Ocean Nowcast/Forecast Systems for Improvement of Naval Undersea Capabilities

The U.S. Navy is a major investor in ocean model development. The pay-off of such an investment is the value-added ocean nowcast/forecast systems on naval operations and warfare effectiveness. The purpose of this paper is to investigate the value added of the Navy’s nowcast/forecast system to naval antisubmarine warfare (ASW) and anti-surface warfare (ASUW). The nowcast/forecast versus observational fields were used by the Weapon Acoustic Preset Program (WAPP) to determine the suggested presets for Mk 48 variant torpedo. The metric used to compare the two sets of outputs is the relative difference in acoustic coverage area generated by WAPP. Output presets are created for five different scenarios, two ASUW scenarios and three ASW scenarios in the South China Sea. The same metrics used in the nowcast/forecast case were used to generate and compare the acoustic coverage. Analysis of the output reveals that the ocean forecast system outperformed the nowcast system in most scenarios. (MODAS) is a commonly used nowcast system, which is built on the base of the optimal interpolation (statistical model). The Navy Coastal Ocean Model (NCOM) is a Navyused ocean forecast system, which is built on the base of the Princeton Ocean Model (POM). MODAS uses climatology as an initial guess and assimilates satellite and in situ measurements such as altimetry, conductivity-temperature-depth (CTD), expendable bathythermographs (XBT), and ARGO casts. NCOM (physical model) forecasts the ocean environment using observational data such as temperature, salinity, and velocity. Representation of the Navy’s nowcast (MODAS) and forecast (POM) systems for ocean environment (SSP through T, S profiles) was verified using the CTD data collected from the South China Sea Monsoon Experiment (SCSMEX) in April – June 1998 (Chu et al., 2001, 2004b). The errors have a Gaussian-type distribution with mean temperature nearly zero and mean salinity of -0.2 ppt. However, evaluation of a value-added ocean nowcast/forecast system on the naval undersea capability has yet been conducted. At the combat level, acoustic detection of torpedoes is extremely important for undersea warfare. This is because undersea warfare has changed considerably since Admiral Farragut gave his famous battle order over a century ago. Human ingenuity and advancements in technology have taken underwater weapons from floating mines and spar torpedoes to the fast-moving, self-guided, homing torpedoes we have in the fleet today. From submarine warfare to warship design and tactics development, the modern torpedo is one of the fundamental drivers of 20th century naval warfare (cited from http://www.navy.mil/ navydata/cno/n87/usw/ i s sue_14/ torpedoes.html). In this study, the Weapon Acoustic Preset Program (WAPP) for the Mk48 torpedo is used for such an evaluation. 2. Oceanographic Observations 2.1. South China Sea The South China Sea (SCS) is a semi-enclosed tropical sea located between the Asian land mass to the north and west, the Philippine Islands to the east, Borneo to the southeast, and Indonesia to the south (Figure 1), covering a total area of 3.5× 10 km. It includes the shallow Gulf of Thailand and connections to the East China Sea (through Taiwan Strait), the Pacific Ocean (through Luzon Strait), Sulu Sea, Java Sea (through Gasper and Karimata Straits) U