Objective (1), the Realistic Test Case
This focusses on the period November 1988 to October 1989 and spatially on the southern North Sea. The models are taken as they are presently used, i.e. varying from one another in the detail of included physics (2D/3D, barotropic/baroclinic, dispersion formulation), numerical solution techniques, grid sizes, calibration and underlying hydrodynamics. Thus the one constraint is that the models must all have the common spatial coverage of the North Sea basin between 51deg N and 55deg 40’N. To enable model simulations to be carried out a comprehensive [data set] has been constructed which includes bathymetry, tides, time- varying meteorology, boundary forcing and river discharge. Within the intercomparison there are four 2D models and nine 3D models, some of these are tidally-resolving (mesoscale) (9) and some are not (macroscale) (3).
Four experiments were defined:
– Experiment 1 (for 2D models only): continuous release of conservative and non-conservative tracers from the six locations (see figure) for 180 days starting 1 March 1989. Parameters examined include the age and concentration of the tracer.
– Experiment 2: single particle release from each location at four intervals in the year (releases at surface, mid-depth and bottom for 3D models). Parameters examined include distance travelled and relative tracer position.
– Experiment 3: instantaneous release of a 1kg tracer from each location at 2 times, 1 March and 1 August 1989. Parameters examined include trajectory of the centre of mass, total mass and patch shape and area.
– Experiment 4: Salinity and accumulated volume flux across 2 east-west sections at 52.5deg N and 54.5deg N (see figure). Parameters examined include time slice values and spatial variability.
The 2D models were run for the full 12 month period, the 3D models were run for either a) a six-month period, March-September 1989, or b) two 1-month periods, March and August 1989, depending on partners computational ability. Grid sizes of the models ranged from 2.4 km up to 20 km.
Objective (2), the Idealised Test Case
The experiment is the development of a fresh water eddy. A cylinder of fresher water is placed in the surface layer of an ambient fluid and allowed to mix. As the eddy forms under the influence of rotation, instabilities (secondary eddies) develop, the order of which depends on the friction (numerical and interfacial) in the system. The experiment is based on laboratory measurements (Griffiths & Linden, 1981, Journal of Fluid Mechanics, 105, 283- 316) and the numerical design of James (1996, Journal of Marine Systems, in press).
Five partners have participated in this experiment, producing 2 distinct sets of instability related to their numerical schemes.
Together, both experiments have generated more than 900 datasets for intercomparison. The intercomparison is presently in the analysis phase.
NOMADS will produce four Technical Reports:
• TR-1 Project rationale and experimental setup
• TR-2 Overview of the models to be used
• TR-3 Partner interpretations of the simulations
• TR-4 Intercomparison and conclusions.
The NOMADS dataset
The dataset has been constructed to enable partners to run the NOMADS simulations for the period 1 November 1998 to 31 October 1989. It contains the following data:
1) Covering the European Continental Shelf
Gridded bathymetry, 1/12deg latitude by 1/8deg longitude
Ten tidal constants around the shelf break
Gridded meteorological data at 3-hourly intervals (approx. 75km resolution) –
- Surface winds
- Atmospheric pressure
- Relative humidity
- Air temperature
- Daily estimates of cloud cover derived from AVHRR imagery
Annual mean discharge of 43 rivers.
2) Covering the Common Area (Southern North Sea, 51deg N to 55deg 40’N)
Thirty tidal constants across north and south boundaries
Residual elevation and current at hourly intervals across the boundaries
Monthly discharges from 16 rivers
Initial conditions for Temperature and Salinity for March and August 1989.
The data is available via anonymous ftp, subject to owner conditions. Please contact the NOMADS co-ordinator for details (@pol.ac.uk).