Hong Kong International Airport (HKIA) is located in a coastal environment and is occasionally affected by sea breezes. High air temperatures relative to sea surface temperature, coupled with light winds or moderate east to northeasterly winds are favourable conditions for the development of a sea breeze at the airport. In the afternoon of 27 April 2000, sea breezes affected the airport and a sea breeze front was formed by convergence between the westerly sea breeze and strengthening easterlies. An aircraft landing at HKIA encountered the sea breeze front and reported wind shear with airspeed gain of up to 15 kt at a height of 30 m (100 feet) above ground. Surface wind observations and Doppler velocity data of a Terminal Doppler Weather Radar (TDWR) allow quantitative analysis of the wind shear event. Shortly after the wind shear event, the TDWR also captured interesting interactions between the sea breeze front and an area of passing showers. This paper presents a quantitative analysis of the wind shear event and observations of the sea breeze interactions by the TDWR.

The prediction of severe convection is a major forecasting problem in Australia during the summer months. In particular, severe convection in the Sydney basin frequently produces heavy rain or hail, flash flooding, and destructive winds. Convective activity is a forecasting challenge for the Sydney basin, mainly from October to April. Currently, there is a need for improved numerical model guidance to supplement the official probabilistic convective outlooks, issued by the operational forecasters. In this study we assess the performance of a very high resolution (2 km) numerical weather prediction (NWP) model in terms of how well it performed in providing guidance on heavy rainfall and hail, as well as other mesoscale features such as low level convergence lines. Two cases are described in which the operational forecasts were incorrect on both occasions. Non-severe thunderstorms were predicted on 1 December 2000 but severe convection occurred. Severe convection was predicted on 8 December 2000, but no convection was reported. In contrast, the numerical model performed well, accurately predicting severe convection on 1 December and no convection on 8 December. These results have encouraged a program aimed at providing an enhanced numerical modelling capability to the operational forecasters for the Sydney basin.

It is a well-known fact that adverse weather significantly influences the safety and operating efficiency of air traffic, particularly in the terminal area. The unavoidable consequences of reduced operating efficiency are delays, diversions and cancellations of flights. In this paper the impact of thunderstorms on landing aircraft at Frankfurt Airport (Germany) during 1997 and 1998 is analysed. Thunderstorms are among the major weather phenomena affecting airport operations. Weather and operating conditions permitting, Frankfurt Airport is operated at more than 100% of its nominal capacity, which in turn lowers the actual capacity during periods of adverse weather. To the authors' knowledge this is the first quantitative study of weather impact on aviation in Germany. To identify thunderstorms as the unambiguous cause of delays, days without thunderstorms were used for reference and the difference in delay time was determined. The study shows that in 1997 thunderstorms increased the accumulated regular delay time by a factor of 6.3 and in 1998 by a factor of 1.1. On average, a thunderstorm typically caused about 740 delay minutes in addition to the 280 regular delay minutes for a sum of 100 arriving aircraft within four hours of average impact time.

This paper uses an alternative to the usual cost-avoidance approach to estimating the value of weather forecast products. Value is estimated via a demand-based approach based on the willingness to pay of those who use weather forecast services. Contingent valuation is used to estimate the benefits generated by an automated telephone-answering device that provides weather forecast information to commercial users in the Toronto area of Ontario, Canada. Commercial sectors included in the study are construction, landscaping/snow-removal businesses, TV and film, recreation and sports, agriculture, hotel and catering, and institutions such as schools and hospitals. Average value per call varied by commercial sector, from $2.17 for agricultural users to $0.60 per call for institutional users, with an overall mean of $1.20 per call. At roughly 13,750,000 commercial calls annually, this would result in an estimate of benefits generated by the service to commercial users of $16,500,000 per year.

Extensive studies of extra-tropical cyclones over the past sixty years have led to a plethora of conceptual models that describe the structure and evolution of storms. These conceptual (or structural) models blend observational, experimental and mathematical studies so as to represent the physical phenomena within a simple, yet flexible framework. The first three sections of this report review these conceptual models by stepping through the evolution of a mid-latitude cyclone and aim to illustrate how at each stage, conceptual models give a description of the physical processes occurring within the system. The fourth section reviews a recent cyclone classification scheme in the context of these conceptual models and attempts to group the cyclone classes into those arising through similar mechanisms. Conceptual models are then combined with this classification scheme so as to offer a unified perspective of cyclogenesis out of which a cyclone continuum develops.

Long records of hydrological and hydrometeorological variables are of inestimable value for the planning, design and management of water resource systems. Unfortunately, such long series invariably exhibit inconsistencies and non-homogeneities arising from a wide variety of causes - for example, changes in instruments and observation practices, and alterations to the general environment of the instruments themselves. Both non-parametric and parametric statistical tests are available to detect the presence of long-term movements in recorded time series, but the interpretation of results from such testing has often to be carried out in the absence of sufficient station metadata. A case study of precipitation records for 20 stations in Scotland over the period from 1961-99 has demonstrated that the results from statistical testing for inconsistency and non-homogeneity should also be interpreted in the context of prevailing weather systems. In particular, the increases in both annual and winter (Dec-Feb) rainfall totals recorded during this period at sites in western Scotland appear to reflect the changes in the winter atmospheric circulation over the Northern Hemisphere as encapsulated in the North Atlantic Oscillation Index (NAOI). Sustained periods of positive values of the NAOI, albeit less severe than during those of the 1970s and 1980s, are evident in the historical record of this index. The trends in rainfall totals identified could therefore be interpreted as arising from natural variability or even greenhouse gas forcing rather than from any inconsistency and non-homogeneity.

In Finland, winter road maintenance has been privatised, and the use of reliable predictors to measure the harshness of winter conditions has become even more important. Several indices have been developed to estimate the difficulty of winter conditions; however, many of these indices require relatively complicated input data and are in that sense vulnerable - for example, to changes in the meteorological station network. In the current study the year-to-year variation of the climatological variables describing maintenance costs were compared with the variation in costs. This made it possible to make a qualitative interpretation of the costs. This approach is ‘simple enough’ for its use to be easily accepted both by the contractors responsible for winter road maintenance and also by the administrators responsible for covering the maintenance costs.

A three-dimensional numerical model for mountain waves is described and validated against radiosonde observations over the Isle of Arran, south-west Scotland. The model is based on the Boussinesq equations of motion linearised about a background profile of wind, potential temperature and density. The equations are discretised using finite-difference approximations and the time dependence is retained. Solutions are then computed by integrating forward the equations in time until a stationary mountain-wave field is obtained. Data from radiosondes, launched during fieldwork on Arran during February and March 2000, are used to represent the background flow and model simulations of the wave field are compared directly with fluctuations in the ascent rates of the balloons. On average, the model reproduces the observed maximum peak-to-peak vertical velocity to within about 0.6 ms-1. In many cases the accuracy of the simulations is even better than this: the model vertical velocity along the path of the radiosondes agree well with fluctuations in the balloon ascent rate. The sensitivity of the model results to the wave ‘launching height’ is discussed. The model was also used to provide forecasts of wave activity during the field campaign. In this case, rather than using radiosonde data to represent the background flow, data were extracted from a global forecast model. The accuracy of these forecasts is examined and the viability of using such a model to provide routine operational high-resolution mountain-wave forecasts for specific sites is discussed.

On 16 May 2000, a low pressure system produced very intense winds over the Río de la Plata (RP) river, giving rise to floods within the region. In order to study this event, the synoptic evolution of the cyclone of 14-17 May 2000 is documented using conventional observations and the NCEP analysed data sets. The synoptic-dynamic overview reveals that the system starts as a usual cyclogenesis on the Chilean coast. The low level cyclone favours a convectively unstable environment within the central Argentinean region triggering convective storms and, consequently, a large amount of latent-heat release. The latent heat and the vorticity advection both play an important role in determining the depth of the system and its movement. Another unusual feature of this event is the north-eastward displacement of the cyclone, which is attributed to the interaction with other low-pressure systems located in the South Atlantic Ocean.