Publications


A DYNAMIC INTERPRETATION OF THE GLOBAL RESPONSE TO EQUATORIAL PACIFIC SST ANOMALIES

JOURNAL OF CLIMATE 6 (1993) 777-795

F MOLTENI, L FERRANTI, T PALMER, P VITERBO


PREDICTABILITY AND FINITE-TIME INSTABILITY OF THE NORTHERN WINTER CIRCULATION

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 119 (1993) 269-298

F MOLTENI, T PALMER


MODELING INTERANNUAL VARIATIONS OF SUMMER MONSOONS

JOURNAL OF CLIMATE 5 (1992) 399-417

T PALMER, C BRANKOVIC, P VITERBO, M MILLER


THE SENSITIVITY OF THE ECMWF MODEL TO THE PARAMETERIZATION OF EVAPORATION FROM THE TROPICAL OCEANS

JOURNAL OF CLIMATE 5 (1992) 418-434

M MILLER, A BELJAARS, T PALMER


A real-time scheme for the prediction of forecast skill

Monthly Weather Review 119 (1991) 1088-1097

F Molteni, TN Palmer

During the winter of 1988/89, a real-time experimental scheme to predict skill of the ECMWF operational forecast was devised. The scheme was based on statistical relations between skill scores (the predictands) and a number of predictors including consistency between consecutive forecasts, amplitude of very short-range forecast errors, and indices of large-scale regime transitions. The results of the experiment are assessed with particular attention to a period with large variations in the skill of the operational forecast. -Authors


Tropical-extratropical interaction associated with the 30-60 day oscillation and its impact on medium and extended range prediction

Journal of the Atmospheric Sciences 47 (1990) 2177-2199

L Ferranti, TN Palmer, F Molteni, E Klinker

First, an empirical orthogonal function (EOF) analysis is made of outgoing longwave radiation (OLR) in the tropics over seven winters. Having removed the seasonal cycle and interannual variability, the two leading EOFs describe the 30-60 day oscillation. A composite of extratopical 500 mb geopotential height correlated simultaneously with this mode of tropical variability is constructed. The 500 mb height composite is compared succesfully, with the Simmons, Wallace and Branstator (SWB) mode of barotropic instability, which has similar periodicity and similar spatial structure in both its phase-quadrature components. In the final phase of this study, the ECMWF model has been integrated over four wintertime 20-day periods. -from Authors


The European Centre for Medium-Range Weather Forecasts (ECMWF) program on extended-range prediction

Bulletin - American Meteorological Society 71 (1990) 1317-1330

TN Palmer

The topics discussed include 1) The evolution of extended-range systematic error and skill in forecasting large-scale weather regime transitions; 2) The dependence of extended-range systematic error and skill on model horizontal resolution; 3) Monthly mean forecasts of tropical rainfall; 4) Tropical/extratropical interaction, and the influence of tropical low-frequency variability on extratropical forecast skill; 5) Ensemble forecasting, including the impact of ensemble averaging on forecast skill, and ensemble dispersion as a measure of forecast reliability; and 6) Probabilistic forecasting using phase-space cluster analysis. We believe that operational extended-range forecasting using the ECMWF model may be viable to day 20 - and possibly beyond - following further research on techniques for Monte Carlo forecasting, and when model systematic error in the tropics has been reduced significantly. -from Authors


Regimes in the wintertime circulation over northern extratropics. I: Observational evidence

Quarterly Journal - Royal Meteorological Society 116 (1990) 31-67

F Molteni, S Tibaldi, TN Palmer

Regimes of the northern extratropical circulation in winter are identified in this paper as clusters of atmospheric states in a low-dimensional phase space generated by the leading EOFs of eddy geopotential fields. Our algorithm seeks points corresponding to local maxima for the density of atmospheric states; subsequently, a cluster is defined around each density maximum as that portion of the phase space in which the observed density can be locally approximated by a unimodal function. Six clusters were found. The largest cluster includes 40% of the field in our sample; but it possesses a positive projection on the Pacific-North American (PNA) pattern. The other five clusters represent anomalous flow regimes and include 52% of the fields. One of them shows a low amplitude of the planetary waves; the remaining four represent states with large wave amplitude but different phases. -from Authors


EXTENDED-RANGE PREDICTIONS WITH ECMWF MODELS - INFLUENCE OF HORIZONTAL RESOLUTION ON SYSTEMATIC-ERROR AND FORECAST SKILL

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 116 (1990) 835-866

S TIBALDI, T PALMER, C BRANKOVIC, U CUBASCH


EXTENDED-RANGE PREDICTIONS WITH ECMWF MODELS - INTERANNUAL VARIABILITY IN OPERATIONAL MODEL INTEGRATIONS

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 116 (1990) 799-834

T PALMER, C BRANKOVIC, F MOLTENI, S TIBALDI


EXTENDED-RANGE PREDICTIONS WITH ECMWF MODELS - TIME-LAGGED ENSEMBLE FORECASTING

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 116 (1990) 867-912

C BRANKOVIC, T PALMER, F MOLTENI, S TIBALDI, U CUBASCH


Extratropical response to SST anomalies and the barotropic model

Climate-ocean interaction. Proc. workshop, Oxford, 1988 (1990) 225-232

TN Palmer

Recent GCM integrations with El Nino sea surface temperature anomalies are reviewed, and the question of whether the extratropical response can be explained in terms of simple barotropic model dynamics is examined. -Author


A WEATHER EYE ON UNPREDICTABILITY

NEW SCIENTIST 124 (1989) 56-59

T PALMER


The 1988 US drought linked to anomalous sea surface temperature

Nature 338 (1989) 54-57

TN Palmer, C Brankovic

The 1988 drought in the United States has been widely reported as an indicator of the reality of the 'greenhouse effect'. On the other hand, drought is a naturally occurring phenomenon. Here we have studied 30-day forecasts of the atmospheric flow over the United States using a complex numerical weather-prediction model initialized with data from May 1987 and May 1988. The results indicate that much of the difference between the 1987 and 1988 forecasts was associated with interannual variability in sea surface temperature. These results suggest that the US drought was linked to anomalous oceanic conditions in the tropical Pacific. -from Authors


PARAMETRIZATION AND INFLUENCE OF SUBGRIDSCALE OROGRAPHY IN GENERAL-CIRCULATION AND NUMERICAL WEATHER PREDICTION MODELS

METEOROLOGY AND ATMOSPHERIC PHYSICS 40 (1989) 84-109

M MILLER, T PALMER, R SWINBANK


THE 1988 UNITED-STATES DROUGHT LINKED TO ANOMALOUS SEA-SURFACE TEMPERATURE

NATURE 338 (1989) 54-57

T PALMER, C BRANKOVIC


On the prediction of forecast skill

Monthly Weather Review 116 (1988) 2453-2480

TN Palmer, S Tibaldi

Using 10-day forecast 500 mb height data from the last 7 yr, the potential to predict the skill of numerical weather forecasts is discussed. Four possible predictor sets are described. The skill of the predictors are tested, and the regression coefficients derived, on data from six winters, for both regional and hemispheric skill scores. As an independent test, the predictors are also applied separately to the seventh winter period 1986/87. It is concluded that some aspects of the low-frequency component of forecast skill variability can be satisfactorily predicted, though significant high frequency variability remains unpredicted. In discussing the physical mechanisms that underlie the use of these predictors, three important components of forecast skill variability are discussed: the quality of the initial analysis, the intrinsic instability of the flow, and the role of model systematic errors. -from Authors


Numerical simulations of the Madden and Julian oscillation

Journal of the Atmospheric Sciences 45 (1988) 774-788

R Swinbank, TN Palmer, MK Davey

The structure of the disturbances resembles a Kelvin wave, although the speed of propagation is slower than anticipated from theory as applied to a dry atmosphere. However, a simple model of the tropical atmosphere demonstrates that the wave speed is sensitive to moisture effects. This notion is confirmed by two further general circulation model experiments in which the latent heat release is increased; in both cases the intrinsic speed of the wave is reduced in inverse proportion to the vertical gradient of equivalent potential temperature. The time-mean circulation of the basic aqua-planet integration exhibits some unusual features; for example a double Hadley cell, with ascending branches displaced some 15° either side of the equator. Dynamical reasons for the maintenance of the aqua-planet circulations are discussed, since these shed some light on the general circulation of the earth's atmosphere. -from Authors


Medium and extended range predictability and stability of the Pacific/North American mode

Quarterly Journal - Royal Meteorological Society 114 (1988) 691-713

TN Palmer

It is shown from an assessment of a small set of extended range forecasts from two centres, and from a much larger set of medium range forecasts from one centre, that variability in predictive skill is strongly related to fluctuations in the Pacific/North American (PNA) mode of low frequency variability. A hypothesis is put forward that this is associated with the dependence of large-scale instability of the forecast flow on the amplitude of the PNA mode. The hypothesis is tested in a barotropic model using as basic states, composite skilful and unskilful cases from the set of medium range forecasts and individual monthly mean fields. Results from the barotropic stability analysis suggests possible reasons for the asymmetric nature of the response of general circulation models to sea surface temperature anomalies of opposite sign, relevant to forecasting on monthly to seasonal timescales. Observational evidence for the stability hypothesis is also discussed. -Author


ANALOGS OF POTENTIAL VORTICITY IN ELECTRICALLY-CONDUCTING FLUIDS

GEOPHYSICAL AND ASTROPHYSICAL FLUID DYNAMICS 40 (1988) 133-145

T PALMER