Optimal numerical strategy for unsteady natural convection in two and three dimensions

Analyses of accuracy and computational cost of finite difference methods in computational fluid dynamics have illustrated a criterion for the minimum order for efficient calculations. This criterion favours the use of higher than second order methods when modelling greater than two space-time dimensions. These analyses have assumed the dominant length scale to be homogeneous throughout the model domain. Natural convection in a cavity can exhibit inhomogeneity of the smallest dominant length scales in identifiable sub-domains. Any inhomogeneity of this nature is shown to have a significant impact on the computational efficiency. This extended analysis suggests that an optimally efficient numerical calculation for unsteady natural convection requires: a non-uniform grid that complements the distribution of length scales to obtain a homogeneous non-dimensional grid scale; and a numerical order equal to or greater than the space-time dimension

Forecast Errors, Goodness, and Verification in Ocean Forecasting

Verification is an essential part of the forecast process that provides guidance on the statistical behavior of the system and a framework by which a forecast can be assessed for its goodness. Much of the framework applicable to ocean forecasting has been developed within the atmospheric community. A review of the available material is given with some commentary on its relevance in the context of ocean forecasting. A statistical theory is presented for errors in an ocean forecast system (both deterministic and ensemble) and for a number of verification metrics. Theoretical results are demonstrated with empirical models and results from an operational ocean forecast system. Some new results are presented comparing the mean absolute error and root mean square error and the inference hypothesis testing of ensemble forecast systems. The progress in ocean verification is discussed, as are advances in technology to analyze international verification databases

Sea level pressure response to the specification of eddy-resolving sea surface temperature in simulations of Australian east coast lows

Four east coast lows (ECLs) were simulated with the Weather Research and Forecast model to investigate the influence of the sea surface temperature (SST) distribution on the sea level pressure (SLP). Each ECL was simulated with two different SST datasets: the Bluelink SST field and NCEP skin temperature field. The former resolved eddies in the East Australian Current while the latter did not. The simulated SLP fields in the eddy-resolving SST runs were compared with those in the non-eddy-resolving SST runs. On time-scales of about 48 hours, higher SSTs were asso-ciated with lower SLPs. The spatial scale of the SLP response was similar to that of the ocean eddies, indicative of the rapidity and robustness of the response given the rapidly evolving conditions within the storms. On shorter time-scales, the SLP response to SST change can become substantially larger. The largest reductions in SLP in the eddy-resolving SST runs were associated with regions of deep atmospheric convection that warm the tropospheric column. These areas were shown to be related to the SST distribution with the greatest SLP reductions associated with convection over strong SST gradient regions. The landfall of a damaging convective mesoscale low pressure system on 8 June 2007 was also investigated. It was found that a region of strong SST gradients on the southern flank of a large warm ocean eddy was associated with lower pressures at the time of formation of this meso-low. In addition, the only case that simulated the low pressure at the correct time (albeit at not quite the correct location) was the eddy-resolved SST run. It was hy-pothesized that the development of this meso-low that impacted the coast around Newcastle, was enhanced because of the eddy-scale SST distribution at the time

ACCESS-OM2 v1.0: a global ocean-sea ice model at three resolutions

We introduce ACCESS-OM2, a new version of the ocean–sea ice model of the Australian Community Climate and Earth System Simulator. ACCESS-OM2 is driven by a prescribed atmosphere (JRA55-do) but has been designed to form the ocean–sea ice component of the fully coupled (atmosphere–land–ocean–sea ice) ACCESS-CM2 model. Importantly, the model is available at three different horizontal resolutions: a coarse resolution (nominally 1∘ horizontal grid spacing), an eddy-permitting resolution (nominally 0.25∘), and an eddy-rich resolution (0.1∘ with 75 vertical levels); the eddy-rich model is designed to be incorporated into the Bluelink operational ocean prediction and reanalysis system. The different resolutions have been developed simultaneously, both to allow for testing at lower resolutions and to permit comparison across resolutions. In this paper, the model is introduced and the individual components are documented. The model performance is evaluated across the three different resolutions, highlighting the relative advantages and disadvantages of running ocean–sea ice models at higher resolution. We find that higher resolution is an advantage in resolving flow through small straits, the structure of western boundary currents, and the abyssal overturning cell but that there is scope for improvements in sub-grid-scale parameterizations at the highest resolution

Sea Level Forecasts Aggregated from Established Operational Systems

A system for providing routine seven-day forecasts of sea level observable at tide gauge locations is described and evaluated. Forecast time series are aggregated from well-established operational systems of the Australian Bureau of Meteorology; although following some adjustments these systems are only quasi-complimentary. Target applications are routine coastal decision processes under non-extreme conditions. The configuration aims to be relatively robust to operational realities such as version upgrades, data gaps and metadata ambiguities. Forecast skill is evaluated against hourly tide gauge observations. Characteristics of the bias correction term are demonstrated to be primarily static in time, with time varying signals showing regional coherence. This simple approach to exploiting existing complex systems can offer valuable levels of skill at a range of Australian locations. The prospect of interpolation between observation sites and exploitation of lagged-ensemble uncertainty estimates could be meaningfully pursued. Skill characteristics define a benchmark against which new operational sea level forecasting systems can be measured. More generally, an aggregation approach may prove to be optimal for routine sea level forecast services given the physically inhomogeneous processes involved and ability to incorporate ongoing improvements and extensions of source systems

Observations of a transient stratified laminar intrusion

A transient, stratified laminar intrusion in a side-heated cavity is observed for an extended period by using a cavity of reduced aspect ratio. Shadowgraph observations are made of the jump-like feature and trailing waves immediately downstream from the vertical boundary and of the associated development of the secondary circulation. The stratified intrusion front is initially driven by momentum flux from the vertical boundary layer and horizontal pressure gradients due to the intrusion buoyancy. When the intrusion propagates further along the horizontal boundary buoyancy losses due to conduction and momentum losses due to viscosity lead to a deceleration and a reduced heat transfer efficiency. Numerical simulations demonstrate that horizontal momentum of the intrusion is partially compensated by a horizontal pressure gradient due to a thickening of the stratified layers. The thickening is ocalised to a position downstream from the intrusion waves and upstream from the intrusion front. Downstream from the layer thickening the observations are consistent with those of a viscous gravity current. Upstream from this layer thickening an adverse pressure gradient leads to a regression and steepening of the jump-like feature and intrusion waves. For parameter values illustrated the regression leads to an interaction with the vertical boundary layer prior to the arrival of the intrusion fronts to the opposing boundaries