HYCOM
415
2.
The ocean model
HYCOM is designed to provide a significant improvement over the
existing global operational ocean products, since it overcomes design
limitations of present systems as well as limitations in vertical resolution.
The ultimate goal is a more streamlined system with improved performance
and an extended range of applicability (e.g., the present U.S. NAVY systems
are seriously limited in shallow water and in handling the transition from
deep to shallow water). The generalized coordinate (hybrid) ocean model
HYCOM retains many of the characteristics of its predecessor, the isopycnic
coordinate model MICOM (Miami Isopycnic Coordinate Model), while
allowing coordinate surfaces to locally deviate from isopycnals wherever the
latter may fold, outcrop, or generally provide inadequate vertical resolution
in portions of the model domain. The freedom to adjust the vertical spacing
of the coordinate surfaces in HYCOM simplifies the numerical
implementation of several physical processes (mixed layer detrainment,
convective adjustment, sea ice modeling, …) without robbing the model of
the basic and numerically efficient resolution of the vertical that is
characteristic of isopycnic models throughout most of the ocean's volume.
The implementation of the generalized coordinate in HYCOM follows
the theoretical foundation set forth in Bleck and Boudra (1981) and Bleck
and Benjamin (1993): i.e., each coordinate surface is assigned a reference
isopycnal. The model continually checks whether or not grid points lie on
their reference isopycnals and, if not, attempts to move them vertically
toward the reference position. However, the grid points are not allowed to
migrate when this would lead to excessive crowding of coordinate surfaces.
Thus, vertical grid points can be geometrically constrained to remain at a
fixed depth while being allowed to join and follow their reference isopycnals
in adjacent areas (Bleck, 2002). The default configuration in HYCOM is one
that is isopycnal in the open stratified ocean, but smoothly reverts to a
terrain-following (V) coordinate in shallow coastal regions and to fixed
pressure-level coordinates (hereafter referred to as p) in the surface mixed
layer and/or unstratified seas (Figure 1). In doing so, the model combines the
advantages of the different types of coordinates in optimally simulating
coastal and open-ocean circulation features. It is left to the user to define the
coordinate separation constraints that control regional transitions among the
three coordinate choices. Figure 1 illustrates the transition that occurs
between p/V and isopycnic (U) coordinates in the fall and spring in the upper
400 meters and over the shelf in the East China and Yellow Seas. In the fall,
the water column is stratified and can be represented with isopycnals; in the
spring, the water column is homogenized over the shelf and is represented
by a mixture of p and V coordinates. A particular advantage of U coordinates
415
2.
The ocean model
HYCOM is designed to provide a significant improvement over the
existing global operational ocean products, since it overcomes design
limitations of present systems as well as limitations in vertical resolution.
The ultimate goal is a more streamlined system with improved performance
and an extended range of applicability (e.g., the present U.S. NAVY systems
are seriously limited in shallow water and in handling the transition from
deep to shallow water). The generalized coordinate (hybrid) ocean model
HYCOM retains many of the characteristics of its predecessor, the isopycnic
coordinate model MICOM (Miami Isopycnic Coordinate Model), while
allowing coordinate surfaces to locally deviate from isopycnals wherever the
latter may fold, outcrop, or generally provide inadequate vertical resolution
in portions of the model domain. The freedom to adjust the vertical spacing
of the coordinate surfaces in HYCOM simplifies the numerical
implementation of several physical processes (mixed layer detrainment,
convective adjustment, sea ice modeling, …) without robbing the model of
the basic and numerically efficient resolution of the vertical that is
characteristic of isopycnic models throughout most of the ocean's volume.
The implementation of the generalized coordinate in HYCOM follows
the theoretical foundation set forth in Bleck and Boudra (1981) and Bleck
and Benjamin (1993): i.e., each coordinate surface is assigned a reference
isopycnal. The model continually checks whether or not grid points lie on
their reference isopycnals and, if not, attempts to move them vertically
toward the reference position. However, the grid points are not allowed to
migrate when this would lead to excessive crowding of coordinate surfaces.
Thus, vertical grid points can be geometrically constrained to remain at a
fixed depth while being allowed to join and follow their reference isopycnals
in adjacent areas (Bleck, 2002). The default configuration in HYCOM is one
that is isopycnal in the open stratified ocean, but smoothly reverts to a
terrain-following (V) coordinate in shallow coastal regions and to fixed
pressure-level coordinates (hereafter referred to as p) in the surface mixed
layer and/or unstratified seas (Figure 1). In doing so, the model combines the
advantages of the different types of coordinates in optimally simulating
coastal and open-ocean circulation features. It is left to the user to define the
coordinate separation constraints that control regional transitions among the
three coordinate choices. Figure 1 illustrates the transition that occurs
between p/V and isopycnic (U) coordinates in the fall and spring in the upper
400 meters and over the shelf in the East China and Yellow Seas. In the fall,
the water column is stratified and can be represented with isopycnals; in the
spring, the water column is homogenized over the shelf and is represented
by a mixture of p and V coordinates. A particular advantage of U coordinates