Project Description

S.E. TASMANIA (LMSN/INFORMD + INFORMD2) Project Description

Download the INFORMD2 Final Report 18.4 MB

Introduction.

This project aims to provide near real-time hydrodynamic modelling of the south-east Tasmania, including the Huon and Derwent Estuaries, D’Entrecasreaux Channel and Storm Bay (Figure 1). It is anticipated that input data provided by a sensor network deployed throughout the region will be used by the hydrodynamic model in a data assimilating capacity. The sensors would provide sea-level, salinity and temperature data in near real-time, while the hydrodynamic modelling system (SHOC) would then provide near real-time fields of currents, sea-level, water temperature and salinity.

The hydrodynamic model is forced by global and regional data, which is readily available in real-time. A prototype model has been developed (SETAS2) which extended from South-West Cape to the Freycinet Peninsula. The model domain included a proposed IMOS measuring station near Maria Island. This model is nested directly into the global model, which resolves the Australasian region at 10 km, and supplies sea level, temperature and salinity on the open boundaries. The model is forced with river flow from the Huon and Derwent Rivers, and uses MesoLAPS atmospheric products for surface flux specification. SETAS2 is well-behaved dynamically, and is successfully implemented in near real-time.

A high resolution model is nested within this larger scale model, capable of supplying output at ~100 m resolution withing the estuaries and increasing to ~2 km offshore. This offshore resolution is suitable for nesting within the larger scale model. Due to the high resolution, the model operates with a run-time ratio of ~3:1, making it suitable for now-cast operation only. An archive of output is created which may be used in conjunction with a transport model to run biogeochemical and sediment transport models of the region.

Model Grids

The SETAS model grid uses a orthogonal curvilinear grid for horizontal discretization which allows high resolution to be placed in areas of interest (e.g. the estuaries) and coarser resolution offshore. The grid size is 175×120 horizontal grid cells and is shown in Figure 2. The grid resolution ranges from 2.5km on the south-east offshore boundary, to 1.4km on the boundary of the STORM grid. In the Derwent at Hobart resolution is 400m, 300m further up the estuary at Bridgewater, and 250m in the middle Huon Estuary. The water depth over the model domain ranges from a minimum of 2 m to 200 m at the offshore boundary. The model uses a geopotential vertical coordinate system with 21 model layers.

The SETAS model has a total of 5 open boundaries; two of these are the represent the Derwent and Huon River upstream boundaries, which are forced in identical fashion using river flow and temperature data. The other 3 boundaries are offshore boundaries (Figure 2) which are forced identically except for a radiation condition and sponge layer imposed on the north-east boundary to maintain stability. The surface elevation for forcing these 3 boundaries is composed of 3 components – the BlueLINK low-frequency component, the tides (derived from a global tide model) model, and an ‘inverse barometer’ component calculated from the MesoLAPS pressure data. The latter is included because the low-frequency component from the global model does not include sea level changes due to atmospheric pressure, whereas the SETAS model interior does, hence this must be compensated for on the open boundary. Salinity and temperature forcing for the offshore boundaries is provided by BlueLINK. The meteorological forcing is provided entirely from the MesoLAPS data, and both salt and heat fluxes across the air-sea interface are included.

Output from the SETAS model currently consists of a daily record of all variables (used for hot-starting subsequent runs), hourly records of all surface variables, and 12-minute records of elevation, salinity and temperature along the path of the STORM offshore boundary.

The STORM model grid also uses an orthogonal curvilinear grid, with size 700×398 cells. The grid resolution ranges from 600m on the south-east offshore boundary, to 100m in the Derwent at Hobart, 120m at Bridgewater, and 125m in the middle Huon Estuary. The water depth over the model domain ranges from a minimum of 2 m to 145m (Figure 3). There are 24 model layers in the vertical, discretized on a geopotential vertical coordinate system. The model uses cell wetting and drying, which allows large surface fluctuations (e.g. from the tide) to exist in the presence of fine vertical resolution.

The STORM model has a total of 3 open boundaries. Two of these are the Derwent and HuonRiver upstream boundaries, which are forced exactly as for the SETAS model. The other boundary is an offshore boundary which is forced with surface elevation, salinity and temperature provided by the SETAS model. A radiation condition is imposed on this boundary to maintain stability. The meteorological forcing is provided entirely from the MesoLAPS data, and both salt and heat fluxes across the air-sea interface are included.

Output from the STORM model currently consists of a daily record of all variables (used for hot-starting subsequent runs), hourly records of all surface variables and also hourly records of hourly means of velocity, T/S sea level and vertical diffusivity in a sparse format file for that may be used offline to run a transport model coupled to biogeochemical and sediment transport libraries.

Near Real Time Operation

The objective of this system is to create near-real time output that is archived indefinitely without user intervention. To achieve this the models will be integrated in the ROAM framework, which will manage data aquisition, automated model deployment and sequencing. The ROAM system is described seperately on this site.

ROAM accesses OceanMAPS global ocean model products from the Bureua of Meteorology (BoM). These products consist of forecast, near-real time and reanalysis product, the latter of which is deemed more accurate due to inclusion of data assimilation. This product is spans the week ending at the present time. The near-real time product spans the period ending on or just before the present day. To optimise accuracy, the near real time operation attempts to use the most accurate data available, which in this case is the OceanMAPS reanalysis. These products are only available twice weekly. In principle, OceanMAPS foecast data may be used to derive up to a 24 hour forecast of ocean state.

River levels are downloaded from the BoM website and automatically converted into appropriate formats for forcing the model. MesoLAPS data is downloaded from BoM, and is available in near-real time and a forecast out to 48 hours.