The Continuous Improvement of an Offline Particle Tracking Model: Nested grid applications.

The analysis of coastal ocean circulation using Lagrangian tools is a useful method of investigating phenomenon such as, but not limited to, transport pathways of water masses, the fate of pollution discharged into the environment, and the transport/recruitment of larva. A common approach is to run a hydrodynamic model of the coastal ocean, then run an offline particle tracking model using the hydrodynamic output, and finally analyze the resulting trajectories. Although there are a number of software packages supporting offline particle tracking, in differing stages of development, as ocean modeling systems advance, the corresponding particle tracking systems do not always follow suit. In this study, we use output from an instance of the regional ocean modeling system (ROMS). In recent years, ROMS development moved forward, including such features as wetting and drying of coastal areas, and the nesting of higher resolution grids in larger scale model domains. At the same time, the offline particle tracking model LTRANS (http://northweb.hpl.umces.edu/LTRANS.htm#Northetal2008), which uses ROMS output exclusively, was not updated to account for these features. Here we present a series of updates to LTRANS, including the ability to handle nested grids, wetting and drying, and advection scheme which uses the ROMS native grid coordinate system. The result is a more flexible particle tracking system, with modest improvements in performance.