Chung-Chun Ma

Peruvian Stratus Clouds and the Tropical Pacific Circulation: A Coupled Ocean-Atmosphere GCM Study

Abstract Extensive and persistent stratus cloud decks are prominent climatic features off the Peruvian coast. They are believed to play a key role in the coupled atmosphere-ocean processes that determine the sea surface temperature (SST) throughout the eastern tropical Pacific. This notion is examined and further developed using a coupled ocean-atmosphere general circulation model (GCM): a control simulation, in which the simulated amount of Peruvian stratus clouds is unrealistically low, is compared with an experiment in which a stratus cloud deck is prescribed to persistently cover the ocean off the Peruvian coast. Beneath the prescribed cloud deck SSTs are reduced by up to 5 K, as expected from decreased solar radiation reaching the surface. In addition, there is significant cooling over much of the eastern tropical Pacific south of the equator, and even along the equator well into the central Pacific. The prescribed stratus deck largely alleviates the coupled GCMs warm bias in SST in the southeastern...

Simulation of the tropical Pacific climate with a coupled ocean - atmosphere general circulation model. Part II: Interannual variability

Abstract Two multiyear simulations with a coupled ocean-atmosphere general circulation model (GCM)-totaling 45 years-are used to investigate interannual variability at the equator. The model consists of the UCLA global atmospheric GCM coupled to the GFDL oceanic GCM, dynamically active over the tropical Pacific. Multichannel singular spectrum analysis along the equator identifies ENSO-like quasi-biennial (QB) and quasi-quadrennial (QQ) modes. Both consist of predominantly standing oscillations in sea surface temperature and zonal wind stress that peak in the central or east Pacific, accompanied by an oscillation in equatorial thermocline depth that is characterized by a phase shift of about 90° across the basin, with west leading east. Simulated interannual variability is weaker than observed in both simulations. One of these is dominated by the QB, the other by the QQ mode, although the two differ only in details of the surface-layer parameterizations.

Sensitivity of a Coupled Ocean–Atmosphere Model to Physical Parameterizations

Abstract The sensitivity of a coupled ocean–atmosphere general circulation model to parameterizations of selected physical processes is studied. The parameterizations include those of longwave radiation and surface turbulent fluxes in the atmospheric model, and those of vertical turbulent mixing and penetration of solar radiation in the ocean model. It is shown that the performance of the coupled model is highly sensitive to the parameterization of longwave radiation. This sensitivity is not solely due to the difference in surface radiative flux but involves interactions among radiation, convection, and large-scale dynamics of the atmosphere and ocean. It is concluded that differences in parameterizations can have large impacts on the performance of the coupled model, and these impacts can be very different from what may be expected from uncoupled model simulations.

Simulation of the Tropical Pacific Climate with a Coupled Ocean-Atmosphere General Circulation Model. Part I: The Seasonal Cycle

Abstract A multiyear simulation with a coupled ocean-atmosphere general circulation model (GCM) is presented. The model consists of the UCLA global atmospheric GCM coupled to the GFDL oceanic GCM; the latter is dynamically active over the tropical Pacific, while climatological time-varying sea surface temperatures (SST) are prescribed elsewhere. The model successfully simulates the main climatological features associated with the seasonal cycle, including the east-west gradient in SST across the equatorial Pacific. The most apparent deficiencies include a systematic cold bias (∼2 K) across most of the tropical Pacific and underestimated wind stress magnitudes in the equatorial band. Multichannel singular spectrum analysis is used to describe the multivariate structure of the seasonal cycle at the equator in both the model and observed data. The annual harmonic in equatorial SST is primarily wind driven, while air-sea interaction strongly affects the semiannual harmonic.

Parallelization and Distribution of a Coupled Atmosphere–Ocean General Circulation Model

Abstract The distribution of a climate model across homogeneous and heterogeneous computer environments with nodes that can reside at geographically different locations is investigated. This scientific application consists of an atmospheric general circulation model (AGCM) coupled to an oceanic general circulation model (OGCM). Three levels of code decomposition are considered to achieve a high degree of parallelism and to mask communication with computation. First, the domains of both the gridpoint AGCM and OGCM are divided into subdomains for which calculations an carded out concurrently (domain decomposition). Second, the model is decomposed based on the diversity of tasks performed by its major components (task decompositions). Three such components are identified: (a) AGCM/physics which computes the effects on the grid-scale flow of subgrid-scale processes such as convection and turbulent mixing; (b) AGCM/dynamics, which computes the evolution of the flow governed by the primitive equations; and (c) ...

Comparison of tropical pacific temperature and current simulations with two vertical mixing schemes embedded in an ocean general circulation model and reference to observations

The Pacanowski-Philander (PP) and Mellor-Yamada (MY) parameterization models of vertical mixing by turbulent processes were embedded in the Geophysical Fluid Dynamics Laboratory high-resolution ocean general circulation model of the tropical Pacific Ocean. All other facets of the numerical simulations were the same. Simulations were made for the 1987–1988 period. At the equator the MY simulation produced near-surface temperatures more uniform with depth, a deeper thermocline, a deeper core speed of the Equatorial Undercurrent, and a South Equatorial Current with greater vertical thickness compared with that computed with the PP method. Along 140°W, between 5°N and 10°N, both simulations were the same. Moored-buoy current and temperature observations had been recorded by the Pacific Marine Environmental Laboratory at three sites (165°E, 140°W, 110°W) along the equator and at three sites (5°N, 7°N, 9°N) along 140°W. Simulated temperatures were lower than those observed in the near-surface layer and higher than those observed in the thermocline. Temperature simulations were in better agreement with observations compared to current simulations. At the equator, PP current and temperature simulations were more representative of the observations than MY simulations.

Distribution of a climate model across high-speed networks

No abstract available

Toward a high performance distributed memory climate model

As part of a long range plan to develop a comprehensive climate systems modeling capability, the authors have taken the atmospheric general circulation model originally developed by Arakawa and collaborators at UCLA and have recast it in a portable, parallel form. The code uses an explicit time-advance procedure on a staggered three-dimensional Eulerian mesh. They have implemented a two-dimensional latitude/longitude domain decomposition message passing strategy. Both dynamic memory management and interprocess communication are handled with macro constructs that are preprocessed prior to compilation. The code can be moved about a variety of platforms, including massively parallel processors, workstation clusters, and vector processors, with a mere change of three parameters. Performance on the various platforms as well as issues associated with coupling different models for major components of the climate system are discussed.<<ETX>>

Distributing a climate model across gigabit networks

The authors investigate the distribution of a climate model across homogeneous and heterogeneous computer environments with nodes that can reside at geographically different locations. The application consists of an atmospheric general circulation model (AGCM) coupled to an oceanic general circulation model (OGCM). Three levels of code decomposition are considered to achieve a high degree of parallelism and to mask communication with computation. First, the domains of both the grid-point AGCM and OGCM are divided into sub-domains for which calculations are carried out concurrently (domain decomposition). Second, the model is decomposed based on the diversity of tasks performed by its major components (task decomposition). Last, computation and communication are organized in such a way that the exchange of data between different tasks is carried out in subdomains of the model domain (I/O decomposition). In a dedicated computer/network environment, the wall-clock time required by the resulting distributed application is reduced to that for the AGCM/Physics, with the other two model components and interprocessor communications running in parallel.<<ETX>>