Publications


THE MONTE CARLO FORECAST

Weather 45 (1990) 198-207

TN Palmer, R Mureau, F Molteni


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


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 of the 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. In order to define the clusters, 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. Two analyses were performed, using a 5‐dimensional and a 3‐dimensional space respectively, and they provided consistent results. Six clusters were found. the largest cluster includes 40% of the fields in our sample; its centroid is close to the climatological winter state, 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. the variability between clusters accounts for the bimodality in the amplitude of planetary waves detected in previous observational studies. Our analysis reveals that this bimodality is much enhanced in the region of the phase space where the PNA index is negative, and the separation among the clusters is stronger. Finally, frequencies of transitions between clusters are presented, which show an asymmetric behaviour in the transitions between regimes with low and high amplitude of planetary waves. Copyright © 1990 Royal Meteorological Society


The 1988 US drought linked to anomalous sea surface temperature

Nature 338 (1989) 54-57

TN Palmer, C Branković

THE 1988 drought in the United States has been widely reported, not least as an indicator of the reality of the "greenhouse effect" - see for example ref. 1. 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. In addition, an experiment with 1988 initial conditions and 1987 sea surface temperatures was made. 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 provide complementary evidence to that in ref. 2, which suggested that the US drought was linked to anomalous oceanic conditions in the tropical Pacific. © 1989 Nature Publishing Group.


A weather eye on unpredictability

NEW SCIENTIST 124 (1989) 56-59

T Palmer

The implications of the inherent unpredictability of weather patterns, as part of a chaotic system, are discussed. The work of the meteorologist Edward Lorenz and his development of the concept of the Lorenz attractor are used to illustrate the application of chaos dynamics to weather forecasting problems. (D.W.T.)


A WEATHER EYE ON UNPREDICTABILITY

NEW SCIENTIST 124 (1989) 56-59

T PALMER


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

METEOROLOGY AND ATMOSPHERIC PHYSICS 40 (1989) 84-109

MJ MILLER, TN PALMER, R SWINBANK


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

NATURE 338 (1989) 54-57

TN PALMER, C BRANKOVIC


Large-scale tropical, extratropical interactions on time-scales of a few days to a season

Australian Meteorological Magazine 36 (1988) 107-125

TN Palmer

The observational studies focus on teleconnections between tropical outgoing longwave radiation and Northern Hemisphere extratropical height in wintertime, firstly using five-day mean fields, and secondly using monthly mean fields. On the month to seasonal time-scales, correlations between tropical sea surface temperature and extratropical geopotential height are discussed. The modelling studies include 10-day forecast experiments in which the tropical belt is relaxed towards the analysed state, seasonal time-scale GCM studies with imposed sea surface temperature anomalies, and results from simple nondivergent barotropic models. Results from the barotropic model indicate that the impact of the tropics on the extratropics is strongly dependent on the flow regime obtained in the extratropics. It is suggested that this may partially account for the variable impact of tropical forecast errors found in the forecast relaxation experiments. -from Author


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


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

Quarterly Journal of the 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. Copyright © 1988 Royal Meteorological Society


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


ANALOGS OF POTENTIAL VORTICITY IN ELECTRICALLY-CONDUCTING FLUIDS

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

TN PALMER


THE IMPACT OF EL NINO ON AN ENSEMBLE OF EXTENDED-RANGE FORECASTS

MONTHLY WEATHER REVIEW 115 (1987) 2103-2117

JA OWEN, TN PALMER


ESSO ENERGY AWARD LECTURE, 1986 - ADVANCES IN NUMERICAL WEATHER PREDICTION FOR AVIATION FORECASTING

PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON SERIES A-MATHEMATICAL AND PHYSICAL SCIENCES 410 (1987) 255-268

PW WHITE, MJP CULLEN, AJ GADD, CR FLOOD, TN PALMER, K POLLARD, G SHUTTS


ALLEVIATION OF A SYSTEMATIC WESTERLY BIAS IN GENERAL-CIRCULATION AND NUMERICAL WEATHER PREDICTION MODELS THROUGH AN OROGRAPHIC GRAVITY-WAVE DRAG PARAMETRIZATION

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 112 (1986) 1001-1039

TN PALMER, GJ SHUTTS, R SWINBANK


A STUDY OF WINTERTIME CIRCULATION ANOMALIES DURING PAST EL-NINO EVENTS USING A HIGH-RESOLUTION GENERAL-CIRCULATION MODEL .2. VARIABILITY OF THE SEASONAL MEAN RESPONSE

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 112 (1986) 639-660

TN PALMER, DA MANSFIELD


Influence of the Atlantic, Pacific and Indian Oceans on sahel rainfall

Nature 322 (1986) 251-253

TN Palmer

Folland et al.1 have reported that persistently dry and wet periods of several years in the Sahel have been accompanied by global-scale patterns of sea-surface temperature (SST) anomaly. They also demonstrated that the response of a general circulation model (GCM) of the atmosphere to an observed composite SST difference field between a number of such dry and wet periods showed substantial reduction in Sahel rainfall compared with values from a simulation with climatological SSTs. I examine here the same model's response to the individual components of the composite SST difference field in the Atlantic, Pacific and Indian Oceans. It is found that over the western Sahel, the Atlantic and Pacific fields have a comparable effect in reducing rainfall whereas the Indian Ocean field produces a slight enhancement. Results suggest that, over the eastern Sahel, the Indian Ocean has the dominant role in reducing rainfall. © 1986 Nature Publishing Group.


Sahel rainfall and worldwide sea temperatures, 1901-85

Nature 320 (1986) 602-607

CK Folland, TN Palmer, DE Parker

Using the comprehensively quality-controlled Meteorological Office Historical Sea Surface Temperature data set (MOHSST)1,2 we show for the first time that persistently wet and dry periods in the Sahel region of Africa are strongly related to contrasting patterns of sea-surface temperature (SST) anomalies on a near-global scale. The anomalies include relative changes in SST between the hemispheres, on timescales of years to tens of years, which are most pronounced in the Atlantic. Experiments with an 11-level global atmospheric general circulation model (AGCM) support the idea that the worldwide SST anomalies modulate summer Sahel rainfall through changes in tropical atmospheric circulation3-6. El Nino events may also play a part. We do not discount the effects of soil moisture and albedo changes in the Sahel7,8, although Courel et al.9 have questioned the importance of albedo changes, but we do suggest that worldwide SST anomalies may have a more fundamental influence on Sahel rainfall. © 1986 Nature Publishing Group.

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