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.

This is the first of a pair of papers covering the production and use of probability forecasts at ranges of 3 to 10 days at the Met Office using the ECMWF Ensemble Prediction System (EPS). The use of ensembles is intended to provide a set of forecasts which cover the range of possible uncertainty, recognising that it is impossible to obtain a single deterministic forecast which is always correct. We present a brief review of ensemble forecasting techniques and their use for the generation of probability forecasts. A wide range of probability forecasting products and tools are now available to forecasters at the Met Office, generated from the EPS. These will be described, and their use and interpretation discussed, both for site-specific forecast data and for fields of data covering a wider area. An important part of any forecasting system is verification: this is covered in some detail using several different methods, for the site-specific forecasts of surface weather parameters. Comparisons are made between the probabilistic forecasts and equivalent deterministic forecasts generated from the high-resolution ECMWF model, and it is evident that the former are more skilful according to most assessment methods for forecasts more than three days ahead. The companion paper shows how forecasters use the vast amounts of information available in forecast production (Young and Carroll 2002).

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.

A diagnostic study of two cases of severe thunderstorms in Catalonia (the north-eastern region of Spain) is presented. The first occurred on 24 August 1993. A squall line, with bow echo configuration in the radar images, crossed Catalonia from west to east. Strong winds, large hail up to 7 cm in diameter, and heavy rain producing floods were observed. The meteorological situation was characterized by an upper-level trough approaching Spain from the north-west. This upper trough developed a low, which displaced the warm, dry air located over the Iberian Peninsula towards the east and north-east. The warm, dry air overran the cold, humid Mediterranean air producing a strong inversion (LID) capped by an elevated mixed layer over the western Mediterranean and also over the Ebro valley. Hand-drawn analyses permitted the identification of a secondary trough; a combination of this trough and a jet streak located over Spain was responsible for the mesoscale lifting mechanism that was able to break the LID and maintain the deep convection. The second case occurred on 31 August 1994. A tornado developed over south Catalonia and large hail, up to 5 cm in diameter, also fell. The meteorological situation was also characterised by an upper-level trough advancing from the west towards Spain. Accurate hand-drawn analyses identified a small thermal low over Catalonia leading to strong convergence at low levels. Over the same area, two jet streaks overlapped their left exit and right entrance zones, producing an enhancement of the ascending motion forced by the low-level convergence. However, the typical environment for supercell development was not present over Catalonia. The tornado was probably produced by a non-supercell thunderstorm.

Agrometeorological information and services from the National Meteorological and Hydrological Services (NMHSs) are increasingly being demanded by the farming community to cope more efficiently with climate variability and the increasing incidence of extreme meteorological events such as droughts, floods, frosts and wind erosion. While considerable advances have been made in the collection, archiving and analysis of weather and climate data, their transformation into information that can be readily used by the farm sector has lagged behind, especially in developing countries where such information needs are the greatest. One of the important reasons is the lack of adequate interaction with the user community in assessing the appropriate dissemination and communication procedures that can enhance the value of the agrometeorological information and services. A brief review of the present status of dissemination and communication of agrometeorological information by the NMHSs and associated agencies in different regions around the world is presented. A description of the user communities for agrometeorological information and their varying needs is also presented. Opportunities and challenges in the dissemination and communication of agrometeorological information by the NMHSs are described with suitable examples which emphasize that continued improvements are necessary to make agrometeorological information more accessible and useful to the user community.

The correlation between Meteosat water vapour (WV) data and positive anomalies of potential vorticity (PV) is investigated using a case study involving cyclogenesis in the Alpine region and a north-west African depression. Correlation analyses are performed at different isobaric surfaces in the middle and upper troposphere for two latitudinal belts (25–35° N – ‘southern’, and 35–52.5°N – ‘northern’). A significant correlation is obtained at 500 hPa (correlation coefficient range: 0.58–0.64). Linear regression models are fitted to the samples from the northern and southern belts. On the basis of these results, the use of WV data as a tool for detection of errors in model-derived PV fields is discussed. It is concluded that, though some of the discrepancies between PV fields and WV images could be due to errors in the model analysis or prognosis, some mismatches are possibly due to the variable sensitivity of the WV channel because of the changes in pressure over the commonly used PV fields. It is suggested that the comparison of WV data with the field of positive PV anomalies at 500 hPa might be another approach to help solve the problem of verifying and validating numerical output.

Fog is the most important physical process that reduces surface visibility, having a major negative impact on air traffic. In this study, a one-dimensional boundary layer model was developed in order to predict the occurrence and intensity of fog. The model uses a selection of parameterisations, suggested by different authors, and uses the finite element method for the vertical discretisation of the equations. The 1977 Cabauw fog event is used to validate the model and eight fog events at Lisbon Airport are simulated. The validation shows that the model is able to predict realistically the fog's onset, growth and dissipation. The fog forecasts at Lisbon Airport are, in general, quite reasonable, and a simple simulation of a fog event, where advection played a fundamental role, also gives realistic results.

The Cooperative Research Centre for Southern Hemisphere Meteorology in collaboration with the Bureau of Meteorology has developed an ultraviolet index (UVI) forecast system to support the Australian UVI forecast issued to the public by the Bureau of Meteorology. In an effort to alert the public to the possibility of harmful levels of ultraviolet radiation the UVI forecast has been issued daily as part of the meteorological weather report since spring 1996. The predicted UVI indicates the amount of ultraviolet radiation that will reach the Earth's surface at local noon next day. The forecast includes the effects of cloud cover. The global clear sky UVI is calculated using a two-stream radiative transfer model and a deterministic ozone forecast scheme. The system utilises meteorological fields from the global numerical weather forecast model of the Bureau of Meteorology. This paper describes the UVI forecast system, and its results are compared with observational data from the Australian Radiation Laboratory for several Australian cities for the period January–December 1997. Possible improvements are outlined since the UVI forecast system is the subject of ongoing development.

Wind direction is a circular variable. This makes the algorithms used to find its standard deviation different from that of the linear variables. In particular, the requirement for storing all the data points before the standard deviation can be computed limits the storage capacity and puts great strain on remote data acquisition systems. Various algorithms have therefore been developed to estimate the standard deviation in order to reduce the number of terms stored. The following work consists of a comparative analysis of such estimators together with the parameters used. It emerges that some of the assumptions adopted to produce the equations being analysed do not hold in practice, even though this does not affect significantly the performance of the estimators that depend on them. On the other hand, the parameter that has the best trend with the algorithm adopted is the magnitude of the vector to the centre of gravity of the system. However, such a result gives rise to some concerns since it does not account for the ‘vectorial’ nature of the angle being treated.

It is part of British sailing and forecasting folklore that the wind speed increases in the Dover Straits when there is an established wind-flow - westerly/south-westerly or easterly/north-easterly - along the English Channel. However the underlying mechanism of the phenomenon is unclear. We have used the Met Office ‘New Dynamics’ mesoscale model to perform a case study on an occasion when this phenomenon was observed in the Channel but not forecast well by the operational model, UM 4.5. Results are presented showing the sensitivity of forecasts to horizontal resolution (down to 2km) and to vertical resolution. We probe the physical mechanism of the Channel jet by altering the surrounding orography and the land or sea surface roughness. Both the orography and the surface roughness are shown to influence the jet formation.

When high pressure is located near the Korean peninsula, a diffluent wind regime generally occurs over the Yellow Sea. At night or early morning, diffluent westerly winds occur on the western side of the Korean peninsula near Inchon city and encounter a combined land breeze and katabatic easterly offshore wind, resulting in conditions ranging from calm to a moderate westerly wind near the coast. Nocturnal radiational cooling of the land surface and the moisture laden westerly winds can cause air near the coast to become saturated, resulting in coastal advection fog. During the day, on the other hand, the synoptic-scale westerly wind is reinforced by a westerly sea breeze and is further reinforced by a westerly valley wind directed upslope towards the mountain top. Even if the resulting intensified onshore wind could transport a large amount of moisture from the sea over the land, it would be very difficult for fog to form because the daytime heat flux from the ground would develop the convective boundary layer inland from Inchon city sufficiently to reduce significantly the moisture content of the air. Therefore, fog does not generally form in situ over the inland coastal basin. When an area of cold sea water (10 °C average) exists approximately 25–50 km offshore and the sea surface temperature increases towards the coast, air parcels over the cool sea surface are cooled sufficiently to saturation, resulting in the formation of advection sea fog. However, at the coast, nocturnal cooling of the ground further cools the advected moist air driven by the westerly wind and causes coastal advection fog to form.

A weather radar and processing system has been developed in conjunction with North West Water Ltd., a water utility responsible for the supply of potable water and the treatment of wastewater in the north-west of England. The primary objective has been to develop a radar system which can produce accurate and reliable quantitative estimates of rainfall at a temporal and spatial resolution commensurate with the requirements of urban drainage system modelling. This paper describes procedures for removing ground clutter and adjusting radar estimates of rainfall (using the Probability Matching Method). An archive of stratiform rainfall event data has supported an objective assessment of the radar data quality using a dense gauge network and simulated flow hydrographs for an urban drainage system. The results indicate that the adjusted radar data are of comparable accuracy to the dense raingauge data and are therefore of sufficient accuracy to be used for the management of water resources within an urban environment. The data processing and adjustment procedures have been incorporated into a real-time data processing and display software package (ARTIST) which is briefly described in the paper.

The story of numerical weather prediction (NWP) is a great story. It deals with the first successful attempts to see into the chaotic future, to predict non-period events. It is to a large extent a story of computers, mathematics and numerical schemes. But the development cannot be fully understood unless non-mathematical and non-technological facts are also included. Meteorological understanding, political considerations and human emotions are difficult to quantify, but they make the picture complete and the narrative more interesting for readers outside the ‘NWP community’.

We study the contribution of surface data to convection nowcasting over regions of modest orography and under weak synoptic forcing. Hourly mesoscale analyses are performed using the CANARI optimal interpolation analysis scheme, which combines first-guess fields from the fine mesh (10 km) ALADIN model with hourly routine observations arising from a mesonet of automated ground stations. These analyses then allow the computation of diagnostic parameters that quantify convective instability, low-level lifting processes and moisture supply: these are the convective available potential energy (CAPE) and the moisture convergence (MOCON). A tuning of the analysis scheme is needed first for it to fit the meso-?-scale. Then, the skill of the computed diagnostics for convection nowcasting is evaluated by comparing their fields with radar reflectivities observed between one and four hours after the analysis time. This is done for four selected convective situations. With regard to thunderstorm triggering, results show that this usually happens over areas of persistently high values of CAPE which undergo convergence continuously from four to one hour before the event; on the other hand, areas of persistent divergence are never associated with convective developments. In addition, the proposed criteria allow a significant reduction in the areal extent of predicted thunderstorms (i.e. decreasing the false-alarm rate) compared with what can be currently done on an operational basis, while maintaining a low non-detection rate. As to convection monitoring, we find that the organization of convective systems into a reflectivity line is preceded by a similar organization in the MOCON field from one to three hours ahead.

In Part I of this study recent versions of the ECWMF Integrated Forecasting System (IFS) are used together with historical observational data to carry out reanalyses and deterministic reforecasts of three major north-west European wind storms of the twentieth century. The storms considered are the Dutch storm of 1 February 1953, the Hamburg storm of 17 February 1962, and the British October storm of 1987 (Great October Storm). Common to all these storms is their severity, which caused large loss of life and widespread damage.

Reanalysis of the storms is based on a 3D-Var and 4D-Var assimilation scheme at a horizontal resolution of T$_L$159 ($\approx$125 km) and T$_L$511 ($\approx$50 km), respectively. Similarly, two different horizontal resolutions (T$_L$159 and T$_L$511) are used to investigate the deterministic predictability of these storms. The lower-resolution system is exactly that used in the ERA-40 reanalysis project. The high-resolution system is a more recent version of the ECMWF IFS.

It is shown that the basic characteristics of the Dutch and Hamburg storms that gave rise to the storm surge are well predicted by the single deterministic forecasts up to about 48 and 84 hours, respectively, in advance.

Our capability to predict the Great October Storm is more difficult to assess. On the one hand, even recent versions of the ECMWF IFS underestimate the severity of the storm in the very short-range (12–24-hour forecasts started at 12 UTC 15 October 1987). On the other hand, the high-resolution version of the ECMWF IFS provides excellent deterministic forecasts of the track and intensity of the storm up to 96 hours in advance. However, there are errors in the timing of the storm (12 hours for the 96 hour forecast).

From the results presented in this study it is concluded – bearing in mind the limited number of cases considered – that with the current ECMWF forecasting system reliable deterministic predictions of some European wind storms are possible several days in advance.

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.

This report is a summary of the methods and findings of a research project that evaluated the social and economic benefits of meteorological services in Australia. The meteorological services evaluated were the basic public weather services and several specialist user-pay and commercial services used by business firms in several sectors of the national economy. Overall, the results indicated that the quality of the selected services was high. In addition, the benefits of these services were extensive, resulting in high benefit-cost ratios.

High atmospheric temperature and humidity can cause discomfort and even death among people in high-risk groups. Forecasting such meteorological situations can be a useful tool for public health managers. The predictability of discomfort is analysed using a time series of the relative strain index in Buenos Aires. Other authors have found that index values exceeding the thresholds of both nocturnal and diurnal comfort can increase mortality. This study finds that such situations are associated with index values over 0.20 at 14:00 h. local time (RSI14>0.20). Consequently, the meteorological conditions associated with RSI14≥0.20 are studied. The anomalies of the geopotential monthly mean fields show the passage of a low-pressure system south of 45°S. This system is related to a trough, possibly of frontal character, moving across the province of Buenos Aires, and a simultaneous intensification of the north-western low in Argentina. The synoptic situation is associated with a general northerly flow that brings heat and moisture from the tropics. On 81% of days with RSI14=0.20, winds with a northerly component were recorded. This increases to 84.4% when at least one day with northerly wind is previously observed. The synoptic characteristics are examined using a combination of circulation indices: meridional (R), zonal (Z) and curvature (C). The daily sequence of the weighted averages of the anomalies obtained are ΔR<0, ΔZ>0 and ΔC>0. Some 67% of the days when RSI14=0.20 correspond to this daily index pattern estimated at locations near Buenos Aires.

The CERES-Rice version 3.0 crop growth simulation model was calibrated and evaluated for the agroclimatic conditions of the state of Kerala in India. Genetic coefficients were developed for the rice crop variety Jaya and used for the model evaluation studies. In four experiments using different transplanting dates during the virippu season (June to September) under rainfed conditions (i.e. no irrigation), the flowering date was predicted within an error of four days and date of crop maturity within an error of two days. The model was found to predict the phenological events of the crop fairly well. The grain yield predicted by the model was within an error of 3% for all the transplanting dates, but the straw yield prediction was within an error of 27%. The high accuracy of the grain yield prediction showed the ability of the model to simulate the growth of the crop in the agroclimatic conditions of Kerala. It can be concluded from this study that the model can be used for making various strategic and tactical decisions related to agricultural planning in the state.

The atmospheric and fuel conditions that led to the case of extreme fire behaviour registered in Puerto Madryn in north-eastern Patagonia, Argentina, on 21 January 1994 were analysed in this study. The analyses included surface and upper-air meteorological conditions, fuel composition and load, and morphometry of the burnt area. When the fire occurred the total fuel load was 16 688 ± 2 611 kg of dry matter per hectare, with fine fuel representing 66% of the total fuel load. At the time of the major fire run, the air temperature reached 32 °C and relative humidity decreased to 13%. The fuel load and its high proportion of fine fuels, the extreme temperatures, the low relative humidity, and the changes in wind directions which resulted from the conjunction of synoptic and local-scale phenomena, enhanced the development of a large convection column which favoured the interaction of surface and upper-air winds. As in similar cases reported for other ecosystems, the combination of these phenomena led to this case of extreme fire behaviour. It is intended that this analysis will contribute to the future development of fire alert systems for the region and to the worldwide knowledge of extreme fire behaviour in different scenarios.