Robert C. Malone

Parallel ocean general circulation modeling

Abstract We have developed a global ocean model on the massively parallel CM-2 Connection Machine based on the Bryan-Cox-Semtner ocean general circulation model. This paper discusses the Connection Machine implementation of the model and its performance, as well as major changes that were made in the algorithms and numerical methods. In order to increase the parallel efficiency, we reformulated the barotropic equations to solve for the surface-pressure field rather than the volume-transport streamfunction. In addition to being more efficient, this method has several other advantages over the streamfunction formulation: (1) any number of islands can be included in the computational grid at no extra cost; (2) the model can handle steep gradients in the bottom topography; and (3) the free-surface height is a prognostic variable, which simplifies assimilation of altimetric data into the model. We have also developed a new parallelizable preconditioning method for the solution of the elliptic equation for the surface-pressure field. In order to use a standard conjugate gradient algorithm, an approximate-factorization technique is used to split off the Coriolis terms and obtain a symmetric operator. We then apply a symmetric local approximate-inverse operator as a preconditioning matrix, which is very effective in accelerating convergence to a solution.

A multidimensional model for aerosols: description of computational analogs

Abstract The numerical algorithms which we use to simulate the advection, diffusion, sedimentation, coagulation and condensational growth of atmospheric aerosols are described. The model can be used in one, two, or three spatial dimensions. We develop the continuity equation in a generalized horizontal and vertical coordinate system which allows the model to be quickly adapted to a wide variety of dynamical models of global or regional scale. Algorithms are developed to treat the various physical processes and the results of simulations are presented which show the strengths and weaknesses of these algorithms. Although our emphasis is on the modeling of aerosols, the work is also applicable to simulations of the transport of gases.

Global eddy‐resolving ocean simulations driven by 1985–1995 atmospheric winds

Results are presented from a high-resolution global ocean model that is driven through three decadal cycles of increasingly realistic prescribed atmospheric forcing from the period 1985–1995. The model used (the Parallel Ocean Program) is a z level primitive equation model with active thermohaline dynamics based on the formulation of Bryan [1969] rewritten for massively parallel computers. Improvements to the model include an implicit free-surface formulation of the barotropic mode [Dukowicz and Smith, 1994] and the use of pressure averaging for increasing the numerical time step. This study extends earlier 0.5° simulations of Semtner and Chervin [1992] to higher horizontal resolution with improved treatments of ocean geometry and surface forcing. The computational grid is a Mercator projection covering the global ocean from 77°N to 77°S and has 20 vertical levels. Three successive simulations have been performed on the CM-5 Connection Machine system at Los Alamos using forcing fields from the European Centre for Medium-Range Weather Forecasts (ECMWF). The first run uses monthly wind stresses for 1985–1995 and restoring of surface temperature and salinity to the Levitus [1982] seasonal climatology. The second run is the same but with 3 day-averaged rather than monthly averaged wind stress fields, and the third is the same as the second but uses the monthly climatological ECMWF heat fluxes of Barnier et al. [1995] instead of restoring to climatological sea surface temperatures. Many features of the wind-driven circulation are well represented in the model solutions, such as the overall current patterns, the numerous regions of hydrodynamic instability which correspond to those observed by satellite altimetry, and the filamented structure of the Antarctic Circumpolar Current. However, some features such as the separation points of the Gulf Stream and Kuroshio and the transport through narrow passages such as the Florida Straits are clearly inaccurate and indicate that still higher resolution may be required to correct these deficiencies. Water mass properties and some aspects of the thermohaline circulation are also not always well reproduced, which is partly due to the relatively short length of the integrations. The use of the ECMWF heat fluxes, rather than restoring to climatological surface temperatures, leads to stronger and more realistic surface and deep western boundary currents (primarily in the Atlantic) as well as more realistic meridional heat transport; this is primarily because the equilibrium meridional heat transport implied by the ECMWF surface fluxes is quite large. The ECMWF heat fluxes also produce improved seasonal cycles of sea surface temperature and height in both the northern and southern hemispheres. The 3-day wind forcing gives rise to modes of model variability that are clearly seen in synoptic observations, such as the large-scale 20–100-day oscillations seen in the TOPEX/POSEIDON data, which are barotropic oscillations induced by the high-frequency wind forcing. Additional studies on other aspects of the simulations described here are being conducted by a variety of investigators, and some of these are briefly described.

A Reformulation and Implementation of the Bryan-Cox-Semtner Ocean Model on the Connection Machine

Abstract Certain aspects of the Semtner-Chervin version of the Bryan-Cox-Semtner global ocean model are reformulated for improved efficiency on parallel computer architectures and on the Connection Machine CM-2 in particular. These changes involve (a) the switch from a streamfunction to a surface pressure formulation in the barotropic equations, (b) the splitting off of the Coriolis terms from the barotropic equations to produce a symmetric surface pressure equation, which then permits (c) the use of a preconditioned conjugate-gradient method for the solution of this equation. The switch to a surface pressure formulation (a) eliminates global equations associated with island boundary conditions and therefore improves performance as well as allows an unlimited number of islands, (b) reduces sensitivity to rapidly varying bottom topography and therefore obviates the need for smoothing the topography, and (c) makes the surface pressure a prognostic variable, thus potentially making it easier to assimilate su...

The Simulation of Stationary and Transient Geopotential-Height Eddies in January and July with a Spectral General Circulation Model

Abstract We examine the characteristics of stationary and transient eddies in the geopotential-height field as simulated by a spectral general circulation model. The model possesses a realistic distribution of continents and oceans and realistic, but smoothed, topography. Two simulations with perpetual January and July forcing by climatological sea surface temperatures, sea ice, and insulation were extended to 1200 days, of which the final 600 days were used for the results in this study. We find that the stationary waves are well simulated in both seasons in the Northern Hemisphere, where strong forcing by orography and land-sea thermal contrasts exists. However, in the Southern Hemisphere, where no continents are present in midlatitudes, the stationary waves have smaller amplitude than that observed in both seasons. In both hemispheres, the transient eddies are well simulated in the winter season but are too weak in the summer season. The model fails to generate a sufficiently intense summertime midlati...

Influence of Solar Heating and Precipitation Scavenging on the Simulated Lifetime of Post—Nuclear War Smoke

The behavior of smoke injected into the atmosphere by massive fires that might follow a nuclear war was simulated. Studies with a three-dimensional global atmospheric circulation model showed that heating of the smoke by sunlight would be important and might produce several effects that would decrease the efficiency with which precipitation removes smoke from the atmosphere. The heating gives rise to vertical motions that carry smoke well above the original injection height. Heating of the smoke also causes the tropopause, which is initially above the smoke, to reform below the heated smoke layer. Smoke above the tropopause is physically isolated from precipitation below. Consequently, the atmospheric residence time of the remaining smoke is greatly increased over the prescribed residence times used in previous models of nuclear winter.

Numerical modeling of tracer transport within and out of the lower tropospheric Arctic region

The dispersion of passive tracer within and out of the Arctic region is studied using a three-dimensional general circulation model with a Lagrangian particle module. Emphasis is placed on the lower troposphere by releasing numerical tracer particles in a layer near the surface within the Arctic circle to mimic the potential release of aerosol particles from the Arctic Ocean and the observed variability of Arctic haze. This study is also intended to investigate the characteristics of the quasi-horizontal mixing in transporting the air from high latitudes to mid- and low latitudes; a process which has extensively been studied mostly for the stratospheric regime. The model calculation shows that the Lagrangian passive tracer contours can correlate well with the isentropic potential vorticity field. This indicates that the identified large-scale mixing processes due to Rossby wave breaking and chaotic advection in the stratosphere may also occur in the lower troposphere.

On testing the significance of atmospheric response to smoke from the Kuwaiti oil fires using the Los Alamos general circulation model

The response of the Los Alamos atmospheric general circulation model to the smoke from the Kuwaiti oil fires set in 1991 is examined. The model has an interactive soot transport module that uses a Lagrangian tracer particle scheme. The statistical significance of the results is evaluated using a methodology based on the classic Students t test. Among various estimated smoke emission rates and associated visible absorption coefficients, the worst- and best-case scenarios are selected. In each of the scenarios, an ensemble of 10 30-day June simulations are conducted with the smoke and are compared to the same 10 June simulations without the smoke. The results of the worst-case scenario show that a statistically significant wave train pattern propagates eastward-poleward downstream from the source. The signals favorably compare with the observed climate anomalies in summer 1991, albeit some possible El Nino-Southern Oscillation effects were involved in the actual climate. The results of the best-case (i.e., least-impact) scenario show that the significance is rather small but that its general pattern is quite similar to that in the worst-case scenario.

Ocean modeling on the connection machine

No abstract available

Computing climate change: can we beat nature?

No abstract available