Spectral Primitive Equation Model Research Articles

What Constrains Spread Growth in Forecasts Initialized from Ensemble Kalman Filters?

Abstract The spread of an ensemble of weather predictions initialized from an ensemble Kalman filter may grow slowly relative to other methods for initializing ensemble predictions, degrading its skill. Several possible causes of the slow spread growth were evaluated in perfect- and imperfect-model experiments with a two-layer primitive equation spectral model of the atmosphere. The causes examined were the covariance localization, the additive noise used to stabilize the assimilation method and parameterize the system error, and the model error itself. In these experiments, the flow-independent additive noise was the biggest factor in constraining spread growth. Preevolving additive noise perturbations were tested as a way to make the additive noise more flow dependent. This modestly improved the data assimilation and ensemble predictions, both in the two-layer model results and in a brief test of the assimilation of real observations into a global multilevel spectral primitive equation model. More generally, these results suggest that methods for treating model error in ensemble Kalman filters that greatly reduce the flow dependency of the background-error covariances may increase the filter analysis error and decrease the rate of forecast spread growth.

Modification of the Baroclinic Instability Associated with Positive and Negative Arctic Oscillation Index: A Theoretical Proof of the Positive Feedback

The modification of the baroclinic instability associated with positive and negative Arctic Oscillation Index (AOI) is theoretically investigated using a linearized 3D spectral primitive equation model. The linear instability analysis shows that the most unstable Charney mode MC changes its structure to intensify (weaken) the polar jet by the eddy momentum flux associated with the positive (negative) AOI. More importantly, the meridionally dipole Charney mode M2 is modified into the monopole Charney mode M1 to transport eddy momentum flux northward under the positive AOI condition. It is found that this modification is essential to intensify the polar jet during the AOI positive phase. Hence, we have theoretically confirmed that there are positive feedbacks between the baroclinic instability waves and the Arctic Oscillation characterized by the intensity of the polar jet.

Similarities and Differences among the South Indian Ocean Convergence Zone, North American Convergence Zone, and Other Subtropical Convergence Zones Simulated Using an AGCM

I examined features of the South Indian Ocean convergence zone (SICZ) and the North American convergence zone (NACZ) simulated using an atmospheric general circulation model (AGCM; T106L56: a spectral primitive-equation model with 56 σ levels and triangular spectral truncation at wave-number 106). The 24-year model integration from 1979 to 2002 was constrained by observed sea-surface temperature and sea-ice distribution. I selected a typical case for each zone (SICZ and NACZ) from the 1985-1996 simulation. The AGCM properly simulates African and Indian Ocean monsoon circulation and precipitation. The precipitation zone of the SICZ extends southeastward from the southeastern part of Africa to the southwestern rim of the Mascarene high during Southern Hemisphere summer. North American summer monsoon circulation and precipitation were also correctly reproduced. The precipitation zone of the NACZ extends northeastward along the southeastern coast of North America to the northwestern rim of the Bermuda high during the North American summer monsoon season. I compared the features of the simulated SICZ and NACZ with features of the South Atlantic convergence zone (SACZ) and the Baiu frontal zone (BFZ) simulated using the same AGCM. The SACZ, SICZ, NACZ, and BFZ were characterized as subtropical convergence zones (STCZs) and are commonly sustained along their respective subtropical anticyclones that form over the ocean east of continents. However, their geographical environments differ significantly. Whereas the respective cool oceans at the poleward sides of the SACZ and SICZ provide significant baroclinicity for the SACZ and SICZ, the respective hot continents to the poleward sides of the BFZ and NACZ create weak baroclinicity for the BFZ and NACZ.

Differences between the Intense Precipitation Associated with Subsynoptic-Scale Baiu Frontal Depression Simulated by an AGCM and Described in Observational Studies

We compared features of the precipitation associated with subsynoptic-scale Baiu frontal depression simulated by an atmospheric general circulation model (AGCM; T106L56: a spectral primitive-equation model with 56 σ levels and triangular spectral truncation at zonal wavenumber 106) with the features had been described in observational studies. The 21-year model integration from 1979 to 1999 was constrained by observed sea-surface temperature and sea-ice distribution. As typical examples, this paper examines simulation cases for June 1991.Comparisons with past observational results showed that the AGCM properly simulated the Baiu front and the Baiu frontal precipitation in the averaged elds for a 15-day period in June 1991. However, the area of the largest 1-hour precipitation is simulated at about 500 km northeastward from the area of the largest averaged precipitation. We also found signicant differences between the simulations and the ob servations when we examined the daily precipitation and the maximum 1-hour precipitation for each day simulated by the model. While precipitation associated with subsynoptic-scale Baiu frontal depressions was relatively well simulated, intense precipitation in the trailing portion of these depressions was signi cantly underestimated.The result of the present study indicates that the ability of the AGCM to reproduce such extreme pre cipitation events must be examined in detail by comparing simulated daily and hourly precipitation with observational studies as well as by statistical analyses.

Similarity and Difference between the South Atlantic Convergence Zone and the Baiu Frontal Zone Simulated by an AGCM

Features of the South Atlantic convergence zone (SACZ) and the Baiu frontal zone (BFZ) simulated by an AGCM (T106L56: a primitive equation spectral model which has 56 s-levels and triangular spectral truncation at wave-number 106) are studied. The 24-year integration, from 1979 to 2002, by the model constrained by observed sea-surface temperature and sea-ice distribution, is used for this study. The detailed analysis is made for the typical case of SACZ and BFZ selected from data in 1985-1996.The South American monsoon circulation and the associated precipitation are reasonably reproduced. The precipitation zone of the SACZ, which extends southeastward from the southern part of Brazil to the South Atlantic, is sustained along the southwestern rim of the South Atlantic subtropical anticyclone during the southern summer. The Asian monsoon circulation and the associated precipitation are also reasonably reproduced. The precipitation zone of the BFZ, which extends northeastward from the southern part of China to Japan is formed along the northwestern rim of the North Pacific subtropical anticyclone in June.Many common features are found in the simulated SACZ and BFZ, in regard to the frontal structure and the associated synoptic- and meso-α-scale disturbances, and the relation to the respective subtropical anticyclone. However, an important difference is seen between their geographical environments. The cold south Atlantic in the poleward side of the SACZ provides the significant baroclinicity for SACZ, while the existence of warm land mass to the poleward side of the BFZ brings on weak baroclinicity of the BFZ. Another significant difference is seen in the large-scale environmental condition. The SACZ is influenced, in essence, by the trade winds and the subtropical anticyclone over the South Atlantic. The BFZ is strongly influenced by the moisture transport due to the Indian monsoon westerly and variation of the North Pacific subtropical anticyclone related to the convective activity over the western North Pacific.

Polar-Air Outbreak and Air-Mass Transformation over the East Coast of Asia as Simulated by an AGCM

The present report studies features of the polar-air outbreak and the associated air-mass transformation over the east coast of Asia simulated in an AGCM (T106L52: a primitive equation spectral model, which has 52 σ-levels and triangular spectral truncation at wave-number 106) that used climatological SST in comparison with the features described in several observational studies.The large-scale circulations are properly reproduced for January Y07 (the 7th year after the spin up integration). A typical case of polar-air outbreak simulated in January is studied in detail. During the polar-air outbreak, cyclonic northwesterly polar-air streams over the Sea of Japan and the northwestern Pacific, and anticyclonic northeasterly polar-air streams over the East China Sea and the South China Sea are reasonably simulated. During the polar-air outbreak, the sensible and latent heat fluxes simulated over the Sea of Japan reach to ∼150 and ∼250 W m−2, while those over the East China Sea reach to ∼75 and ∼250 W m−2, respectively. These simulated fluxes agree with the fluxes evaluated in observational studies. The multi-layer structure of the transformed air-mass, including the unstable surface layer, the subcloud layer, and the cloud layer capped by the stable layer, simulated over the coastal areas of Asia is consistent with the observations for the cases of typical polar-air outbreak.However, the precipitation over the coastal areas of Japan simulated during the polar-air outbreak is significantly small as compared with the observation. This is due to the insufficient horizontal resolution of the T106L52 to simulate mesoscale circulation systems which induce snowfalls. In addition, the duration of the polar-air outbreak simulated in the AGCM is relatively short as compared with that in the real atmosphere, since the anticyclone over the continent tends to extend eastward in the model.

Analysis of Recent Extreme Events Measured by the Barotropic Component of the Atmosphere

In this study, recent extreme events over the Northern Hemisphere are quantified in terms of an energy norm of the anomaly of the state variables. Since persistent low-frequency variabilities are characterized, in most cases, by their barotropic structure, the energy norm of the anomaly is measured for the barotropic component of the atmosphere. The norm is then normalized by its climatology to assess the abnormality of the extreme events. In this study the norm is evaluated in the framework of the 3-D spectral primitive equation model to assess the external forcing as well as the state variabiles. According to the analysis of the monthly mean anomaly data for 50 years from 1953 to 2002, the most abnormal months appear to be Apr. 1997, Jan. 1963, Jan. 1977, Mar. 1983, Apr. 1967, Feb. 1989, and Jan. 1989. Those are well known abnormal months in the past studies. In this study, the top 3% of the extreme events are listed as the abnormal months. The quantification of the abnormality is further extended to the external forcing of the barotropic component of the atmosphere and also to the SST anomaly. It is found that only 3 cases of the abnormal months (Jan. 1963, Feb. 1989 and Jan. 1989) are associated with the abnormal external forcing, and the rest of the abnormal months are associated with the non-abnormal external forcing. Likewise, it is found that most of the abnormal external forcing result in a non-abnormal month. The SST forcing anomaly is not directly related to the external forcing. It is concluded from the result that more than 80% of the abnormal months are induced by the natural variability of the barotropic component of the atmosphere under the non-abnormal external forcing for the last 50 years. For the monthly time scale, the chaotic nonlinear behavior is quantitatively larger than a linear response to the external forcing or that to the SST anomaly.

Polar Low Genesis over the East Coast of the Asian Continent Simulated in an AGCM

This report shows genesis of a polar low by an AGCM without specialized initial condition. A case of polar low genesis over the eastern coast of the Asian Continent, simulated in the seasonally varying climatological SST run by an AGCM (T42L52, primitive equation spectral model with 42 wave-number and 52 layers), is presented. A polar low simulated in January 22-23 of the fourth year (Y04) integration after the 10-year period of the spin-up is studied in comparison with polar lows described in several observational studies. In January Y04, large-scale circulation systems, such as Asian winter monsoon, extratropical cyclones and upper cold lows are reasonably simulated. A typical polar low is formed in January 22 over the coastal sea area ∼1500 km west of the major extratropical cyclone that developed over the Northwestern Pacific Ocean, under the influence of a short wave trough, which propagates along the rim of an upper cold low. The polar low genesis takes place first in connection with a deepening of the surface trough, which extends westward from the major cyclone. The deepening of the surface trough in the zone of strong low-level thermal gradient over the coastal sea area suggests the important role of the low-level baroclinicity for the polar low genesis. The strong heating due to the energy supply from the sea surface contributes for the genesis of the polar low through the decreasing of the vertical stability, and the sustaining thermal gradient. Meanwhile, the heating around 700 hPa associated with the precipitation concentrated within the polar low indicates the influence of the condensation heating for the development of the polar low. Aforementioned various processes contribute together to generate and develop the polar low. The structure and the evolution process of the simulated polar low are consistent with those of the observed polar lows. It is concluded that the realistic polar low genesis takes place in the model, when the large-scale phenomena such as the upper cold low, the short-wave trough, parent major cyclone and polar air outbreak are reasonably simulated. The present study is significant in presenting an AGCM simulation of a polar low for the first time.

Synoptic- and Meso-.ALPHA.-Scale Variations of the Baiu Front Simulated in an AGCM

The present paper studies features of the Baiu front and associated precipitation systems simulated in the climatological SST run by an AGCM T106L52 (primitive equation spectral model, the maximum wave-number of 106, with 52 vertical levels) in comparison with the features found in observational studies. The Baiu front is properly simulated in 1-20 June Y07 (the 7th year after the spin-up integration). In this “Baiu phase”, the large-scale circulation systems, such as the upper cold lows and blocking ridge in the northern latitudes, westward extending Pacific subtropical anticyclone, monsoon westerly, and subtropical jet stream, are simultaneously maintained. Under this condition, the synoptic- and meso-ascale variations of the Baiu front are simulated. The southward shift of the front occurs when a synopticscale cyclone develops in association with a westerly trough. When westerly troughs are inactive, weak subsynoptic-scale depression is formed in the frontal zone. A few meso-α-scale precipitation systems are generated in the trailing portion of the preceding depression, and form a “Baiu precipitation system family” with a length of∼2000 km. This indicates that the simulation of the realistic Baiu front depends on the maintenance of proper large- and synoptic-scale circulation systems. However, the Baiu precipitation zone disappears in the early July, as the west-east elongating barotropic anticyclone, which is usually seen over Japan in August in the real atmosphere, is situated over ∼37°N.

Vertical Structure of Convective Heating and the Three-Dimensional Structure of the Forced Circulation on an Equatorial Beta Plane*

In this paper, the three-dimensional structure of the thermally forced atmosphere on an equatorial b plane is investigated. Special emphasis is placed on the relations between the vertical structure of heating and the horizontal structure of the forced response. By solving the vertical eigenvalue‐eigenfunction problem in a vertically semi-infinite domain, the authors obtain a complete set of vertical eigenfunctions that includes a single barotropic (external) mode and a continuous spectrum of baroclinic (internal) modes. These eigenfunctions are used to decompose vertical heating profiles for two types of tropical heating: 1) deep heating representing the convective plume (CP) heating and 2) shallow heating representing mature cloud (MC) cluster heating. By examining the spectral energy density of the heating profile, the contributions of each vertical mode (spectral interval) to the overall structure are explored for each case, and the difference between the responses to these two profiles of heating is discussed. A dry spectral primitive equation model of the atmosphere is employed to verify the analytical results. The results from both the analytical approach and the numerical simulations are consistent in showing that the vertical structure of the heating is fundamental to the structure of the forced response. The CP is deep relative to the MC. Thus, the CP projects onto the vertical eigenfunctions of relatively larger equivalent depth more so than does the MC. As a result, the CP-forced signals propagate away from the heat source much faster than those forced by the MC. Hence, when the atmosphere is subjected to the same linear dampings (Rayleigh friction and Newtonain cooling), the spatial (mainly in the horizontal) decay rate of the CP-forced signals is significantly smaller than that of the MC-forced signals, and the CP-forced signals extend farther. To what extent a shallow-water system of a specified vertical mode (e.g., the Gill model) can approximate the three-dimensional response is also examined. Results show that the effective gravity wave speed of the multimode system varies greatly with location. Hence, it is extremely difficult to select a globally suitable equivalent depth so that a one-mode shallow-water system can approximate the spatially three-dimensional structure of the response to a given heating.

Rayleigh Friction, Newtonian Cooling, and the Linear Response to Steady Tropical Heating*

A series of studies are performed to examine the response of the tropical atmosphere to a prescribed steady, large-scale, elevated heat source (i.e., a region of persistent precipitation). Special emphasis is placed on the surface wind response in two idealized cases in which dissipation is achieved exclusively by Rayleigh friction or by Newtonian cooling. Starting from the linearized equations on an equatorial beta plane, theoretical arguments are presented that suggest there are qualitative differences in the solutions of these two models. A dry spectral primitive equation model of the atmosphere is employed and confirms the results obtained from the analytical studies. The results from both the analytical study and the numerical simulations are consistent in showing that Rayleigh friction and Newtonian cooling play totally different roles in the tropical atmosphere. Newtonian cooling homogenizes the atmospheric motion in the vertical direction, and a strong, vertically uniform wind is found below the base of the heat source. When Rayleigh friction dominates, the circulation driven by the heat source is confined to the layer where the heat source is located. It is also shown that a strong Hadley circulation is associated with reasonable strong Rayleigh friction, but not with Newtonian cooling alone. Finally, the numerical solution is found for the case where Newtonian cooling acts uniformly in the vertical and Rayleigh friction is included in the lower atmosphere to mimic crudely the dissipation of momentum in the boundary layer. The introduction of the simple boundary layer dramatically reduces the surface circulation that was supported in the Newtonian cooling alone case. Together these results suggest a significant surface circulation is unlikely to be driven by an elevated heat source if it resides above the top of the boundary layer.

A Simple GCM Based on Dry Dynamics and Constant Forcing

A dry spectral primitive equation model is used to simulate the global atmospheric circulation during northern winter. The resolution is T21 in the horizontal, with five equally spaced sigma layers. The only additional terms in the equations are those describing linear damping, linear scale-selective diffusion, and time-independent forcing. The damping and diffusion act on temperature and momentum. The forcing acts on all prognostic variables. It is calculated objectively from the tendencies produced when the model is initialized with a long time series of observational analyses, and separated into components to ease comparison with time-dependent perturbation models. The simulated climate reproduces observed features of the circulation, both time-mean fields and transient-eddy covariances, with remarkable success. The accurate simulation of tropical divergent flow is a particularly useful result. The main deficiencies are an underestimation of transient-eddy kinetic energy and a lack of transient activity in the Southern Hemisphere. In an attempt to reduce the forcing of divergent flow, a modified vertical scheme and modified forcing functions based on a calculation of balanced flow are introduced. The former still has significant divergence forcing and makes little difference to the final result. The latter tends to give solutions that are unrealistic in the Tropics. The model’s sensitivity to variations in forcing functions and damping parameters is further explored. The Southern Hemisphere transient behavior can be improved by boosting the local forcing of baroclinicity by up to a factor of 2, and a simulation of the Southern Hemisphere winter is relatively successful. The applications and limitations of such a simple fast-running climate model with a relatively realistic simulated climate are discussed.

A life-cycle of nonlinear baroclinic waves represented by a simple 3-D spectral model

In this study, a life-cycle experiment for baroclinic disturbances is carried out for Simmons and Hoskins' 45° jet by integrating a three-dimensional spectral primitive equation model. The utility of the spectral representation in the vertical direction is examined for a fully nonlinear well-established phenomenon. The energy evolution and corresponding energy transformation are analyzed in the framework of the baroclinic-barotropic decomposition of atmospheric energy. According to the result, the initial perturbations of n= 6 grow exponentially drawing zonal baroclinic energy. This early evolution is reasonably described by linear baroclinic instability of the 45° jet. Both of baroclinic energy and barotropic energy of n= 6 increase simultaneously since the unstable mode maintains its consistent structure to grow. The energy flow is characterized as from zonal baroclinic energy via eddy baroclinic energy to eddy barotropic energy. These energy transformations are also synchronized since they are proportional to the eddy energy levels in the linear framework. When the waves reach the finite amplitude, the barotropic conversion increases, tranferring eddy barotropic energy toward zonal barotropic energy. It is shown by the result that the zonal barotropic energy increases when the waves decay, and the zonal jet is accelerated so that the structure becomes more barotropic. It is found by this study that the important baroclinic–barotropic interactions are coupled with baroclinic instability rather than the barotropic conversion. The results are consistent with previous studies. Therefore, we confirm also that the vertical spectral representation is applicable to simulate the nonlinear phenomenon. DOI: 10.1034/j.1600-0870.1995.00113.x

Approximations and Sensitivity Experiments with a Baroclinic Semi-Lagrangian Spectral Model

This study examines the extensions that have been made to a basic semi-Lagrangian semi-implicit multilevel spectral primitive equation model in preparing it for use as an operational data assimilation and medium-range forecast model. The authors present an optimized formulation using accurate approximations to alternate trigonometric calculations for finding the upstream positions and performing the transformations for treating a vector form of the equation of motion in spherical geometry. The impact of the order of accuracy of the interpolators used in the semi-Lagrangian algorithms is also examined. It is shown that the recommended approximations have no significant meteorological consequences, but in a typical model step they reduce the time spent in the semi-Lagrangian calculations from about 50% to about 30%. Through a series of sensitivity tests, the authors establish the viability of the semi-Lagrangian semi-implicit method for spectral models with a more comprehensive physical parameterization package, and on a wider range of meteorological situations than in the previous study. First of all it is confirmed that even in these global runs with full physical parameterizations the sensitivity to the conversion from an Eulerian to a semi-Lagrangian formulation is acceptably small. In time truncation error tests with a T79 horizontal resolution it is found that, on average and with full physics, 30 min is about the upper limit for the time step based on acceptable time truncation errors. Since the Courant-Friedrichs-Lewy limit for the corresponding Eulerian model is about 10 min and the overhead of the semi-Lagrangian calculations is less than 30% per time step, there is a significant gain in efficiency by using the optimized semi-Lagrangian formulation. Results are also presented showing the improvement in the accuracy of the, 5-day forecasts obtained by raising the model top and by increasing the horizontal resolution.

Control Of Gravitational Oscillations in Variational Data Assimilation

Abstract Variational four-dimensional data assimilation, combined with a penalty method constraining time derivatives of the surface pressure, the divergence, and the gravity-wave components is implemented on an adiabatic version of the National Meteorological Center's 18-level primitive equation spectral model with surface drag and horizontal diffusion. Experiments combining the Machenhauer nonlinear normal-mode initialization procedure and its adjoint with the variational data assimilation are also presented. The modified variational data-assimilation schemes are tested to assess how well they control gravity-wave oscillations. The gradient of a penalized cost function can be obtained by a single integration of the adjoint model. A detailed derivation of the gradient calculation of different penalized cost functions is presented, which is not restricted to a specific model. Numerical results indicate that the inclusion of penalty terms into the cost function will change the model solution as desired. Th...

The impact of doubled CO2 on the energetics and hydrologic processes of mid-latitude transient eddies

Atmospheric transient eddies contribute significantly to mid-latitude energy and water vapor transports. Changes in the global climate, as induced by greenhouse enhancement, will likely alter transient eddy behavior. Unraveling all the feedbacks that occur in general circulation models (GCMs) can be difficult. Here, we isolate the transient eddies from the feedbacks and focus on the response of the eddies to simulated zonal-mean climate change that results from CO2-doubling. Using a primitive-equation spectral model, we examine the impact of such climate change on the life cycles of transient eddies. We compare transient eddy behavior in experiments with initial conditions that are given by the zonal-mean climates of GCMs with current and doubled amounts of CO2. The smaller meridional temperature gradient in a doubled CO2 climate leads to a reduction in eddy kinetic energy, especially in the subtropics. The decrease in subtropical eddy energy is related to a substantial reduction in equatorward flux of eddy activity during the latter part of the life cycle. The reduction in equatorward energy flux alters the moisture cycle. Eddy meridional transport of water vapor is shifted slightly poleward and subtropical precipitation is reduced. The water vapor transport exhibits a relatively small change in magnitude, compared to changes in eddy energy, due to the compensating effect of higher specific humidity in the doubled-CO2 climate. An increase in high-latitude precipitation is related to the poleward shift in eddy water vapor flux. Our experiments indicate that the simulation of climate change in eddy water vapor transport and eddy-generated precipitation is sensitive to model resolution and requires a minimum truncation of rhomboidal 30 in a spectral model. Surface evaporation amplifies climatic changes in water vapor transport and precipitation in our experiments.

Variational Data Assimilation with an Adiabatic Version of the NMC Spectral Model

Abstract Variational four-dimensional (4D) data assimilation is performed using an adiabatic version of the National Meteorological Center (NMC) baroclinic spectral primitive equation model with operationally analyzed fields as well as simulated datasets. Two limited-memory quasi-Newton minimization techniques were used to iteratively find the minimum of a cost function, with the NMC forecast as a constraint. The cost function consists of a weighted square sum of the differences between the model forecast and observations over a time interval. In all the experiments described in this paper, observations are available for all degrees of freedom of the model. The derivation of the adjoint of the discretized adiabatic NMC spectral model is presented. The creation of this adjoint model allows the gradient of the cost function with respect to the initial conditions to be computed using a single backward-in-time integration of the adjoint equations. As an initial evaluation of the variational data-assimilation ...

Simulation of a winter circulation over India using a global spectral model

A primitive equation spectral model has been successfully integrated for five days starting with the initial data of 26 February 1982. The geopotential heights and wind strengths are well predicted up to day 3. The 24-hour accumulated precipitation of the model reasonably agrees only up to two days, with the observed rainfall under the influence of western disturbance that prevailed over Afghanistan, Pakistan and north India till 3 March 1982. For one day global integration of the model, the CPU time requirement in Cyber 170/730 is about one hour.

A Simple Implementation of the Semi-Implicit Time Scheme in a Primitive Equation Spectral Model

A Simple Implementation of the Semi-Implicit Time Scheme in a Primitive Equation Spectral Model

A primitive equation spectral model for study of planetary-scale atmospheric flow

The developments in numercial modelling for the atmosphere in recent years have shown that the spectral models are well suited for general circulation studies of the planetary-scale atmosphere. The use of transform method developed during last decade has made the spectral models comparable in computational speed with the grid point models.
 A global primitive equation model using spectral formulation is under development in the India Meteorological Department. The spectral representation is in terms of a truncated series of surface spherical harmonic functions. The computational facllitiesavaila1Jle at present do not perlt1it the development of a high-resolution model. The horizontal resolution, therefore, is limited to six Fourier waves in the zonal direction, and six zeros of Legendre functions in the meridional direction. In the vertical, the model has three layers in sigma coordinate.
 The model is in Its early stage of development. The model formulation, the results of some experiments performed with It and the future development plans are discussed in this paper.