Preparing for assimilation of acoustic data into a ice-ocean coupled model using EnKF

This report describes the preparation for assimilation of acoustic travel time data into the ice-ocean data and model system TOPAZ. The conceptual development of the assimilation scheme is elaborated, and most components of the measurement matrix have been linked together and tested. The acoustic model RAY has been linked to the ensemble mean and the full ensembles from the TOPAZ-3 system. Using this concept two main numerical studies have been carried out. The first was to feed one year of ensemble means at a weekly resolution into the acoustic model to produce acoustic arrival times. The second group of simulations were made by selecting 4 ensembles from different seasons of the year: 25 July 2007, 23 January 2008, 25 May 2008 and 16 June 2008. Each of the 100 members in each ensemble was used as input to RAY, which was run to produce eigenrays and corresponding travel times (and arrival angles).
The major result of the simulations was that horizontally launched and early arriving rays can be difficult to analyze, as they show a very chaotic arrival time pattern. Simulations show that in some cases array processing might help. The steeper rays are found to fall into well defined clusters characterized by arrival time and arrival angle. In many cases, the cluster of arrivals are very close or even overlapping with other cluster in time domain, but they are often easily resolvable if take arrival angle into account. This indicate that future tomography systems in the Fram Strait, as within ACOBAR, should be designed to enable arrival angle detection.