There is a new global model (OLAM) developed by two outstanding talented scientists, Professor Roni Avissar and Dr. Robert L. Walko which provides original and important tool to study the climate system. This new global model is reported on in two accepted peer-reviewed papers for the journal, Monthly Weather Review.
The first paper is
Walko, R.L., and R. Avissar, 2008: The Ocean-Land-Atmosphere Model (OLAM): Shallow Water Tests. Mon. Wea. Rev., accepted.
The abstract reads
The Ocean-Land-Atmosphere Model (OLAM) has been developed to extend the capabilities of the Regional Atmospheric Modeling System (RAMS) to a global model domain. OLAM adopts many features of its predecessor, including physical parameterizations, initialization methods, data assimilation, logic and coding structure, and I/O formats. However, its dynamic core is new and is based on a global geodesic grid with triangular mesh cells and a finite volume discretization of the full compressible Navier Stokes equations. Local mesh refinement can be activated in OLAM to cover selected geographic areas at very high resolution and hence to simulate atmospheric systems typically studied in regional models. This paper is one in a series whose purpose is to describe the formulation of OLAM and to validate the model’s performance through test results. This paper focuses on global scale dynamics that can be represented by the shallow water equations, and provides details of the spatial and temporal discretizations, which contain some unique algorithms that previously have not been applied to atmospheric models. Validation tests are performed using five standard shallow water simulations that are commonly-used benchmarks for global models. OLAM results for all tests are comparable to results from other global models.”
The summary and conclusions are
“This paper has described the formulation of the Ocean-Land-Atmosphere Model (OLAM), which is based on the Regional Atmospheric Modeling System (RAMS) but covers the global domain. Many components of RAMS were incorporated into OLAM with little modification, which saved considerable development effort. However, OLAM’s dynamic core is new and consists of (1) a global triangular-cell grid mesh with local refinement capability, (2) the full compressible nonhydrostatic Navier-Stokes equations, (3) a finite volume formulation of conservation laws for mass, momentum, and potential temperature, and (4) numerical operators that include time splitting for acoustic terms. The global domain greatly expands the range of atmospheric systems and scale interactions that can be represented in the model, which was the primary motivation for developing OLAM.
The new dynamic core includes some elements that have not previously been applied to atmospheric modeling. These include (1) a spatial discretization scheme introduced by WN1 and WN2 and previously applied to flow around an aircraft wing, (2) modifications and additions to the WN1 discretization to accommodate 3D spherical earth geometry, the Coriolis force, and a higher (second) order advection scheme, and (3) a time discretization scheme that combines time splitting with consistent use of advecting mass flux between all transported quantities. Therefore, an important purpose of this study was to subject OLAM to a series of numerical tests to determine whether the new dynamic core could successfully reproduce expected features of global dynamics.
The present study focused on the shallow water system of equations in order to evaluate how well OLAM’s new dynamic core simulates fundamental processes such as advection, gravity wave propagation, and geostrophic adjustment. The relatively simple shallow water system tests only a subset of the full OLAM dynamic core, but the numerical methods described and used in this paper are themselves a proper subset of those used for flows with vertical motion and multiple vertical levels. Therefore, the numerical simulations in this study provide a valid test of the full system. OLAM was tested with five of the seven well-known global shallow water simulations described by WI. OLAM performed each test successfully, giving results that compare well with other models and with analytic solutions where applicable. Tests at different resolution showed error convergence rates to generally lie between 1.5 and 2.0. The simulation tests give confidence that OLAM’s dynamic core is correctly formulated to represent basic features of the global circulation. A companion paper (Walko and Avissar, jointly submitted) describes and tests additional aspects of the OLAM dynamic core, focusing on vertical nonhydrostatic motion and the acoustic time-splitting scheme. We are in the process of examining OLAM’s performance with moist and radiative processes, surface water and energy exchange, and local mesh refinement. Results will be reported in forthcoming papers.”
The second paper is
Walko, R.L., and R. Avissar, 2008: The Ocean-Land-Atmosphere Model (OLAM): Formulation and Tests of the Nonhydrostatic Dynamic Core. Mon. Wea. Rev, accepted.
The abstract reads
“We describe and test the dynamic core of the Ocean-Land-Atmosphere Model (OLAM), which is a new global model that is partly based on the Regional Atmospheric Modeling System (RAMS). OLAM adopts many features of its predecessor, but its dynamic core is new and incorporates a global geodesic grid with triangular mesh cells and a finite volume discretization of the nonhydrostatic compressible Navier-Stokes equations. The spatial discretization of horizontal momentum is based on a C-staggered grid and uses a method that has not been previously applied in atmospheric modeling. The temporal discretization uses a unique form of time-splitting that enforces consistency of advecting mass-flux among all conservation equations. OLAM grid levels are horizontal, and topography is represented by the shaved-cell method. Aspects of the shaved-cell method that pertain to the OLAM discretization on the triangular mesh are described, and a method of conserving momentum in shaved cells on a C-staggered grid is presented.
The dynamic core was tested in simulations with multiple vertical model levels and significant vertical motion. The tests include an idealized global circulation simulation, a cold density current, and mountain-wave flow over an orographic barrier, all of which are well-known standard benchmark experiments. OLAM gave acceptable results in all tests, demonstrating that its dynamic core produces accurate and robust solutions.”
The summary and conclusions of the paper are
“The Ocean-Land-Atmosphere Model (OLAM) is a new global model based on RAMS that is designed to represent all scales of atmospheric flows, including mesoscale and microscale systems typically simulated in limited-area models. Accordingly, its dynamic core has been developed for nonhydrostatic compressible flow, with efficient representation of acoustic modes using numerical time splitting. This paper described the details of the OLAM dynamic core and demonstrated its performance in standard test simulations, with a particular focus on flows with significant vertical motion in a multi-level atmosphere.
The time-split temporal discretization method in OLAM is unique, and enforces mass-flux consistency of advective transport terms in all equations. The spatial discretization method, first presented in WN, is implemented and tested here for the first time in an atmospheric model, with modifications for a rotating, spherical earth with parameterized turbulent flow. Representation of topography, which is via the shaved-grid-cell method with height coordinates, also contains unique elements that relate to OLAM’s triangular mesh and to momentum conservation principles in a C-staggered grid. Specific details of these and other aspects of the OLAM dynamic core have been presented.
The following test simulations were performed in order to validate the new dynamic core: (1) the Held-Suarez (1994) experiment, which simulates an idealized general circulation of the global atmosphere, (2) the density current experiment described by Straka et al. (1993), and (3) a series of mountain-wave flow experiments described by Dudhia (1993). OLAM simulation results were compared with previous studies and showed that OLAM successfully represented each case with solutions comparable to other models.
The main purpose of developing OLAM was to produce a global-scale model that has the capability to simulate specific regions at very high resolutions so that interactions and feedbacks between all atmospheric scales can be simulated and better studied. For example, such a modeling capability is particularly useful for the simulation of severe storms (e.g., hurricanes) and/or interactions between regional and global climate. Given that objective, it is essential to demonstrate that this new model is capable of performing the typical tests carried out to evaluate the performance of global scale, mesoscale or microscale models independently. The tests performed here and in our companion paper (WA) contribute to this demonstration. Another essential capability is to run large simulations on distributed memory massively parallel computers, and to this end, an MPI-based domain decomposition of OLAM was recently implemented and is undergoing testing and evaluation.”