MPIPOM-TC: A new ocean modeling system with flexible initialization for improved coupled hurricane-ocean model forecasts

URI's version of the Princeton Ocean Model (POM), frequently designated POM for Tropical Cyclones (POM-TC), has been the ocean component of NOAA's operational GFDL and HWRF coupled hurricane-ocean models since 2001 and 2007, respectively. Some significant improvements to POM-TC have been made since 2001, including implementation of a feature-based ocean initialization for the western Atlantic, westward expansion of the ocean domain in the eastern Atlantic, and an NCODA-based initialization worldwide outside of the Atlantic in GFDN, the U.S. Navy's version of the GFDL model. However, no upgrades have been made to the ocean model resolution, and none of the community-based upgrades to POM have been incorporated into POM-TC since 1994. Since POM-TC is a serial (single processor) code, upgrades to the ocean model resolution are not computationally feasible. Hence, URI has now developed a new version of POM-TC, called MPIPOMTC, which incorporates many of the community-based upgrades to POM from 1994 to 2012 by blending the Message Passing Interface (MPI) enabled sbPOM (also known as mpiPOM) with the existing version of POM-TC. MPIPOMTC allows for higher spatial resolution and a larger domain size than POM-TC. In fact, one of the key improvements in MPIPOMTC is the replacement of the two overlapping POM-TC domains in the North Atlantic Ocean, each of which have ~1/6° horizontal grid spacing, with a single, new, transatlantic domain, which has ~1/12° horizontal grid spacing. MPIPOMTC is computationally efficient and scalable, and it has NetCDF I/O, which is more user-friendly than POM-TC's FORTRAN binary I/O.

POM-TC utilizes different initialization procedures in different ocean basins in the GFDL, GFDN, and HWRF coupled systems, and the antiquated architecture of the POM-TC code varies from basin to basin to accommodate the various initialization procedures and grid specifications. URI has now developed a new methodology to initialize a unified, basin-independent MPIPOMTC code from different global real-time ocean products available at NOAA and the U.S. Navy. By separating the ocean initialization and grid specification module from the core MPIPOMTC code, the initializations and grid specifications become a plug-and-play feature that facilitates simplified testing, evaluation, and operational implementation. As an alternative to the feature-based initialization in the Atlantic and NCODA elsewhere, for example, MPIPOMTC initialization modules for NOAA's Global HYCOM RTOFS and the U.S. Navy's Global HYCOM products have been developed. By continuing to perform careful evaluation of various ocean initialization products in all ocean basins, it will be possible to ensure that MPIPOMTC is initialized with the best product available for subsequent GFDL, GFDN, and HWRF operational implementations.

MPIPOMTC, like the sbPOM/mpiPOM, is suitable for community distribution for ocean research applications. However, MPIPOMTC is uniquely suited for simulating coupled hurricane-ocean interaction. A key feature of MPIPOMTC is that it can be easily configured to run in an idealized mode rather than real-case mode to isolate the impact of specific physics changes in a scientifically meaningful way. Idealized options include removal of 3D physics (thereby isolating 1D vertical mixing) and using an idealized initial ocean condition, grid specification, bathymetry, and/or forcing. Some of these capabilities were included in research mode in POM-TC, but MPIPOMTC has the new capability to run in a purely 1D mode in which POM's C-grid architecture has be reconfigured to the A-grid, thereby allowing all variables to be calculated at the same horizontal location, with no horizontal averaging or differencing, so each vertical profile is completely independent of the others.