Tim Palmer

Abstract:

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

Abstract:

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

Abstract:

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

Abstract:

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