Search Results

You are looking at 1 - 7 of 7 items for

  • Author or Editor: R. Fiedler x
  • All content x
Clear All Modify Search
Brian H. Fiedler and R. Jeffrey Trapp

Abstract

The continuous dynamic grid adaption (CDGA) technique is applied to a compressible, three-dimensional model of a rising thermal. The computational cost, per grid point per time step, of using CDGA instead of a fixed, uniform Cartesian grid is about 53% of the total cost of the model with CDGA. The use of general curvilinear coordinates contributes 11.7% to this total, calculating and moving the grid 6.1%, and continually updating the transformation relations 20.7%. Costs due to calculations that involve the gridpoint velocities (as well as some substantial unexplained costs) contribute the remaining 14.5%. A simple way to limit the cost of calculating the grid is presented. The grid is adapted by solving an elliptic equation for gridpoint coordinates on a coarse grid and then interpolating the full finite-difference grid. In our application, the additional costs per grid point of CDGA are shown to be easily offset by the savings resulting from the reduction in the required number of grid points. In the simulation of the thermal, we are able to reduce costs by a factor of 3, as compared with those of a companion model with a fixed, uniform Cartesian grid.

Full access
Andrew R. Dean and Brian H. Fiedler

Abstract

In this study, both linear regression and a nonlinear neural network are used to forecast burnoff of low clouds in the warm season at San Francisco International Airport (SFO). Both forecast systems show skill scores between 0.2 and 0.25 in comparison with use of climatological values. The neural network is slightly more skillful. The forecast systems are derived from 45 yr of NCEP–NCAR reanalysis data and SFO surface observations. A forecast is attempted for both the time of burnoff and the probability of being burned off by 1000 Pacific standard time. The lack of significant superiority of the neural network over linear regression is not due to a failing of the neural network as a method. When both methods are applied to a statistical prediction of the afternoon temperature at SFO, based on early morning conditions, the neural network has a skill score of 0.446 and the linear regression has a skill score of 0.290.

Full access
R. Jeffrey Trapp and Brian H. Fiedler

Abstract

A novel approach to the modeling of tornado-like vortexgenesis has been developed and is used to articulate the sequence of events that leads to tornadogenesis. The “pseudostorm” is an idealized thunderstorm representation and emulates the storm-relative flow, into an updraft, of the horizontal streamwise vorticity that is baroclinically generated in cold air outflow. By not explicitly simulating the morphology of a tornadic thunderstorm, but instead concentrating on the development of low-level rotation and tornado-scale vortices, the authors are able to transcend many of the experimental limitations encountered by cloud modelers.

Intense, near-ground, cyclonic vortices, which are classified herein as “tornado like,” evolve solely from horizontal streamwise vorticity due to buoyancy gradients and friction, if present, at the lower boundary. Regardless of the lower boundary condition, none of the vortices exceed the thermodynamic speed limit based on the vertically integrated buoyancy (or convective available potential energy). Sensitivity experiments reveal that the tornado-like pseudostorm vortices develop only when certain updraft propagation rates (and thus storm-relative flow strengths), downdraft intensifies, and fluid viscosities are well matched. The simplicity of the pseudostorm allows one to look closely at the actual genesis of a vortex. In particular, it is found that the such genesis is not “triggered,” but is instead the outcome of a continuous (albeit rapid) process of amplification of vertical vorticity generated initially through tilting. Also, vertical vorticity intensification proceeds with a large degree of vertical uniformity, obviating the need for a mechanism like the “dynamic pipe effect” to advance the incipient tornado-like vortex toward the ground.

Full access
J. Stuart Godfrey, Rui-Jin Hu, Andreas Schiller, and R. Fiedler

Abstract

Annual mean net heat fluxes from ocean general circulation models (OGCMs) are systematically too low in the tropical Indian Ocean, compared to observations. In the models, only some of the geostrophic inflow replacing southward Ekman outflow is colder than the minimum sea surface temperature (MINSST). Observed heat fluxes imply that much more inflow is colder than MINSST. Since inflow below MINSST can only join the surface Ekman transport after diathermal warming, the OGCMs must underestimate diathermal effects.

A crude analog of the annual mean Indian Ocean heat budget was generated, using a rectangular box model with a deep “Indo–Pacific” gap at 7°–10°S in its eastern side. Wind stress was zonal and proportional to the Coriolis parameter, so Ekman transport was spatially constant and equaled Sverdrup transport. For three experiments, zonally integrated Ekman transport was steady and southward at 10 Sv (Sv ≡ 106 m3 s−1). In steady state, a 10 Sv “Indonesian Throughflow” fed a northward western boundary current of 10 Sv, which turned eastward along the northern boundary at 10°N to feed the southward Ekman transport. Most diathermal mixing occurred within an intense eddy in the northwest corner. Some of the geostrophic inflow was at temperatures colder than MINSST (found at the northeast corner of the eddy); it must warm to MINSST via diathermal mixing. Northern boundary upwelling exceeded the 10-Sv Ekman transport. The excess warms as it recirculates around the eddy, apparently supplying the heat to warm inflow below MINSST. In an experiment using the “flux-corrected transport” (FCT) scheme, diathermal mixing occurred in the strongly sheared currents around the eddy. However the Richardson number never became low enough to drive strong diathermal mixing, perhaps because (like that of other published models) the present model’s vertical resolution was too coarse. In three experiments, the dominant mixing was caused by horizontal diffusion, spurious convective overturn, and numerical mixing invoked by the FCT scheme, respectively. All three mixing mechanisms are physically suspect; such model problems (if widespread) must be resolved before the mismatch between observed and modeled heat fluxes can be addressed. However, the fact that the density profile at the western boundary must be hydrostatically stable places a lower limit on the area-integrated heat fluxes. Results from the three main experiments—and from many published OGCMs—are quite close to this lower limit.

Full access
Fabio Boeira Dias, C. M. Domingues, S. J. Marsland, S. M. Griffies, S. R. Rintoul, R. Matear, and R. Fiedler

Abstract

Ocean thermal expansion is a large contributor to observed sea level rise, which is expected to continue into the future. However, large uncertainties exist in sea level projections among climate models, partially due to intermodel differences in ocean heat uptake and redistribution of buoyancy. Here, the mechanisms of vertical ocean heat and salt transport are investigated in quasi-steady-state model simulations using the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2). New insights into the net effect of key physical processes are gained within the superresidual transport (SRT) framework. In this framework, vertical tracer transport is dominated by downward fluxes associated with the large-scale ocean circulation and upward fluxes induced by mesoscale eddies, with two distinct physical regimes. In the upper ocean, where high-latitude water masses are formed by mixed layer processes, through cooling or salinification, the SRT counteracts those processes by transporting heat and salt downward. In contrast, in the ocean interior, the SRT opposes dianeutral diffusion via upward fluxes of heat and salt, with about 60% of the vertical heat transport occurring in the Southern Ocean. Overall, the SRT is largely responsible for removing newly formed water masses from the mixed layer into the ocean interior, where they are eroded by dianeutral diffusion. Unlike the classical advective–diffusive balance, dianeutral diffusion is bottom intensified above rough bottom topography, allowing an overturning cell to develop in alignment with recent theories. Implications are discussed for understanding the role of vertical tracer transport on the simulation of ocean climate and sea level.

Open access
A. Schiller, J. S. Godfrey, P. C. McIntosh, G. Meyers, and R. Fiedler

Abstract

A global ocean circulation model, driven by observed interannual fluxes, is used to gain insight into how sea surface temperature anomalies (SSTAs, i.e., variations from the mean seasonal signal) in the tropical and subtropical Indian and Pacific Ocean are maintained and changed on interannual timescales. This is done by investigation of heat in the upper ocean at five selected sites and by comparison to observations based on expendable bathythermograph data and the TOGA/TAO moored buoys. A 6-yr simulation between 1985 and 1990 reveals that the model’s simulated interannual temperature variability in the upper 450 m of the ocean is in reasonable agreement with observations. However, the model overestimates the meridional extent and amplitude of SST variability in parts of the equatorial Pacific and Indian Oceans. The problem is associated with the choice of heat flux boundary condition: the ratio of air humidity to saturated humidity over freshwater at SST in the latent heat flux term is independent of the spatial scale of SSTA pattern, which implies a weaker negative feedback on SST change.

In the central Pacific at (0°, 140°W), budgets for the surface mixed layer and over the top 300 m both show the primary causes of temperature change to be zonal and vertical advection, with their sum generally less than half of either term individually. At (0°, 110°W), the mixed layer is much thinner so that the temperature changes result from a small disturbance of a basic balance between the vertical convergence of heat flux and vertical and zonal advection. At both sites the zonal flow (and hence the zonal heat advection) is determined by a sum of several terms, none of which are small. It is therefore difficult to find a clear physical basis in the model for the Kessler–McPhaden empirical rule for SSTAs, which correlates highly with observed SSTAs. However, this rule suggests that differences between wind stress products that exceed 0.04 N m−2 over several months (as occurs at 140°W in 1989) could lead to differences in SSTAs of up to 4°C. This may help explain the occurrence of a short but intense La Niña episode that occurred in the model, but not in the observed SST. Comparison with earlier model results tends to confirm that FSU winds were in error in the east Pacific in late 1989 and suggests that the use of a realistic (thin) surface mixed layer exacerbates the problem by strengthening the sensitivity of SSTAs to wind errors.

A simple time integral of the depth-averaged (0–350 m) current at 140°E, near the western boundary of the equatorial Pacific, shows a clear correlation with the zonal movements of the eastern edge of the warm pool, lagged by about six months. This is qualitatively as expected from “delayed oscillator” theory and confirms that the basic current structure of our model is in close agreement with observations.

Model and XBT observations show strong similarities in the depth of the 20°C isotherm and SSTA along the IX1 section from western Australia to Java during 1985–90. SST close to the southern end of this section (23°S, 112°E) is dominated by the annual signal with a superimposed weak interannual signal. The time rate of change of accumulated temperature anomalies in the top 450 m is dominated by anomalous cold vertical advection from late 1986 to early 1988 with the opposite happening from late 1988 to early 1990. Both signals indicate the arrival of the ENSO signal along the northwest Australian coast with a reduced (increased) thermocline thickness during the El Niño (La Niña) event. SSTA at 23°S, 112°E in the model is controlled by a balance between anomalous vertical advection and total diffusion; SSTA is not driven by local heat fluxes.

Full access
Fabio Boeira Dias, R. Fiedler, S. J. Marsland, C. M. Domingues, L. Clément, S. R. Rintoul, E. L. Mcdonagh, M. M. Mata, and A. Savita

Abstract

Ocean heat storage due to local addition of heat (“added”) and due to changes in heat transport (“redistributed”) were quantified in ocean-only 2xCO2 simulations. While added heat storage dominates globally, redistribution makes important regional contributions, especially in the tropics. Heat redistribution is dominated by circulation changes, summarized by the super-residual transport, with only minor effects from changes in vertical mixing. While previous studies emphasized the contribution of redistribution feedback at high latitudes, this study shows that redistribution of heat also accounts for 65% of heat storage at low latitudes and 25% in the midlatitude (35°–50°S) Southern Ocean. Tropical warming results from the interplay between increased stratification and equatorward heat transport by the subtropical gyres, which redistributes heat from the subtropics to lower latitudes. The Atlantic pattern is remarkably distinct from other basins, resulting in larger basin-average heat storage. Added heat storage is evenly distributed throughout midlatitude Southern Ocean and dominates the total storage. However, redistribution stores heat north of the Antarctic Circumpolar Current in the Atlantic and Indian sectors, having an important contribution to the peak of heat storage at 45°S. Southern Ocean redistribution results from intensified heat convergence in the subtropical front and reduced stratification in response to surface heat, freshwater, and momentum flux perturbations. These results highlight that the distribution of ocean heat storage reflects both passive uptake of heat and active redistribution of heat by changes in ocean circulation processes. The redistributed heat transport must therefore be better understood for accurate projection of changes in ocean heat uptake efficiency, ocean heat storage, and thermosteric sea level.

Open access