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James S. Boyle

Abstract

The 200-hPa divergence and streamfunction from the 30 models of the Atmospheric Model Intercomparison Project (AMIP) are compared. The data used are in the form of monthly averages and are filtered to a spatial resolution of T10, although the actual spatial resolution of the models varies from R15 to T42. The tools of the analysis are principal components analysis (PCA) and common principal components (CPC). These analyses are carried out on the 120 months of data with the climatological annual cycle removed and in the case of the streamfunction with the zonal average also removed. The AMIP period (1979–88) encompasses two El Niño–Southern Oscillation (ENSO) events (1982–83 and 1986–87), and as could be expected the ENSO characteristic response has a prominent impact in the model simulations.

The results indicate the following.

  1. The PCA of the divergence has a dominant mode that is similar for all the models and has the signature of an ENSO response. It has an east–west dipole of divergence anomaly centered on the equator in the western Pacific. The streamfunction PC analysis also exhibits an ENSO-type response as the dominant mode, but this accounts for only 8%–21% of the variance.

  2. The CPC analysis allows a direct comparison of the data from all the models on a common set of vectors. These results indicate that the models share a basic common pattern but there is a strong variation in the amplitude of the corresponding modes. There is less commonality in the higher components for the CPC streamfunction than seen in the divergence. This appears to be related to the stronger streamfunction response in the midlatitudes, which is presumably more affected by nonlinearity and intrinsic variability of the model integrations.

  3. Based on results using an ensemble of five decadal runs using the European Centre for Medium-Range Forecasts (ECMWF) GCM an estimate is made of the variation of explained variance due to intrinsic variability for a single model. It is found that in general the intermodel variation is somewhat greater than the intramodel ensemble variation using the ECMWF model.

  4. A probability density function (PDF) analysis in the space spanned by the first two CPCs for the velocity potential (which explain over 70% of the variance for all but one model) yields distinctive dynamical signatures. Some models populate a somewhat larger PDF space than others.

There is an implication that the models differ beyond the variations due to intrinsic variability in the dynamical system. Some of the models have distinctly different responses to a common SST forcing. The disparate results indicate that consensus on the representation of the physics of the atmosphere has not been reached, and the present uncertainty in the parameterizations is greater than the intrinsic uncertainty of the model system as shown by ensemble simulations.

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James S. Boyle

Abstract

The European Centre for Medium-Range Weather Forecasts (ECMWF) model (cycle 33) was integrated for a full seasonal cycle at four horizontal resolutions, T21, T42, T63, and T106. Within the limits imposed by the varying horizontal resolution all other aspects of the model were identical for each integration. In this paper a comparison is made of the dynamical aspects of the simulations. Fields of zonally averaged zonal wind, eddy heat and momentum fluxes, global divergent wind and vorticity, and stationary wave patterns are presented and compared for each resolution.

The conclusions reached by this study are (i) T21 is qualitatively different from the higher resolutions; (ii) the highest resolution does not provide a simulation that is egregiously superior to T42. There are aspects of the T21 simulation, especially in tropical convection, that are clearly better than the higher resolutions; (iii) aside from differences in smaller-scale boundary forcing at the higher resolutions, the T42. T63, T106 simulations are nearly identical in the dynamical aspects of the circulation considered here, and (iv) increasing horizontal resolution alone will not result in an overall superior simulation, unless other aspects of the model are modified

at an equal pace.

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James S. Boyle

Abstract

This is the second of two papers dealing with midlatitude initiation of East Asian cold surges. The effects of individual cold surges on the circulation an the East Asian coast are studied for the months of December 1974 and December 1978. These months represent a contrast with respect to the strength of cold surges occurring in that month. The surges occurring in December 1978 were weak, those in 1978 were strong.

The circulation features and processes that were considered were: 1) the 200 mb zonal momentum budget, 2) the 400 mb frontogenesis forcing, 3) the low-level meridional eddy heat fluxes, and 4) the thermally direct circulation cell in the entrance region to the East Asian jet maximum. The times of the surge events are shown to be periods which dominate in computing the longer term monthly statistics of the aforementioned circulation features. The monthly mean features computed here are also typical of other studies using even much longer term winter averages. The implication is that surges also dominate these statistics, and long-term averages of the East Asian winter monsoon circulation at the midlatitudes and subtropics are dominated by the surge characteristics.

During the cold surge event the balance of the 200 mb zonal momentum budget is between the zonal advection of momentum and the coriolis acceleration. Between the events the balance is somewhat mixed, with no terms clearly dominating.

The climatological maximum in confluence at 400 mb (quasi-geostrophic frontogenesis) found over East Asia is seen to be, for the most part, the result of intensified confluence accompanying the surge event. This frontogenesis is necessary for the temperature field to remain in thermal-wind balance with the accelerating jet. There is also marked low-level frontogenesis taking place at lower levels as the cold air sweeps southward.

The low-level eddy heat fluxes, which have a large maxima on the East Asian coast, are shown by means of time–longitude plots to be largely the results of the surge circulation. Cold air is continuously moving southward in the Asian winter monsoon, but the surge fluxes are intense and are focused on the coast.

The thermally direct circulation cell in the entrance region to the East Asian jet maximum reaches its peak intensity during the cold surges.

The two months show very similar patterns, only the intensity of the circulation is somewhat reduced in the month of the weaker surges (December 1978). Evidently the same processes are at work during the surges of each month only at reduced levels in December 1978.

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James S. Boyle

Abstract

Divergence and convergence centers at 200 hPa and mean sea level pressure (MSLP) cyclones are located every 6 hours for a 10-year GCM simulation for the boreal winters from 1980 to 1988. The simulation used the observed monthly mean SST for the decade. Analysis of the frequency, locations, and strengths of these centers and cyclones give insight into the dynamical response of the model to the varying SST.

It is found that 1) the model produces reasonable climatologies of upper-level divergence and MSLP cyclones. 2) The model distribution of anomalies of divergence/convergence centers and MSLP cyclones is consistent with available observations for the 1982–83 and 1986–87 El Niño events. 3) The tropical Indian Ocean is the region of greatest divergence activity and interannual variability in the model. 4) The variability of the divergence centers is greater than that of the convergence centers. 5) Strong divergence centers are chiefly oceanic events in the midlatitudes but are more land based in the tropics, except in the Indian Ocean. 6) Locations of divergence/convergence centers can he a useful tool for the intercomparison of global atmospheric simulations.

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James S. Boyle

Abstract

East Asian cold surges during two periods of contrasting intensity, December 1974 (strong) and December 1978 (weak), are studied. It is shown that the midlatitude mechanisms initiating cold surges during both months are quite similar in nature. Synoptic scale short waves passing through the long-wave trough position near the East Asian coast act to release the cold air southward from the main reservoir over eastern Siberia. Dynamic descent is initiated by these synoptic scale waves entering the northwesterly flow on the western side of the long-wave trough anchored on the coast. This is the descent center in the local direct circulation in the entrance region of the East Asian jet maximum at 200 mb.

The frequency and intensity of the cold air surges for a given month depend on the large scale circulation pattern. The average flow over Asia for December, 1974 and 1978, presents a contrast with respect to ridging over the midcontinent. In December 1978 the flow was characterized by a low zonal index over Asia and the low-level development forced by the smaller scale waves is not well-focused with respect to position. Strong ridging in the middle of the Asian continent during December 1974 yielded a stable pattern for descent on the East Asian coast.

The results indicate that the surge event is not the result of the expansion of an intensifying Siberian anticyclone but of a separate dynamically forced subsidence to the east and southeast of the Siberian anticyclone mean position; nor is the surge directly the result of cyclonic development off the East Asian coast. Such cyclogenesis follows the surge initiation and is triggered by the same synoptic short wave which initiated the surge as the wave passes to the eastern side of the long-wave trough position.

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James S. Boyle

Abstract

The monthly mean precipitation patterns of the Atmospheric Model Intercomparison Project decadal simulations over the United States and adjoining oceans are intercompared. A simple harmonic analysis of the 12-month seasonal mean precipitation values and a principal component (PC) analysis of the 120 monthly values were carried out. Emphasis is placed on the basic seasonal variation for three subregions: the eastern and central United States and the U.S. west coast. It is vital for GCM simulations to accurately portray the seasonal cycle.

The results indicate the following. 1) There are problems for almost all the models in capturing the seasonal variation of the precipitation over the eastern United States. The models typically overemphasize the summer–spring rainfall amounts. The PC analysis indicates that many of the models tend to extend the precipitation regime typical of the central United States too far to the east, resulting in a precipitation maxima occurring in the summer for the eastern region. 2) The seasonal variation of the West Coast is handled with the greatest fidelity. This result cuts across all the models and may be attributable to the fact the SST forcing is specified and common to all the simulations. The common SST forcing is apparently a dominant factor in determining this region’s precipitation climatology. 3) On the space scales of the regions selected, there is little consistent evidence that points to any specific model feature as a predictor of model performance. None of the obvious candidates such as horizontal resolution, convective closure schemes, or land surface schemes are reliable discriminators of a model’s ability to simulate precipitation. 4) For one smaller subregion centered over Arizona, chosen because of the dominance of the semiannual cycle, there is evidence that increased horizontal resolution has an effect. For this intermountain region the higher-resolution models as a whole do better than the low-resolution models. However, even in this case there is enough variation among the individual simulations to obscure the conclusion that increased horizontal resolution is a necessary or sufficient quality to produce a reliable simulation. 5) The models tend to have less interannual variation than the observations with more variance being explained by the leading (annual cycle) PC, whereas the observations have a less peaked spectrum.

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James S. Boyle

Abstract

Arguments are put forth supporting the contention that the analysis of specific humidity on isentropic surfaces should be considered as the optimum method for the analysis of atmospheric moisture. Autocorrelation curves of relative humidity and specific humidity on isentropic surfaces are presented. These curves indicate that for the case considered: 1) the values of the autocorrelation for specific humidity were greater than that of relative humidity for all separation distances considered; and 2) for distances >300 km the autocorrelation of specific humidity displayed a more gradual decrease with separation distance than did that of relative humidity.

From these results it is deduced that an objective analysis scheme can make more effective use of the available moisture information if the analysis is of specific humidity on isentropic surfaces rather than relative humidity on pressure or isentropic surfaces.

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Sailes Sengupta
and
James S. Boyle

Abstract

The technique of common principal components (CPC) is applied to compare the results of a number of GCM simulations. The data used are the 120 monthly mean fields from 30 Atmospheric Model Intercomparison Project (AMIP) simulations and an ensemble of five AMIP integrations from a single GCM. The spatial grid and 120 time points allows the calculation of up to 31 covariance matrices for input into the CPC analyses.

The CPC methodology is applied to a variety of model comparision problems within the context of the AMIP experiment. The aspects of the simulations used for demonstration are the seasonal cycle of precipitation over the United States, the global 200-hPa velocity potential, and the difference between the 200-hPa divergence of four closely related AMIP models and the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis and a small ensemble of simulations (five) of the European Centre for Medium-Range Weather Forecasts AMIP model.

These analyses demonstrate the utility of the CPC approach in identifying models systematic errors, the reduction of data in ensembles of simulation, and in model parameterization comparisons. The common errors among the models tend to highlight the area in which a gap in knowledge or parameterization implementation exists. In addition CPC analyses provide a more complete statistical picture of an emsemble of simulations within a single model than the traditional means and variances. It is often the common aspects of the ensembles that are sought as a robust signal.

The CPC analyses tend to support the observation that the models often have more in common with each other than with the observations. The CPC has the ability to answer many pertinent questions posed in the arena of model comparison when used in conjunction with other techniques.

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James S. Boyle
and
Lance F. Bosart

Abstract

A detailed case study has been made of a cyclone/anticyclone couplet over North America during early winter, 11–18 November 1969. The anticyclone was the dominant member of the couplet in this case.

Objective analyses of the wind and mass fields were carried out in both the isobaric and isentropic coordinate systems. Calculated quantities which are discussed include quasi-geostrophic vertical velocity and height tendency, potential vorticity, and quasi-Lagrangian kinetic energy budgets.

The movement of the cold, polar anticyclone out of its source region in Alaska southeastward to the Gulf of Mexico is seen to be thermally steered. During this stage the anticyclone moves toward the region of descent forced by the low level cold advection. As the anticyclone recurves and begins to move northeastward along the cut coast of the United States, it becomes a warm, dynamic system. The forcing directing the movement is that of differential vorticity advection. The static stability structure of the anticyclone changes in a manner consistent with this changeover in forcing in that the lower troposphere becomes more stable and the upper troposphere becomes somewhat less stable. Calculations show that the strong cyclogenetic diabatic forcing created by cold air flowing over warm water along the east coast is overwhelmed by the quasi-geostrophically driven height rises.

The mechanism by which the cold air dome associated with the anticyclone moves southward intact is explained from two complementary perspectives. One viewpoint is that of asymmetries in the jet stream about the long wave trough with a jet streak to the west of the cold dome. The other is that a maximum in potential vorticity located above the coldest air will prevent the dome from subsiding. The migration of the jet streak to the eastern side of the cold dome and the movement of the potential vorticity maximum from its position over the cold air lead to collapse of the cold air and induce cyclogenesis on the eastern side of the cold air. Furthermore, the calculations show that the potential vorticity is not at all conserved in the region of the deepening cyclone.

The kinetic energy budgets indicate that the anticyclone region serves as a source of upper level energy for the downstream cyclone. The collapse of the cold air converts potential to kinetic energy and this energy is fed into the jet streak on the eastern side of the long wave trough.

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James S. Boyle
and
Lance F. Bosart

Abstract

The evolution of a major cyclone over Eastern North America in March 1971 is described. A noteworthy feature of the development, in addition to the extreme baroclinity, was the substantial thermal advection upward from the middle troposphere to the lower stratosphere in conjunction with a strongly sloping tropopause.

Strong cold air advection into the trough aloft was associated with subsidence and vortex tube stretching above 500 mb. The vorticity so generated was advected downstream to support exceptional surface development. A deep layer of air with potential vorticity values characteristic of stratospheric air was advected downward to 600 mb immediately upstream of the deepening surface cyclone. The region of positive potential vorticity advection coincided with the area of computed quasi-geostrophic height falls, and lay along the leading edge of the massive cold advection zone aloft.

A strong sloping tropopause led to a sloping level of nondivergence across the trough axis with substantial cyclonic vorticity produced by vortex tube stretching as descent changed to ascent along an isobaric surface crossing the trough axis from west to east. This vorticity was advected downstream over the surface cyclone. The relatively high level of nondivergence (300 mb) immediately upstream of the surface cyclone ensured convergence and cyclone spinup through a deep layer. Immediately downstream of the surface cyclone the level of nondivergence lowered to below 500 mb as convergence and vorticity generation was concentrated in the lower level moist air from which heavy precipitation was falling. Stratospheric values of potential vorticity are generated in the 900–700 mb layer in the region of heavy precipitation beneath the level of maximum diabatic heating.

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