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Li-Chuan Gwen Chen
and
Huug van den Dool

Abstract

In this study, an optimal weighting system is developed that combines multiple seasonal probabilistic forecasts in the North American Multimodel Ensemble (NMME). The system is applied to predict temperature and precipitation over the North American continent, and the analysis is conducted using the 1982–2010 hindcasts from eight NMME models, including the CFSv2, CanCM3, CanCM4, GFDL CM2.1, Forecast-Oriented Low Ocean Resolution (FLOR), GEOS5, CCSM4, and CESM models, with weights determined by minimizing the Brier score using ridge regression. Strategies to improve the performance of ridge regression are explored, such as eliminating a priori models with negative skill and increasing the effective sample size by pooling information from neighboring grids. A set of constraints is put in place to confine the weights within a reasonable range or restrict the weights from departing wildly from equal weights. So when the predictor–predictand relationship is weak, the multimodel ensemble forecast returns to an equal-weight combination. The new weighting system improves the predictive skill from the baseline, equally weighted forecasts. All models contribute to the weighted forecasts differently based upon location and forecast start and lead times. The amount of improvement varies across space and corresponds to the average model elimination percentage. The areas with higher elimination rates tend to show larger improvement in cross-validated verification scores. Some local improvements can be as large as 0.6 in temporal probability anomaly correlation (TPAC). On average, the results are about 0.02–0.05 in TPAC for temperature probabilistic forecasts and 0.03–0.05 for precipitation probabilistic forecasts over North America. The skill improvement is generally greater for precipitation probabilistic forecasts than for temperature probabilistic forecasts.

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Wilbur Y. Chen
and
Huug M. Van den Dool

Abstract

The characteristics of extratropical low-frequency variability are examined using a comprehensive atmospheric general circulation model. A large experiment consisting of 13 45-yr-long integrations forced by prescribed sea surface temperature (SST) variations is analyzed. The predictability of timescales of seasonal to decadal averages is evaluated. The variability of a climate mean contains not only climate signal arising from external boundary forcing but also climate noise due to the internal dynamics of the climate system, resulting in various levels of predictability that are dependent on the forcing boundary conditions and averaging timescales. The focus of this study deviates from the classic predictability study of Lorenz, which is essentially initial condition sensitive. This study can be considered to be a model counterpart of Madden’s “potential” predictability study.

The tropical SST anomalies impact more on the predictability over the Pacific/North America sector than the Atlantic/Eurasia sector. In the former sector, more significant and positive impacts are found during El Niño and La Niña phases of the ENSO cycle than during the ENSO inactive period of time. Furthermore, the predictability is significantly higher during El Niño than La Niña phases of the ENSO cycle. The predictability of seasonal means exhibits large seasonality for both warm and cold phases of the ENSO cycle. During the warm phases, a high level of predictability is observed from December to April. During the cool phases, the predictability rapidly drops to below normal from November to March. The spring barrier in the atmospheric predictability is therefore a distinct phenomenon for the cold phase, not the warm phase, of the ENSO cycle. The cause of the barrier can be traced to the smaller climate signal and larger climate noise generated during cold events, which in turn can be traced back to the rapidly weakening negative SST anomalies in the tropical Pacific east of the date line.

Due to the fact that the signal to noise ratio of this model climate system is very small, an upper bound in atmospheric predictability is present, even when a perfect model atmosphere is considered and large ensemble mean predictions are exploited. The outstanding issues of the dynamical short-term climate prediction employing an atmospheric general circulation model are examined, the current model deficiencies identified, and continuing efforts in model development addressed.

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Huug M. Van Den Dool
and
Robert M. Chervin

Abstract

The output of a 20-year integration of an annual cycle (AC) version of the NCAR Community Climate Model in which the external conditions went through 20 prescribed identical annual cycles is used to study mouth-to-month persistence of anomalies in monthly mean atmospheric circulation fields on a global and a hemispheric scale. Of all fields considered, the height fields (1000–300 mb) are the most persistent and the transient eddy flux fields the least persistent. Persistence in height field anomalies is largest in winter and small throughout the rest of the year. For the area north of 20°N, a comparison is made with the persistence of months mean height and temperature fields observed in the real world (RW) during a 28-yeu interval. On a pooled all month-pairs basis, RW height anomaly fields are significantly more persistent than those appearing in AC but, from a practical point of view, the difference is small. The differences in persistence are larger for temperature anomalies (500–1000 mb thickness) than for height. Differences between RW and AC monthly persistence over the area north of 20°N are largest in summer when the RW has a local maximum in persistence. On the assumption that the model and atmosphere have the same internal dynamics, the differences just described can be attributed to the interaction of the atmosphere with external or boundary conditions (e.g., ocean surface temperature), which was purposely omitted from the AC integration. Interaction with the lower boundary in summer seems, therefore, to be quite important to explain the observed level of month-to-month persistence in circulation anomalies. In winter, however, the internal dynamics of the atmosphere alone produces the required observed level of month-to-month persistence. The output of a 15-year integration of the same model in which the sea surface temperature, on a global scale, had realistic interannual variability, is used to interpret further the differences between RW and AC.

As a by-product of this study we have calculated the spatial degrees of freedom (dof) associated with time mean anomaly fields. The dof for global monthly mean anomaly height fields in the AC model are quite low, i.e., 25–35 on a yearly pooled basis. Over the area north of 20°N, the dof associated with monthly mean anomaly height fields of the AC model and the RW are quite close, varying from 15–20 in winter months to about 40 in summer.

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Ming Cai
and
Huug M. van den Dool

Abstract

We have documented with the observed Northern Hemispheric 500 mb geopotential height data for ten winter seasons that traveling storm tracks exist downstream of the troughs of traveling low-frequency waves. The relation between the low-frequency flow and the traveling storm tracks is discovered with a novel observational technique that records high-frequency activity in a framework traveling along with an identifiable low-frequency structure. The vorticity flux of the high-frequency eddies associated with the traveling storm tracks acts both to reinforce the low-frequency waves and to retard their propagation.

These findings strongly indicate that a substantial amount of the low-frequency variability of the midlatitude atmospheric circulation is attributable to the forcing of the high-frequency eddies. These low-frequency waves organize the high-frequency eddies in such a way that the latter tend to intensify preferentially downstream of the troughs of the former. The symbiotic relation between the low-frequency flow and the traveling storm tracks is dynamically equivalent to the relation between the stationary waves and the stationary storm tracks. This mutual relationship is a necessary although not sufficient condition to parameterize high-frequency eddies in terms of low-frequency flow.

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Wilbur Y. Chen
and
Huug M. van den Dool

Abstract

A substantial asymmetric impact of tropical Pacific SST anomalies on the internal variability of the extratropical atmosphere is found. A variety of diagnoses is performed to help reveal the dynamical processes leading to the large impact. Thirty-five years of geopotential heights and 29 years of wind fields analyzed operationally at the National Centers for Environmental Prediction (NCEP), formerly the National Meteorological Center, and three sets of 10-yr-long perpetual January integrations run with a low-resolution NCEP global spectral model are investigated in detail for the impact of the SST anomalies on the blocking flows over the North Pacific. The impact on large-scale deep trough flows is also examined.

Both the blocking and deep trough flows develop twice as much over the North Pacific during La Niña as during El Niño winters. Consequently, the internal dynamics associated low-frequency variability (LFV), with timescales between 7 and 61 days examined in this study, display distinct characteristics: much larger magnitude for the La Niña than the El Niño winters over the eastern North Pacific, where the LFV is highest in general.

The diagnosis of the localized Eliassen–Palm fluxes and their divergence reveals that the high-frequency transient eddies (1–7 days) at high latitudes are effective in forming and maintaining the large-scale blocking flows, while the midlatitude transients are less effective. The mean deformation field over the North Pacific is much more diffluent for the La Niña than the El Niño winters, resulting in more blocking flows being developed and maintained during La Niña by the high-frequency transients over the central North Pacific.

In addition to the above dynamical process operating on the high-frequency end of the spectrum, the local barotropic energy conversion between the LFV components and the time-mean flows is also operating and playing a crucial role. The kinetic energy conversion represented by the scalar product between the E vector of the low-frequency components and the deformation D vector of the time-mean flow reveals that, on average, the low-frequency components extract energy from the time-mean flow during La Niña winters while they lose energy to the time-mean flow during El Niño winters. This local barotropic energy conversion on the low-frequency end of the spectrum, together with the forcing of the high-frequency transients on blocking flows on the high-frequency end, explain why there is a large difference in the magnitude of low-frequency variability between the La Niña and the El Niño winters.

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Ming Cai
and
Huug M. Van Den Dool

Abstract

A nearly complete vorticity equation is used to diagnose the tendency components of the low-frequency variations of the 500-mb streamfunction induced by various internal linear-nonlinear interaction processes. With the aid of a special composite technique (“phase-shifting” method) that effectively records the observations in a coordinate system moving with an identifiable low-frequency pattern, the authors are able to separate the internal interactions that primarily act to make low-frequency waves propagate from those that are mostly responsible for development/maintenance/decay (“maintenance” for brevity) of low-frequency transients. It is found that the low-frequency transients are maintained primarily by two nonlinear interaction processes: one is the vorticity flux of high-frequency eddies and the other is the interaction of low-frequency transients and stationary waves. It is also found that an individual propagation tendency component may be much larger than a maintenance tendency component. In particular, the beta effect and the advection of the low-frequency vorticity by the zonally averaged climatological wind are the dominant terms among the propagation tendency components. But there is a great deal of cancellation among the propagation tendency components. As a result, the net magnitude of the tendency components describing propagation is only slightly larger than those relating to maintenance of low-frequency waves. From a forecast point of view, both propagation and forcing terms are equally important if an accurate forecast beyond a few days is required.

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Ming Cai
and
Huug M. Van Den Dool

Abstract

A special composite technique (“phase shifting” method) that records both the low- and high-frequency transient activity throughout the troposphere in a framework moving with an individual low-frequency wave of 500-mb geopotential height at 50°N was used to document the three-dimensional structure of the planetary-scale low-frequency waves as well as the attendant traveling storm tracks from the NMC twice-daily analyses of geopotential height and temperature at pressure levels 850, 700, 500, 300, and 200 mb for the ten winters 1967/68 through 1976/77.

The following are the main characteristics of the Northern Hemisphere midlatitude planetary-scale low-frequency waves (zonal wavenumber m = 1, 2, 3, and 4) in winter: (i) The amplitude of the planetary scale low-frequency waves is nearly constant with the zonal wavenumber m, and has a maximum at 300 mb for geopotential height and at 850 mb for temperature; (ii) All low-frequency waves have a nearly equivalent barotropic structure (much more so than the stationary waves); (iii) The instantaneous zonal phase speed of an individual low-frequency wave is nearly independent of height and latitude so that we may identify the three-dimensional structure of a low-frequency wave by following that wave at just one pressure level and one latitude in either geopotential height or temperature.

The traveling storm tracks, defined as the local maxima on the rms map of the phase-shifted high-frequency eddies, are identifiable from both geopotential height and temperature data throughout the troposphere. They are located over the trough regions of the low-frequency waves. The barotropic feedback (i.e., the geopotential tendency due to the vorticity flux) of the traveling storm tracks tends to reinforce the low-frequency waves and to retard their propagation throughout the troposphere. The baroclinic feedback (i.e., the temperature tendency due to the heat flux) of the traveling storm tracks appears to have an out-of-phase relation with the low-frequency waves in temperature from 850 mb to 300 mb. At 200 mb, the baroclinic feedback is nearly in phase with the low-frequency waves in the temperature field.

The mutual dependence between the low-frequency flow and their attendant traveling storm tracks dynamically resembles that between the climatological stationary waves and the climatological storm tracks. Therefore, our observational study seems to lend support for the local instability theory that accounts for the existence of the stationary/traveling storm tracks as the consequence of the zonal inhomogeneity of the climatological mean/low-frequency flow.

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Wilbur Y. Chen
and
Huug van den Dool

Abstract

A low-resolution version of the National Meteorological Center's global spectral model was used to generate a 10-year set of simulated daily meteorological data. Wintertime low-frequency large-amplitude anomalies were examined and compared with those observed in the real atmosphere. The geographical distributions of the mean and variance of model and real atmosphere show some resemblance. However, careful comparisons reveal distinct regions where short-term climate anomalies prefer to develop. The model's low-frequency anomalies (LFAS) over the North Pacific (North Atlantic) tend to occur about 1500 miles east (southeast) of those observed, locating themselves much closer to the western continents. Because of the Displacement of the model's LFA centers, their associated circulation patterns deviate substantially from those observed.

The frequency distributions of the LFAs for both the model and reality display large skewness. The positive and negative large LFAs were, therefore, examined separately, and four-way intercomparisons were conducted between the model, the observed, the positive, and the negative LFAS. The separate analyses resulted in distinguishable circulation patterns between the positive and negative large LFAS, which cannot possibly be identified if a linear analysis tool, such as an empirical orthogonal function analysis, were used to extract the most dominant mode of the circulations. Despite pronounced misplacement of large LFAs of both polarities and a general underestimation of their magnitudes, the model dm have the capability of persisting its short-term climate anomaly at certain geographical locations. Over the North Pacific, the model's positive LFAs persist as long or longer than those found in reality, while its negative LFAs persist only one-fourth as long (10 versus 40 days).

The principal storm tracks and mean zonal wind at 250 mb (U250) were also examined to supplement the low-frequency anomaly investigation. Contrasting with observations, the model's U250s display considerable eastward extension and its storm tracks near the jet exit show substantial equatorward displacement over both the North Pacific and the North Atlantic oceans. These model characteristics are consistent with the behavior that the model's large LFAs also prefer to develop over the regions far east and southeast of those observed in the real atmosphere.

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Kshudiram Saha
,
Huug van den Dool
, and
Suranjana Saha

Abstract

The authors have investigated the climatological annual cycle in surface pressure on the Tibetan Plateau in relation to the annual cycle in surface pressure at the lower surroundings (India and China). It is found that surface pressure on the plateau is low (high) when the surrounding Asian continent has high (low) pressure. This out-of-phase relationship is evident in the NMC analyses and in long runs made with the NMC's global model. The authors have also found a few station observations on the plateau that have partially confirmed these opposing annual cycles in surface pressure. The authors believe this contrast to be real and operative over other parts of the globe as well. Near mean sea level, the surface pressure is low (high) when the temperature is high (low) (relative to its surroundings). At higher elevations, pressure is low (high) when temperatures are low (high). Also, in the datasets studied, the authors found no evidence for a thermal low on top of the plateau in summer.

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Anthony G. Barnston
and
Huug M. van den Dool

Abstract

The field of standard deviation of monthly mean 700-mb geopotential height in the Northern Hemisphere for each of the 12 months over the 1950–1991 period, among other auxiliary statistics, is compiled in an atlas to which this paper is companion. Some of the major features found in the atlas are highlighted and extended here. A comparison is also made to the same statistics derived from a 10-year run of the NMC model.

There are three distinct regions of peak standard deviation (up to 85 geopotential meters in winter), all of which are located over water. Two of them remain positionally relatively stationary throughout the year in the high-latitude Pacific and Atlantic oceans, respectively. A portion of the Pacific region's winter variability comes from interdecadal fluctuations. The third region is over the Arctic Ocean and exhibits some large seasonal changes in location. A roughly north-to-south troughlike minimum in standard deviation (down to less than 20 geopotential meters in summer) is found in west central North America throughout most of the year.

The standard deviation maxima (minima) coincide largely with areas with a high (low) frequency of occurrence of height anomaly centers of both signs. Many of these anomaly centers occur in spatial coherence with other centers, forming familiar teleconnection and principal component patterns. While the high (low) standard deviation areas invest greater (lesser) amounts of variance in these coherent variability clusters than the surrounding regions, their involvement in terms of the strength of the relationships is not substantially greater (smaller). The standard deviation field does not move north and south with the changes in season as do the jets, storm tracks, and the mean flow. In summer the standard deviation peaks are largely detached from spatially coherent variability patterns, suggesting that they may be caused in large part by local interactions related to permanent (spatially fixed) features of the lower boundary at all times of the year.

The observed monthly mean 700-mb flow and the quasi-stationary locations of its interannual standard deviation maxima and minima are reproduced in approximate form in a 10-year run of the NMC medium-range forecast model. This helps provide evidence that the field of standard deviation is related, directly or indirectly, to some of the geographically fixed boundary conditions across the globe such SST, ocean-land interfaces, and terrain.

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