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J. Sirutis and K. Miyakoda

Abstract

Four packages of subgrid scale (SGS) physics parameterization are tested by including them in a general circulation model and by applying the four models to 1-month forecasts. The four models are formulated by accumulatively increasing the elaboration and the sophistication of the physics. The first is the reference model (the A-physics); the second model (the E-physics) uses the Monin–Obukhov similarity theory for the fluxes of surface boundary layer, the turbulence closure scheme for the fluxes in the entire atmosphere, and subsurface soil heat conduction; the third model (the F-physics) replaces the cumulus parameterization by the Arakawa–Schubert method; and the fourth model (the FM-physics) enhances the SGS orography. One-month integrations are performed for eight January cases, with each case consisting of three different forecasts. Originally the forecast performance was expected to be a stepwise improvement with the elaboration of the SGS physics from the A to the FM, but the forecast results do not show up in such a simple way. The impact of these processes on the 1-month integration is subtle and yet significant. The superiority of the F-model over the A- and the E-models is evident in the last 10 days of the 1-month forecasts, though the performance of the E-model is consistently good, in comparison with the other models, in terms of root-mean-square (rms) error of geopotential height. It is likely that 80% condensation criterion in the E (instead of 100%) is at least partly responsible for the forecast deterioration in the last 10 days, compared with the F. The FM-model gives the lowest rms error, but the predicted transient eddies are extremely low, probably due to the excessively enhanced orography. The simulated global precipitation patterns are presented for the different models, and the drawbacks are discussed. The F- and the FM-models produce spatially smooth distribution of tropical rainfall. The 30-day forecast performance appears to be more sensitive to the initial conditions, rather than the SGS physics. The systematic errors in all of the models are substantial in magnitude, though they vary with the SGS physics.

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W. Stern and K. Miyakoda

Abstract

Assuming that SST provides the major lower boundary forcing for the atmosphere, observed SSTs are prescribed for an ensemble of atmospheric general circulation model (GCM) simulations. The ensemble consists of 9 “decadal” runs with different initial conditions chosen between 1 January 1979 and 1 January 1981 and integrated about 10 years. The main objective is to explore the feasibility of seasonal forecasts using GCMS. The extent to which the individual members of the ensemble reproduce the solutions of each other (i.e., reproducibility) may be taken as an indication of potential predictability. In addition, the ability of a particular GCM to produce realistic solutions, when compared with observation, must also be addressed as part of the predictability problem.

A measure of reproducibility may be assessed from the spread among ensemble members. A normalized spread index, σns, can be defined at any point in space and time, as the variability of the ensemble (σn) normalized by the climatological seasonal variability (σs). In the time mean it is found that the reproducibility is significantly below unity for certain regions. Low values of the spread index are seen generally in the Tropics, whereas the extratropies does not exhibit a high degree of reproducibility. However, if one examines plots in time of seasonal mean σns for the U.S. region, for example, it is found that for certain periods this index is much less than unity, perhaps implying “occasional potential predictability.” In this regard, time series of ensemble mean soil moisture and precipitation over the United States are compared with corresponding observations. This study reveals some marginal skill in simulating periods of drought and excessive wetness over the United States during the 1980s (i.e., the droughts of 1981 and 1988 and the excessive wetness during the 1982/83 El Niño). In addition, by focusing on regions of better time-averaged reproducibility-that is, the southeast United States and northeast Brazil-a clearer indication of a relationship between good reproducibility and seasonal predictability seems to emerge.

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K. Miyakoda and A. Rosati

Abstract

Tests of several interface conditions in a one-way nested grid model were undertaken, where the ratio of grid size for the coarse mesh in the large domain and the fine mesh in the small domain was 4:1. The interface values for all parameters are specified by the solutions of the larger domain model, although they are modified in some cases. Scheme A includes a “boundary adjustment” and the consideration of mountain effect for the surface pressure along the interface. Scheme B uses, in addition to Scheme A, a “radiation condition” at the outward propagation boundaries. Scheme C uses viscous damping along five rows adjacent to the border lines in addition to Scheme A. The solutions for the fine-mesh models obtained by these schemes are compared quantitatively with the solution of a control model. The results show how quickly the effect at the interface propagates into the interior. The proper treatment of the mountain effect on the surface pressure along the interface, and the boundary adjustment are important for obtaining reasonable solutions. Schemes A, B and C are all acceptable, though not entirely satisfactory. Scheme B was useful in reducing the false reflection at the interface. Scheme C gave smooth fields of predicted variables, but false reflection sometimes occurred. A combination of these conditions optimally chosen was applied to a 34 km mesh model for a domain covering the whole mainland of the United States. The resulting maps of the time integration show the formation of a front and the detailed structure of intense rainbands associated with the front.

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A. Navarra and K. Miyakoda

Abstract

Anomally models based on a spectral general circulation model (GCM) are formulated and applied to study of low-frequency atmospheric variability in the extratropics, and long-range forecasting research. A steady linear version of the anomaly model is treated by a matrix method. This model consists of nine vertical levels, 15 wave rhomboidal truncation, primitive equation system, and a fixed basic state, which is three-dimensionally variable. The matrix to be handled is extremely large, but can be solved using Krylov's technique. The solutions represent various teleconnection patterns known in the observed atmosphere. The sensitivity of the response of this anomaly model to zonally variability of the temporally fixed basic fields and to the geographical position of tropical heatings is investigated. The solutions of the steady linear anomaly model are compared with those of the original GCM, revealing that there are a few similarities among the solutions, but considerable discrepancies are also evident. A time-dependent nonlinear anomaly model is applied to further investigate the discrepancy. It appears that transient are crucial for explaining the disagreement, although the study with the time-dependent anomaly model is preliminary.

A noteworthy aspect of the overall approach is that the anomaly models are derived, with only small modifications, from the full GCM, and therefore, their relationship can be readily investigated. It is concluded that the steady linear model may be used as a diagnostic tool for investigating the characteristics of the full GCM and the dynamics of a particular state of the atmosphere. However, caution is needed when there is a significant role played by transient eddies, and in the treatment of tropical Rayleigh friction.

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K. Miyakoda and J. Sirutis

Abstract

The capability of blocking prediction is investigated with respect to four models of different subgrid scale parameterization packages, which were described in Part I. In order to assess the capability, blocking indices are defined, and threat and bias scores are set up for the predicted blocking index against the observation. Applying this evaluation scheme to the dataset of one-month forecasts for eight January cases, we conduct a study on the performance of blocking simulation.

First, it is immediately disclosed that the systematic biases in this forecast set are overwhelmingly large, so that the blocking index has to be adjusted to this bias. One, of the major issues, suggested by Tibaldi and Molteni, is whether the systematic bias is generated by the failure of blocking forecasts. Overall, this study supports this assertion, despite the different definitions of blocking. The study also reveals that the A-model is inferior to the other three models, such as the E-model, with regard to blocking forecasts. The reason for this is that the E-model, for example, which includes turbulence closure parameterization, appears to provide an adequate conversion of low-frequency eddy potential to kinetic energy, and thereby produces a more reasonable amount of standing eddies related to the persistent ridges. It is also pointed out that the blocking activity in the winter Northern Hemisphere is manifested by a distinct subpolar peak in the meridional distribution of standing eddy kinetic energy. The E-model tends to generate a well-defined peak of this energy distribution. All models are deficient in expanding the zonal mean westerlies to higher latitudes, particularly the A-model. In this connection, a hypothesis is postulated on a precondition for blocking: the upstream westerlies prior to the onset have to be displaced relatively at lower latitude. In the successful cases of blocking forecast, the upstream westerlies at 40°–60°N are relatively weaker than those in the unsuccessful cases.

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A. Rosati and K. Miyakoda

Abstract

A general circulation model (GCM) of the ocean that emphasizes the simulation of the upper ocean has been developed. This emphasis is in keeping with its future intent, that of an air-sea coupled model. The basic model is the primitive equation model of Bryan and Cox with the additions, of optional usage, of the Mellor-Yamada level 2.5 turbulence closure scheme and horizontal nonlinear viscosity. These modifications are intended to improve the upper ocean simulations, particularly sea surface temperature and heat content. The horizontal grid spacing is 1° latitude × 1° longitude and is global in domain. The equatorial region between 10°N and 10°S is further refined in the north–south direction to ⅓° resolution. There are 12 vertical levels, with six levels in the top 70 m. The model incorporates varying bottom topography.

Prior to coupling the ocean model to an atmospheric GCM, experiments have been carried out to determine the ocean GCM's performance using atmospheric forcing from observed data. The data source was the National Meteorological Center twice daily 1000 mb analysis for winds, temperature, and relative humidity for 1982 and 1983. From these data, wind stress and total heat flux were calculated from bulk formulas and used as surface boundary conditions for the ocean model.

The response of the ocean GCM to mixing parameterization schemes and frequency of atmospheric forcing have been examined. In particular, the use of constant eddy coefficients for both horizontal and vertical mixing (A-model) versus nonlinear horizontal viscosity and turbulence closure schemes (E-model) have been examined, along with comparisons of monthly mean versus 12-hourly forcing. It was found that, in general, the E-physics produces a more realistic mixed-layer structure as compared to A-physics. Using the monthly mean values produces sea surface temperatures that are too warm, presumably because the evaporative flux, which is proportional to the wind speed, is underestimated. The 12-h forcing improves appreciably both the A and E model since the heat flux is better represented; the E-case shows an even greater improvement due to its sensitivity to wind stirring. The near surface heat budget, along with more traditional variables, is examined for a short period during the 1982–83 El Niño event. These results are encouraging considering the many possible sources of error, including those in forcing data, initial conditions, radiative fluxes, and bulk exchange coefficients.

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K. MIYAKODA and L. UMSCHEID JR.

Abstract

The effect of an artificial lateral boundary (the wall) at the Equator on a simulated atmospheric circulation was studied numerically. By comparing the solutions of two 30-day integrations of a global model with and without the wall, we found that the discrepancies of the wind and temperature at the middle and high latitudes became appreciable at approximately 8 days and serious at approximately 12 days. This suggests that the wall (hemispheric) model may be applied as a forecast model for a maximum of about 12 days. The disagreement in the wind between the two cases starts just below the tropopause level at the Equator and spreads toward the higher latitudes. Eventually, the middle latitudes respond to this equatorial effect, and the disagreement is amplified to the natural variability level. Insertion of the wall considerably increases the condensation of water vapor in the Tropics for the winter hemisphere; the reverse is true for the summer hemisphere. The result is that, in the winter hemisphere, the tropical troposphere and the stratosphere are cooler and the higher latitude troposphere is warmer in the wall case than in the control case. The opposite is true for the summer hemisphere.

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K. Miyakoda and R. F. Strickler

Abstract

A diagnostic analysis and an appraisal of the precipitation calculation by the GFDL (Geophysical Fluid Dynamics Laboratory) 1967 version prediction model are presented, using two-week forecasts of 12 January and 12 July cases. The geographical distribution of predicted rainfall, moisture, and snow over the Northern Hemisphere and the contiguous United States was investigated in comparison with climatological maps published by other authors. The agreement of precipitation and dew-point temperature is marginal. The major causes for the deficiencies are 1) a specification of excessive soil moisture over land, 2) probably an improper treatment of moisture diffusion associated with topography, and 3) an inadequate rain generation process in the model. However, the predicted snow distribution over the United States was reasonable.

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K. Miyakoda, J. Sirutis, and J. Ploshay

Abstract

A series of one-month forecasts were carried out for eight January cases, using a particular prediction model and prescribing climatological sea-surface temperature as the boundary condition. Each forecast is a stochastic prediction that consists of three individual integrations. These forecasts start with observed initial conditions derived from datasets of three meteorological centers. The forecast skill was assessed with respect to time means of variables based on the ensemble average of three forecasts. The time or space filter is essential to suppress unpredictable components of atmospheric variabilities and thereby to make an attempt at extending the limit of predictability. The circulation patterns of the three individual integrations tend to be similar to each other on the one-month time scale, implying that forecasts for the 10 day (or 20 day) means are not fully stochastic. The overall results indicate that the 10-day mean height prognoses resemble observations very well in the first ten days, and then start to lose similarity to real states, and yet there is some recognizable skill in the last ten days of the month. The main interests in this study are the feasibility of one-month forecasts, the adequacy of initial conditions produced by a particular data assimilation, and the growth of stochastic uncertainty. An outstanding problem turns out to be a considerable degree of systematic error included in the prediction model, which is now known to be “climate drift.” Forecast errors are largely due to the model's systematic bias. Thus, forecast skill scores are substantially raised if the final prognoses are adjusted for the model's known climatic drift.

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K. Miyakoda, J. Sheldon, and J. Sirutis

Abstract

The GATE analysis was repeated utilizing the full GATE data set in the delayed mode and a revised four-dimensional analysis procedure. The resulting maps wore compared with maps of other author. Based on the new analysis, macroscale circulation features for the tropical African continent and Atlantic Ocean region were calculated, and other characteristic phenomena of this area were investigated. The easterly waves, in particular, were studied with respect to their formation, propagation, associated condensation, and possible conversion to hurricanes. It was possible to trace nine distinct easterly waves throughout their entire life history, and the analyzed tracks of thew easterly waves agreed quite well with the subjective analyses of Sadler and Oda (1978). The time sequences of precipitation over the GATE A/B-array obtained by the present analysis and by satellite estimates were compared with some success.

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