volume,
free-surface, 3-D primitive equation coastal ocean
circulation model coded for full parallelization
(Chen et al., 2003; Chen et al. 2004, Cowles, 2004).
In common with other coastal models, FVCOM uses
the modified Mellor and Yamada level 2.5 (MY-2.5)
and Smagorinsky turbulent closure schemes for vertical
and horizontal mixing, respectively (Mellor and
Yamada, 1982; Galperin et al., 1988; Smagorinsky,
1963), and a sigma coordinate to follow bottom topography.
The General Ocean Turbulent Model (GOTM) developed
by Burchard’s research group in Germany (Burchard
et al., 1999; Burchard, 2002) has been added to
FVCOM to provide optional vertical turbulent closure
schemes. Unlike existing coastal finite-difference
and finite-element models, FVCOM is solved numerically
by calculation of fluxes resulting from discretization
of the integral form of the governing equations
on an unstructured triangular grid. This approach
combines the best features of finite-element methods
(grid flexibility) and finite-difference methods
(numerical efficiency and code simplicity) and provides
a much better numerical representation of momentum,
mass, salt, heat, and tracer conservation. The ability
of FVCOM to accurately solve scalar conservation
equations in addition to the topological flexibility
provided by an unstructured grid and the simplicity
of the coding structure makes FVCOM ideally suited
for interdisciplinary applications in the integrated
NB/MHB system (Chen et al., 2005b-c; Ji et al.,
2005a-c). |
FVCOM can be run
in hind-, now-, or forecast mode using realistic
forcing. To improve the accuracy of these simulations,
in-situ data can be assimilated using Kalman filter
methods. In collaboration with P. Malanotte-Rizzoli
and her MIT colleagues, we have implemented an Ensemble
Kalman Filter (EnKF) (Zang and Malanottee-Rizzoli,
2003) into FVCOM. We plan to use the Ensemble Kalman
Filter for hind- and now-casting in the proposed
work. FVCOM can also be run in prognostic mode with
idealized forcing to investigate specific processes
and “what if” scenarios concerning changes
in tidal currents and circulation associated with
proposed dredging operations and changes in coastlines
and bathymetry due to large storms as well as the
study of river/estuary/bog systems where there is
significant groundwater. Examples can be viewed
on http://codfish.smast.umassd.edu
or http://fvcom.smast.umassd.edu.
MHB/NB FVCOM is configured with
an unstructured triangular mesh covering the entire
MHB/NB region (Fig. 3). The computational domain
is bounded in the inner shelf with an open boundary
line running from Rhode Island Sound to Buzzard
Bay. Our initial experiments show that in order
to resolve current separation due to strong tidal
flows in MHB, a model must have a minimum resolution
of ~50 m or less (Zhao et al., 2006). To resolve
the thermal plume due to the hotwater injection
from the power plant, a model must have a minimum
resolution of 10 m or less. Our present MHB/NB model
grid has a resolution of 10-20 m in upper NB and
GB to fully resolve small- and meso-scale eddies
in PR and GB. 31-sigma levels are used in the vertical,
which corresponds to a vertical resolution of less
than 1 m in the deepest parts. FVCOM also includes
a mass-conservative wet/dry point treatment method
that can simulate the flooding/drying process over
intertidal wetlands. |
FVCOM
Biological Module: Various
ecosystem models have been implemented in FVCOM,
including NPZ, NPZD, NPZDB, and water quality models.
To make FVCOM more flexible for use in ecosystem
studies, we have built a unstructured grid generalized
FORTRAN 95 biological module (GBM) into FVCOM to
allow users to select either a pre-built biological
model (such as NPZ, NPZD, etc) or construct their
own biological model using the pre-defined pool
of biological variables and parameterization functions.
GBM includes seven groups: (1) nutrients, (2) autotrophy,
(3) heterotrophy, (4) detritus, (5) DOM, (6) bacteria,
and (7) auxiliary. A biological model can be constructed
using “function pointers” to select
both model structures and parameterization functions.
GBM can be run simultaneously together with FVCOM
with parallelization (“online” mode)
or driven separately by FVCOM output (“offline”
mode). This module acts like a platform that
allows us to examine the relative importance of
different physical and biological processes under
well-calibrated physical fields. Validation tests
are presently being conducted to use GBM to re-build
NPZ (Franks and Chen, 1996, 2001) and multi-species
NPZD models (Ji et al., 2005a-c) for use in GB/GoM
applications (see Tian and Chen, 2005 for details).
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One of this application
GBM was to use it to build an ecosystem model for
the study of relative influences of natural (river
discharges) and anthropogenic (hot-water plume)
on the occurrence of the low-DO zone (or hypoxia)
in the MHB/NB system. Hypoxia is the ecosystem result
from imbalance between dissolved oxygen replenishment
and demand. Excessive nutrient loading fuels phytoplankton
growth and DO produced through phytoplankton photosynthesis
in the euphotic layer is partly lost to the atmosphere
due to air-sea exchange and utilized by heterotrophic
processes among the food web. The DO concentration
in shallow areas is also influenced by the subsequent
remineralization of the synthesized organic matter
in the deeper layer and in the sediment. An
ecosystem model suitable for the study of hypoxia
should include the key physical (air-sea fluxes,
advection, and mixing) and biological/biogeochemical
processes occurring in NB/MHB. There are two
existing biological models for NB. The first one
is the food web model developed by Kremer and Nixon
(1978) and the other is the water quality model
(WQM) implemented in FVCOM for MHB studies (Chen
et al., 2004). The KN model focuses on the food
web dynamics without explicit inclusion of DO, so
that it cannot be applied directly to study hypoxia.
The FVCOM WQM was built based on the EPA water-quality
analysis simulation program (WASP) by including
the benthic flux from sediment resuspension via
sedimentation processes (Amborse et al., 1993, Zheng
et al., 2004). This is a typical eutrophication
model consisting of eight water-quality state variables:
1) ammonia (NH3); 2) nitrate and nitrite (NO2 and
NO3); 3) inorganic phosphorus (OPO4); 4) organic
nitrogen (ON); 5) organic phosphorus (OP); 6) phytoplankton
(PHYT); 7) carbonaceous biochemical oxygen demand
(CBOD); and 8) dissolved oxygen (DO). The benthic
and sediment suspension processes are incorporated
into the water model by adding a benthic layer and
a sediment pool at the seabed. This biological/chemical
model incorporates basic transformation processes
including photosynthesis, uptake, respiration, nitrification,
denitrification, benthic flux, sediment suspension,
and external loads, etc. This WQM, which is similar
to the water quality model developed by Spaulding
et al. (1999) for NB/MHB, however, does not contain
heterotrophic and bacterial processes, so that it
can not capture realistic food web dynamics in NB/MHB.
We have used the FVCOM GBM to construct
an ecosystem model for the study of hypoxia and
phytoplankton dynamics in NB/MHB (Fig. 4).
This model consists of 3 nutrients [NH4+, NO3- and
Si(OH)4], 2 phytoplankton (small and large sizes),
2 zooplankton (mirco and meso sizes), bacteria,
detritus, DOM and DO. NB is a nitrogen-limited system
in which growth and seasonal succession of diatoms
are regulated by silicate (Kremer and Nixon, 1978;
Oviatt et al., 1995; Townsend and Thomas, 2001,
2002). NH4+ is a key index of the nutrient supply
through the internal regeneration process. Dividing
the phytoplankton into 2 species helps us distinguish
diatoms (silicate-dependent) and flagellates (non-silicate-dependent)
that show distinct functions in the energy flux
among the food web. Microzooplankton feed mostly
on small phytoplankton, whereas meso-zooplankton
mainly consumes diatoms. They have different efficiencies
in energy flow from the pelagic to the benthic system:
fecal pellets of microzooplankton remain suspended
in the water column, while those of meso-zooplankton
sink toward the bottom. Detritus consist of dead
phytoplankton and zooplankton, fecal pellets or
aggregate, which sinks in part into the sediment
and taken directly by bacteria. Carnivores (such
as ctenophores, fish larvae and menhaden) are key
predators regulating zooplankton abundance in NB
(Kremer and Nixon, 1978). Since our focus is on
nutrient-sustained phytoplankton production and
remineralization that leads to hypoxia, we combine
predators into a single term that simulates predation
pressure on zooplankton. Unlike WQM, we introduce
DOM to represent all dissolved organic matter that
is remineralized mainly by bacteria. Based
on a 1-D model experiment, Kremer and Nixon (1978)
suggested that benthos (shellfish) respiration and
sediment oxygen demand are two potential candidates
that are related to rapid consumption of DO in NB.
These two processes are taken in account in the
proposed biological model. DO constitutes
the major component of the model in which DO is
replenished and produced mainly by re-aeration at
the air-sea interface and phytoplankton photosynthesis
and consumed by heterotrophic respiration (micro-
and meso-zooplankton, predators, benthic shellfish
and bacteria). The model also includes the DO demand
due to denitrification and sediment decomposition
(Seitzinger et al., 1984; Rudnick and Doering, 1985).
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