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Navy Real-Time Global Modeling Systems This article has been published in Oceanography, Volume 15, Number 1, a quarterly journal of The Oceanography Society. Copyright 2001 by The Oceanography Society. All rights reserved. Reproduction of any portion of this article by photocopy machine, reposting, or other means without prior authorization of The Oceanography Society is strictly prohibited. Send all correspondence to: [email protected], or 5912 LeMay Road, Rockville, MD 20851-2326, USA. SPECIAL ISSUE – NAVY OPERATIONAL MODELS: TEN YEARS LATER Navy Real-time Global Modeling Systems Robert C. Rhodes, Harley E. Hurlburt, Alan J. Wallcraft, Charlie N. Barron, Paul J. Martin, E. Joseph Metzger, Jay F. Shriver, Dong S. Ko Naval Research Laboratory • Stennis Space Center, Mississippi USA Ole Martin Smedstad Planning Systems, Inc. • Stennis Space Center, Mississippi USA Scott L. Cross Naval Oceanographic Office • Stennis Space Center, Mississippi USA A. Birol Kara Florida State University • Tallahassee, Florida USA Introduction The global ocean has its own “weather” phenome- Oceanographic Office (NAVOCEANO). NRL research na, although with greatly different time and space dating back to the early 1980s, which focused on high scales compared to the atmosphere. Oceanic mesoscale horizontal resolution, satellite altimeter data assimila- eddies are typically about 100 km in diameter which tion, and statistical downward projection of surface makes them 20–30 times smaller than comparable satellite observations has led to a system made up of atmospheric highs and lows. The ocean’s “jet streams” three main components: the Modular Ocean Data are the western boundary currents and their extensions Assimilation System (MODAS), the NRL Layered into the interior ocean. The currents have speeds on the Ocean Model (NLOM), and the NRL Coastal Ocean order of 1 m/s compared to atmospheric speeds that Model (NCOM). This system provides the Navy with can be 100 times this value. The space scales of the the first global ocean analysis/prediction capability for meanders on these high-speed streams are similar to fleet support and is also a contribution to the opera- those for the eddies mentioned above. Knowing and tional phase of the multi-national Global Ocean Data predicting these oceanic mesoscale features have Assimilation Experiment (GODAE; International numerous naval applications which include tactical GODAE Steering Team, 2000). planning, optimum track ship routing, search and res- The delivery of the first generation system began in cue and supplying boundary conditions for high reso- 1999 with a global ocean analysis capability (MODAS), 1 ° lution coastal models, to name a few. followed in 2000 with the transition of a ⁄16 horizontal However, operational global ocean modeling has resolution global ocean model (NLOM) that became the lagged far behind atmospheric modeling because the first operational system to properly resolve mesoscale smaller space scales mean approximately four orders of ocean dynamics. It will be completed by the delivery of 1 ° magnitude more computer time and three orders of a ⁄8 global version of the NCOM, with higher vertical magnitude more computer memory are required. In resolution in the mixed layer, for improved upper- addition, unlike the meteorological radiosonde net- ocean prediction and boundary conditions for higher work that provides initial conditions from the surface resolution coastal models (Figure 1). to the top of the atmosphere, there are very few obser- There is strong evidence that ocean models need to vations below the ocean surface on a daily basis. Thus, use grid cells for each predicted variable that are at surface satellite observations and an effective method most about 7 km across at mid-latitudes. NRL research to assimilate these data are a requirement. has shown that doubling the horizontal resolution to In the past few years, computer technology has 3.5 km per cell gives substantial improvement but dou- developed to the point where a system capable of bling again to 1.7 km gives only modest additional resolving the oceanic mesoscale (i.e. eddy-resolving) improvement (Hurlburt and Hogan, 2000). For the 1 ° 1 ° can be run on a daily basis. This first generation global NLOM grid, these resolutions translate to ⁄16 , ⁄32 and 1 ° ocean prediction capability has been developed at the ⁄64 , respectively. This is for the global and basin-scale. Naval Research Laboratory (NRL) and all components Much finer resolution is required for coastal regions. At of the system are running in real-time at the Naval 3.5 km, the optimal resolution is finer than might be Oceanography • Vol. 15 • No. 1/2002 29 Figure 1. Snapshots of sea surface height from 1 ° ⁄16 real-time NLOM (upper panel) and sea surface temperature from 1 ° ⁄8 NCOM (lower panel) for May 19, 2001. Note that NLOM’s pole- ward extent is 72°S and 65°N, while NCOM is fully global including the Arctic. expected based on the size of eddies. In relation to ter data and for successful forecasting of ocean eddies ocean eddy size it is similar to the resolution currently and the meandering of ocean currents and fronts. used by the leading weather forecasting models in rela- tion to the size of atmospheric highs and lows. NRL Global Ocean Modeling System More specifically, our research has shown that fine Motivation for NLOM-NCOM System resolution of the ocean eddy scale is required to obtain NRL’s global ocean prediction strategy, as dis- dynamic coupling between upper ocean currents and cussed by Harding et al. (1999), is driven by the need seafloor topography via flow instabilities, a mecha- for ocean models with the high horizontal resolution nism that occurs without direct contact between the required to successfully simulate the variability of upper ocean currents and the topography. This cou- mesoscale features and the high vertical resolution pling can strongly affect the pathways of upper ocean required near the surface to resolve the physics of the currents and fronts, including the Gulf Stream in the upper ocean. On global scales it is not yet possible to Atlantic, the Kuroshio in the Pacific (Hurlburt et al., run affordably one ocean model in real time that has 1996; Hurlburt and Metzger, 1998), the East Korea the horizontal and vertical resolution needed to meet Warm Current in the Japan/East Sea (Hogan and these requirements. Thus, the first generation global Hurlburt, 2000) and the Tasman Front between ocean prediction system uses a two-model approach Australia and New Zealand (Tilburg et al., 2001). The based on NLOM and NCOM. high-resolution is also required to obtain sharp fronts NLOM, forced by daily output from the Navy that span major ocean basins (Hurlburt et al., 1996) and Operational Global Atmospheric Prediction System for adequate representation of straits and islands (e.g. (NOGAPS; see Rosmond et al., this issue), assimilates Metzger and Hurlburt, 2001). In addition, it can affect altimeter-derived sea surface height (SSH) with the the large-scale shape of ocean gyres such as the high horizontal resolution needed for the mesoscale Sargasso Sea in the Atlantic (Hurlburt and Hogan, dynamics and serves as an effective dynamic interpola- 2000). As discussed later, simulation skill for ocean tor of the altimeter data in mapping this variability. mesoscale features is crucial for an ocean model to act NCOM, also forced by NOGAPS and deriving its as an effective dynamic interpolator for satellite altime- mesoscale information from the 3-dimensional Oceanography • Vol. 15 • No. 1/2002 30 Figure 2. Wiring diagram of NRL global ocean prediction system baseline showing relationship between MODAS, NLOM, and NCOM modules and the role of each component in the overall system. MODAS temperature and salinity profiles derived generate synthetic temperature and salinity profiles from the NLOM SSH and the MODAS SST analyses, based on observed sea surface temperature (SST) and provides a global oceanic boundary layer prediction SSH. MODAS contains a global bimonthly database of capability from the open ocean and into the coast. With regression coefficients relating deviations of SSH and a sophisticated surface mixed-layer formulation SST from climatology to deviations of subsurface tem- (Mellor and Yamada, 1982) and the ability to run a fully perature from climatological profiles. This synthetic global grid including the Arctic and continental profile capability is part of a global climatology that shelves, NCOM can be used to provide boundary and extends into shallow water using only SST where the initial conditions to nested higher resolution coastal depth is < 200 meters. The current operational global models and can also be used for global air/sea interac- MODAS capability generates synthetic profiles based 1 ° tion applications. Until NCOM, or other similar mod- on ⁄8 global SSH and SST analyses performed daily at els, can run on operational computers at sufficient hor- the NAVOCEANO Altimetry Data Fusion Center izontal resolution to resolve the ocean mesoscale, we (ADFC; see Jacobs et al., this issue). The global MODAS expect the two-model approach will be necessary. capability will be upgraded to perform a full 3-D analy- Figure 2 shows a schematic of the entire global sys- sis by assimilating available in situ data and the analy- tem emphasizing the individual roles of NLOM, sis profiles will be used for assimilation into the global MODAS, and NCOM. The analysis portion of the sys- NCOM (discussed later). tem is based on MODAS (Fox et al., 2001), which is Operational applications for the NLOM system now operational at NAVOCEANO. MODAS is an opti- include improved nowcasts (i.e. the current state of the mum interpolation analysis system with the ability to ocean) and forecasts of mesoscale variability including Oceanography • Vol. 15 • No. 1/2002 31 major front and eddy systems like the Gulf Stream and Shriver and Hurlburt (1997). A bulk mixed layer model Kuroshio Extension. These will be used by NAVO- and sea surface temperature was added to NLOM, CEANO to improve their capability to depict the loca- Wallcraft et al.
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