Publications


Predictability of a coupled model of ENSO using singular vector analysis .2. Optimal growth and forecast skill

MONTHLY WEATHER REVIEW 125 (1997) 2057-2073

Y Xue, MA Cane, SE Zebiak


The ECMWF Ensemble Prediction System

Meteorological Applications 4 (1997) 301-304

TN Palmer, J Barkmeijer, R Buizza, T Petroliagis

A brief description of the ECWMF Ensemble Prediction System is given, including the method for producing initial perturbations, products and verification, and planned developments.


The impact of increased resolution on predictability studies with singular vectors

Quarterly Journal of the Royal Meteorological Society 123 (1997) 1007-1033

R Buizza, R Gelaro, F Molteni, TN Palmer

The dominant singular vectors of the tangent propagator of the European Centre for Medium-Range Weather Forecast (ECMWF) numerical weather-prediction model are an essential component of the ECMWF Ensemble Prediction System (EPS). These singular vectors describe the principal finite-time linear instabilities of the northern extratropical atmospheric circulation. The impact of increasing the horizontal resolution of the tangent model from T21 to T42 on three different types of initial perturbation, which make use of these singular vectors, is considered. The increase of resolution allows the possibility of describing more accurately instabilities with an upscale cascade of energy from subsynoptic to synoptic scales. Two of the perturbations are referred to as the pseudo-inverse and sensitivity vectors. These are both diagnostic, and involve estimating from the short-range forecast error the component of initial error in the unstable subspace. The third type of perturbation is used to construct the set of initial states for the ensemble prediction scheme. Linear and nonlinear integrations are described using these different types of perturbation. All the results point to the conclusion that the higher-resolution calculations lead to more accurate results. Specifically, it is shown that significant amounts of energy in the pseudo-inverse and sensitivity vectors reside in scales smaller than T21, and that the skill of the EPS is improved when generating the initial perturbations using T42 singular vectors. This is found to be especially true in a number of cases when the control forecast was particularly poor. As a consequence, the predictability of synoptic-scale disturbances in the atmosphere on time-scales of a few days is likely to be determined by errors in the initial state on subsynoptic scales. In addition to these conclusions, it is proved that the analysis of the relationship between the EPS and the pseudo-inverse perturbations can be used to define the initial amplitude of the EPS perturbations.


Potential use of the ECMWF Ensemble Prediction System in cases of extreme weather events

Meteorological Applications 4 (1997) 69-84

T Petroliagis, R Buizza, A Lanzinger, TN Palmer

The combined use of the European Centre for Medium-range Weather Forecasts (ECMWF) high-resolution operational model, at T213 spectral triangular truncation and with 31 vertical levels (T213L31), and the Ensemble Prediction System (EPS), during cases of intense Mediterranean storms, is studied. In particular, it is discussed how EPS products can be used to provide a measure of confidence in the high-resolution precipitation forecast. Three case studies (two extreme events plus one false-alarm case) are analysed. The first event took place between 21 and 22 October 1994 over Greece, where heavy rainfall led to local flash-floods in many areas that cost the loss of 12 lives, and caused significant property damage. The second event occurred in northwest Italy (and some parts of southern France) exactly two weeks later. More than 60 lives were lost and the cost of damage was enormous. For both these cases, the EPS probability values for precipitation occurrence supported the medium-range T213L31 prediction, which proved to be successful. A third case is also investigated, where the high-resolution forecast suggested heavy rainfall over northern Italy but was not supported by the EPS. The T213L31 prediction for this case was poor. Finally, the reliability of EPS probability predictions is discussed. It is shown that probability forecasts of low-level temperature and wind show considerable skill. Reliability statistics of clusters of large-scale flow show considerable skill as well. Probabilistic precipitation predictions are less skilful, possibly due to the EPS model resolution.


Sensitivity analysis of atmospheric low-frequency variability

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 123 (1997) 2425-2447

S Corti, TN Palmer


Relations between interannual and intraseasonal monsoon variability as diagnosed from AMIP integrations

Quarterly Journal of the Royal Meteorological Society 123 (1997) 1323-1357

L Ferranti, JM Slingo, TN Palmer, BJ Hoskins

Monsoon variability on intraseasonal and interannual time-scales is analysed using data from five 10-year European Centre for Medium-Range Weather Forecasts Atmospheric Model Intercomparison Project integrations, which differ only in their initial conditions. The results show that monsoon fluctuations within a season and within different years have a common dominant mode of variability. The spatial pattern of the common dominant mode in precipitation has a pronounced zonal structure, with one band of anomalous rainfall extending from 20°N to 5°N, covering most of the land areas, with the other band, of opposite sign, lying between 5°N and 10°S, mostly over the Indian Ocean. This mode therefore describes both the active/break monsoon spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between the continental and the oceanic regime and the principal pattern of interannual variability of monsoon rainfall. In the observations the oscillations between active and break monsoon spells have similar behaviour, although the model is deficient in representing the rainfall variability over India. On the intraseasonal time-scale the transition between the two regimes seems to have a chaotic nature. In addition the probability density function of the principal mode is bimodal for the years in which this mode is particularly dominant. These two results indicate a possible similarity with the Lorenz 3-component chaotic model. Northward-propagating convective regions, simulated by the model, are not clearly associated with the phase transitions of the TCZ regime. The timing of the monsoon onset appears to be modulated by the phase of the El Niño/Southern Oscillation during the preceding season, consistent with observational studies. The results suggest that the dominant mode may also represent some components of the observed monsoon variability. The interannual fluctuations of the dominant mode exhibit only a weak level of reproducibility compared with the relatively large predictability of a broad-scale monsoon wind-shear index.


The impact of increased resolution on predictability studies with singular vectors

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 123 (1997) 1007-1033

R Buizza, R Gelaro, F Molteni, TN Palmer


A study of the predictability of tropical pacific SST in a coupled atmosphere-ocean model using singular vector analysis: The role of the annual cycle and the ENSO cycle

Monthly Weather Review 125 (1997) 831-845

YQ Chen, DS Battisti, TN Palmer, J Barsugli, ES Sarachik

The authors examine the sensitivity of the Battisti coupled atmosphere-ocean model - considered as a forecast model for the E1 Niño-Southern Oscillation (ENSO) - to perturbations in the sea surface temperature (SST) field applied at the beginning of a model integration. The spatial structures of the fastest growing SST perturbations are determined by singular vector analysis of an approximation to the propagator for the linearized system. Perturbation growth about the following four reference trajectories is considered: (i) the annual cycle, (ii) a freely evolving model ENSO cycle with an annual cycle in the basic state, (iii) the annual mean basic state, and (iv) a freely evolving model ENSO cycle with an annual mean basic state. Singular vectors with optimal growth over periods of 3, 6, and 9 months are computed. The magnitude of maximum perturbation growth is highly dependent on both the phase of the seasonal cycle and the phase of the ENSO cycle at which the perturbation is applied and on the duration over which perturbations are allowed to evolve. However, the spatial structure of the optimal perturbation is remarkably insensitive to these factors. The structure of the optimal perturbation consists of an east-west dipole spanning the entire tropical Pacific basin superimposed on a north-south dipole in the eastern tropical Pacific. A simple physical interpretation for the optimal pattern is provided. In most cases investigated, there is only one structure that exhibits growth. Maximum perturbation growth takes place for integrations that include the period June-August, and the minimum growth for integrations that include the period January-April. Maxima in potential growth also occur for forecasts of ENSO onset and decay, while minima occur for forecasts initialized during the beginning of a warm event, after the transition from a warm to a cold event, and continuing through the cold event. The physical processes responsible for the large variation in the amplitude of the optimal perturbation growth are identified. The implications of these results for the predictability of short-term climate in the tropical Pacific are discussed.


Atmospheric seasonal predictability and estimates of ensemble size

MONTHLY WEATHER REVIEW 125 (1997) 859-874

C Brankovic, TN Palmer


Extreme rainfall prediction using the European Centre for Medium-Range Weather Forecasts ensemble prediction system

Journal of Geophysical Research Atmospheres 101 (1996) 26227-26236

T Petroliagis, R Buizza, A Lanzinger, TN Palmer

The combined use of the European Centre for Medium-Range Weather Forecasts high-resolution, at T213 spectral triangular truncation and with 31 vertical levels (T213L31, operational model and ensemble prediction system (EPS), during cases of intense Mediterranean storms, is studied. In particular, it is discussed how EPS products can be used to provide a measure of confidence in the high-resolution precipitation forecast. Three case studies (two extreme events plus one false alarm case) are analyzed. For the first two cases, the EPS probability values for precipitation occurrence supported the medium-range T213L31 prediction, which proved to be successful. By contrast, for the third case, the high-resolution forecast suggested heavy rainfall over northern Italy but was not supported by the EPS. The T213L31 prediction for this case was poor. EPS forecasts of extreme weather events are necessarily compromised by the moderate resolution of the T63L19 model (a version of the operational high-resolution T213L31 model, at T63 spectral triangular truncation and with 19 levels) used to generate the ensembles. In future studies, ensembles will be made using at least T106L31 resolution combined with an increase in ensemble size.


Interannual tropical rainfall variability in general circulation model simulations associated with the atmospheric model intercomparison project

JOURNAL OF CLIMATE 9 (1996) 2727-2750

KR Sperber, TN Palmer


Extreme rainfall prediction using the European centre for medium-range weather forecasts ensemble prediction system

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 101 (1996) 26227-26236

T Petroliagis, R Buizza, A Lanzinger, TN Palmer


Finite-time instabilities of lower-stratospheric flow

Journal of the Atmospheric Sciences 53 (1996) 2129-2143

DL Hartmann, TN Palmer, R Buizza

The linear structures that produce the most in situ energy growth in the lower stratosphere for realistic wintertime flows are investigated using T21 and T42 calculations with the ECMWF 19-level forecast model. Significant growth is found for relatively large scale structures that grow by propagating from the outer edges of the vortex into the strong jet features of the lower-stratospheric flow. The growth is greater when the polar vortex is more asymmetric and contains localized jet structures. If the linear structures are properly phased, they can induce strong nonlinear interactions with the polar vortex, both for Northern Hemisphere and Southern Hemisphere flow conditions, even when the initial amplitudes are small. Large extensions from the main polar vortex that are peeled off during wave-breaking events give rise to a separate class of rapidly growing disturbances that may hasten the mixing of these vortex extensions.


Singular vectors and seasonal predictability

EIGHTH CONFERENCE ON AIR-SEA INTERACTION AND CONFERENCE ON THE GLOBAL OCEAN-ATMOSPHERE-LAND SYSTEM (GOALS) (1996) 69-69

TN Palmer, AM SOC


Intraseasonal oscillations in 15 atmospheric general circulation models: Results from an AMIP diagnostic subproject

CLIMATE DYNAMICS 12 (1996) 325-357

JM Slingo, KR Sperber, JS Boyle, JP Ceron, M Dix, B Dugas, W Ebisuzaki, J Fyfe, D Gregory, JF Gueremy, J Hack, A Harzallah, P Inness, A Kitoh, WKM Lau, B McAvaney, R Madden, A Matthews, TN Palmer, CK Park, D Randall, N Renno


The ECMWF ensemble prediction system: Methodology and validation

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 122 (1996) 73-119

F Molteni, R Buizza, TN Palmer, T Petroliagis


The influence of north‐west Atlantic sea surface temperature: An unplanned experiment

Weather 50 (1995) 413-419

TN Palmer


A local deterministic model of quantum spin measurement

PROCEEDINGS OF THE ROYAL SOCIETY-MATHEMATICAL AND PHYSICAL SCIENCES 451 (1995) 585-608

TN Palmer


SINGULAR VECTORS - THE EFFECT OF SPATIAL SCALE ON LINEAR GROWTH OF DISTURBANCES

JOURNAL OF THE ATMOSPHERIC SCIENCES 52 (1995) 3885-3894

DL HARTMANN, R BUIZZA, TN PALMER


THE SINGULAR-VECTOR STRUCTURE OF THE ATMOSPHERIC GLOBAL CIRCULATION

JOURNAL OF THE ATMOSPHERIC SCIENCES 52 (1995) 1434-1456

R BUIZZA, TN PALMER

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