Variational Data Assimilation for the Global Ocean - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 321

13.6 Global HYCOM

As mentioned in the introduction, the NCODA 3DVAR analysis is currently cycling
with global HYCOM in real-time at NAVOCEANO. The 3DVAR is expected to
replace the MVOI as the data assimilation component in the operational HYCOM,
which is referred to as the Global Ocean Forecast System (GOFS) version 3.
As configured within GOFS v3, HYCOM has a horizontal equatorial resolution

:08 ı 1=12 ı . 7 of km mid latitude) resolution. This makes HYCOM eddy

resolving. Eddy-resolving models can more accurately simulate western boundary
currents and the associated mesoscale variability and they better maintain more
accurate and sharper ocean fronts. In particular, an eddy resolving ocean model
allows upper ocean topographic coupling via flow instabilities, while an eddy-
permitting model does not because fine resolution of the flow instabilities is required
to obtain sufficient coupling ( Hurlburt et al. 2008b ). The coupling occurs when
flow instabilities drive abyssal currents that in turn steer the pathways of upper
ocean currents ( Hurlburt et al. 1996 in the Kuroshio; Hogan and Hurlburt 2000
in the Japan/East Sea; and Hurlburt and Hogan 2008 in the Gulf Stream). In ocean
prediction this coupling is important for ocean model dynamical interpolation skill
in data assimilation/nowcasting and in ocean forecasting, which is feasible on time
scales up to about a month ( Hurlburt et al. 2008a ).
The global HYCOM grid is on a Mercator projection from

or

47 ı N
and north of this it employs an Arctic dipole patch where the poles are shifted
over land to avoid a singularity at the North Pole. This gives a mid-latitude
(polar) horizontal resolution of approximately 7 km (3.5 km). This version employs
32 hybrid vertical coordinate surfaces with potential density referenced to 2,000 m
and it includes the effects of thermobaricity ( Chassignet et al. 2003 ). Vertical
coordinates can be isopycnals (density tracking), often best in the deep stratified
ocean, levels of equal pressure (nearly fixed depths), best used in the mixed layer
and unstratified ocean, and sigma-levels (terrain-following), often the best choice
in shallow water. HYCOM combines all three approaches by choosing the optimal
distribution at every time step. The model makes a dynamically smooth transition
between coordinate types by using the layered continuity equation. The hybrid
coordinate extends the geographic range of applicability of traditional isopycnic
coordinate circulation models toward shallow coastal seas and unstratified parts of
the world ocean. It maintains the significant advantages of an isopycnal model in
stratified regions while allowing more vertical resolution near the surface and in
shallow coastal areas, hence providing a better representation of the upper ocean
physics. HYCOM is configured with options for a variety of mixed layer sub-
models ( Halliwell 2004 ) and this version uses the K-Profile Parameterization (KPP)
of Large et al. ( 1994 ). A more complete description of HYCOM physics can be
found in Bleck ( 2002 ). The ocean model uses 3-hourly Navy Operational Global
Atmospheric Prediction System (NOGAPS) forcing from FNMOC that includes: air
temperature at 2 m, surface specific humidity, net surface short-wave and long-wave
radiation, total (large scale plus convective) precipitation, ground/sea temperature,

78:64 ı Sto