The UK Meteorological office introduced a new bogussing scheme for tropical cyclone (TC) motion prediction in its global model in October 1994 after a series of tests. Before the operational implementation of this scheme, a 19-day period (in August and September 1994) was selected during which both the new and the old schemes were run operationally in parallel for a total of 12 TCs in three ocean basins. Comparisons between the forecast errors of the schemes demonstrated the superiority of the new scheme at all forecast time periods (Heming et al., 1995). This paper presents the results of an investigation into the possible reasons for the improved predictions by the new scheme during this trial period. All three factors of the new scheme (an improved initial positioning of the TC, an imposed steering flow derived from the past 6-h motion of the TC and an empirical vortex structure determined from actual observations and TC warning bulletins) are found to contribute towards the improvement. In particular, a good initial position together with an appropriate steering produce a much better 24-h forecast. For the 48- and 72-h forecasts, the steering flow remains an important factor and the contribution of the vortex structure also becomes significant in some cases. The steering flow appears to be the most crucial factor in determining the accuracy of the model predictions.

Practical monitoring of the amount of salt spread on the road surface for de-icing purposes is technically difficult, especially if it has to be done in a commercially viable way. Possible solutions to this problem are discussed. As it has become customary to refer to the freezing temperature of a salted road, the physical meaning of such a temperature is scrutinised. It turns out that the meaning and the definition of freezing temperature are not clear, and that it also does not have the significance that is often associated with it. Some experimental results of salt measurements on a road are presented.

Freezing rain in the winter of 1995/96 has drawn attention to the severity of black-ice problems on British roads, additional to the normal increased hazards presented to drivers by any form of precipitation. Disruption to traffic was considerable on several days. Weather radar provides improved nowcasting, for both winter and summer conditions, where available real time directly to highway engineers, but this is not yet generally the case in England and Wales. New developments from the Meteorological Office offer improved possibilities at reasonable costs.

An operational lee wave forecasting code is described and tested on a number of actual cases. Although the solution procedure involves a well-known numerical algorithm, its implementation for operational purposes needs to be intelligent and flexible. The technique involves an eigenvalue problem based on the vertical structure equation for stationary, internal gravity waves. Since the orientation of the trapped lee waves is not known a priori, the code solves the eigenvalue problem for a range of wind directions centred on the low-level wind direction and makes a decision as to which direction will have the largest amplitude and most highly trapped lee wave. The amplitude of the lee wave motion depends on the intensity of orographic forcing in a spectral band centred on the resonant wavelength. For complex terrain this is difficult to quantify. Here, it is assumed that the Fourier spectrum of the orography follows a power law in wavenumber and that the wave amplitude is attributed with a ‘wave launching-height’ condition. One of the test cases considered is the notorious ‘Sheffield gale’ of 16 February 1962, and in that case the lee wave code does predict an unusually large-amplitude wave.

A knowledge of minimum temperature during winter and maximum temperature during summer is a very useful for individuals, as well as for organisations whose workers and machines have to operate in the open, e.g. the armed forces, railways, roadways, tourism, etc. Accurate forecasts of minimum temperature during winter help in the prediction of cold-wave conditions and those of maximum temperature during summer help in the prediction of heat-wave conditions over northern India. Models for forecasting the minimum temperature during December and the maximum temperature during May at Delhi have been developed using surface and upper-air meteorological data from 1984-89. The results of testing the models on independent data from recent years (1994-95) are presented. The results are encouraging and more than 80% of the forecasts are correct within ±2 °C. Possible reasons for large deviations are also investigated.

Three severe hail- and windstorms, which occurred in Northern Switzerland, have been investigated. The storms produced hailswaths of 70-170 km in length and 12-25 km in width. Compact tracks of severe wind damages within the hailswaths are documented from public reports. These tracks have a length of 12-25 km. The damage was produced by straight-line winds rather than by tornadoes. Volume-scan Doppler radar data of the storms are available in time steps of 5 min. Radar signatures, such as low-level convergence and shear, mid-level vorticity, and high-level divergence, were attributed to the damage tracks at the ground. The low-level radar signatures allow the deduction of the time of occurrence of the damage tracks with a precision of some minutes.

Striking similarities in the evolution of the three stormes were found. The storms developed in the foothills of the Alps and the Jura mountains and propagated towards the plains of the Swiss midland. The storms can be classified as ‘high-precipitation’ supercell storms, known as producers of severe straight-line winds in the USA. Meso(anti)cyclonic vortex signatures were seen 40-55 min in advance of the heavy wind damage at the ground. The damage tracks were associated with explosive secondary cellular growth aloft. The physical explanation of this behaviour is that the first cells with mid-level rotation produced a gust front outflow that was accelerated to damaging strength at the time when the secondary cellular growth was initiated. The operational implication is that the nowcasting of severe and damaging winds can be improved by considering mid-level rotation in an early stage of the evolving storms.

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.

The physics of climate

An International Conference on ‘The Physics of Climate’' will be held at the Royal Society, London, UK, on 29 and 30 October 1997. The themes of the conference are:

[bull ] Radiation, clouds, humidity and aerosols.

[bull ] Climate, chaos, predictability and entropy.

[bull ] Interactions between the cryosphere, land, atmosphere and biosphere.

[bull ] Climate models, observations and detection of climate change.

Information about the conference is available from: Royal Meteorological Society, 104 Oxford Road, Reading RG1 7LL, UK.

Fax: +44 118 956 8571

E-mail: [email protected]