Orographically enhanced rainfall is the dominant climatic feature of the alpine regions of South Island, New Zealand. Most of this rain occurs in strong north-westerly winds although a forecast difficulty exists in that near-saturated north-westerly flows do not always produce heavy rain. Air masses impinging on the Southern Alps are generally carried over the alpine divide in times that are normally regarded as insufficient for clouds to produce precipitation-sized particles. Hence a possible explanation for these observations is that precipitation occurs only when pre-existing clouds advected onshore contain cloud drops that are sufficiently mature to allow for raindrop growth in the available time. A model combining advection of moist air over the topography and IR satellite data was used to show that synoptic cloud is necessary for the formation of heavy rainfall in the Alps.

Research was performed in Poggio Casciano Estate (Chianti area, central Italy) with the aim of defining a general approach to analyse the spatial variability of temperature at the microscale. Hourly data were collected from a network of 27 temperature stations covering an area of about 120 ha and determination coefficients r between station pairs on the basis of different geo-topographical factors were calculated. The data were analysed in order to investigate trends describing the spatial distribution of temperature inside the study area. The results pointed out a strong effect of some topographical condition on the distribution of thermal patterns, in particular altitude and the distance from valley bottoms. The results are discussed in order to formulate a general approach for the characterisation of climatic conditions at small scale.

The risk of dam failure in India, coupled with better analysis of extreme storm events, has led to the estimation of revised one-day probable maximum precipitation (PMP). For some water conservation projects where the catchment area is very large, there is a need to have estimates of PMP for durations of both two and three days. These are provided in this paper. It was found that the two-day PMP over the Indian region varied from 100 cm over the central peninsula to over 290 cm in eastern India. For a duration of three days the corresponding values are 120 cm and 370 cm. For water management projects which include impounding reservoirs, an estimate has been made of the probable maximum flood (PMF) based on the new PMP estimates. The results of the design flood estimates were compared with existing estimates at the same sites and in most cases were found to be considerably higher. A comparison of the revised estimates of PMF and recorded floods in India shows that the trend of the rates of runoff in relation to catchment area differ by an average of 38% for an area of 1000 km2 and 46% for an area of 10 000 km2. These results show that some dams in India may be unsafe in the event of the probable maximum flood.

This paper examines the commonly used assumption that decision-makers possess accurate prior probability information about climate events that affect their well-being, and illustrates the impact of that assumption on the valuation of prediction information. A survey of large producers in the Mid-western United States is used to recover their prior beliefs about climate variables. It is found that producers systematically misrepresent the probabilities of climate events that materially affect their well-being. In particular, the most common form of the miscalibration between actual and subjective probabilities is to overstate the likelihood of adverse events and understate the likelihood of favourable events. As a result, common methods for valuing prediction information are likely to understate the true value when recipients begin with less accurate prior beliefs.

The diagnosis of a vigorous dry intrusion over the central Mediterranean is performed using water-vapour images from METEOSAT. This dry intrusion was located on the rear side of a cold front (propagating from Italy to Greece) and played an important role in the onset of thunderstorms over the western Greek coasts. A combination of satellite imagery and potential vorticity analyses showed that the dry air originated in the lower-stratospheric and higher-tropospheric layers. The interaction of the dry air with the moist air masses within the warm conveyor belt ahead of the cold front (overrun of warm air by low equivalent potential temperature air) produced a potentially unstable region over the area of reported thunderstorms.

A poor forecast of an ex-tropical depression over the UK is examined. The water vapour imagery presented strongly implied an evolution diverging from the operational NWP forecast; it is suggested that such image sequences can be used to monitor upper level flow evolution for early warning of trough disruption.

Spanish building legislation has recently changed and now requires an updated and restructured Technical Building Code which is in accordance with European directives. The norm contained in this Code is based on studies of extreme values for climatic elements such as temperature, precipitation and wind. Revised maps of extreme values for climatic elements with a 50-year recurrence interval are required. Here, extreme maximum and minimum temperature maps for Spain are evaluated and mapped by means of geographical information technology. The data are extracted from the historical database held by the Spanish Meteorological Institute. Daily extreme temperatures from 1,181 stations with records going back more than 30 years have been used. The maximum and minimum temperatures are determined as 50-year mean recurrence interval values. To obtain these values, a Gumbel distribution is fitted to the extreme annual values extracted from the database. Spatial interpolation in a regular 5 km × 5 km grid of the annual maximum temperature is made by ordinary kriging. Meanwhile, for the annual minimum temperature a residual kriging has been applied due to its strong dependence on altitude.

The daily maximum and minimum temperatures in Melbourne, Australia, for the period 1981–1990 are examined in this article by means of fractional integration techniques. Using a parametric testing procedure (Robinson 1994), the results show that the time series for both datasets can be specified in terms of fractionally integrated statistical models: the maximum temperatures with orders of integration slightly smaller than 0.5 and the minimum temperatures with values slightly higher. Thus, the temperatures seem to be stationary in the case of the maximum values and non-stationary for the minimum ones, but mean reverting in both cases. Moreover, no significant trends in the model were found, implying that there is no evidence of climatic change in the data, at least for the period 1981–1990.

This paper is concerned with the transformation of an upper level trough into a shear line and a tropopause cyclone or cut-off low. During this event, the isentropic potential vorticity of air columns between fixed isentropes approaching the trough axis at its level of maximum intensity from the west is well conserved over a period of about 12 to 24 hours, but there is a significant repartitioning of the constituent parts, from a relatively large static stability to a relatively large absolute vorticity. This indicates stretching of vertical columns of air. The effect that is inducing the vortex stretching is identified within the context of quasi-geostrophic theory as ‘opposite vector-frontogenesis’. This implies that Q-vectors point in opposite directions, respectively, above and below the level of maximum trough-intensity. Isentropic analysis indicates that the criterion for unstable isentropic downgliding is fulfilled in the upper troposphere in a restricted area, covering several hundred thousand square kilometres, near the base of the trough when and where the cut-off low is formed. This area of unstable isentropic downgliding, to which we assign the term ‘baroclinic downburst’, and which maintains its identity during a period of time in the order of 24 hours, coincides approximately with the dry intrusion as identified on a ‘water vapour’ image. It is argued that unstable isentropic downgliding is in fact necessary in order to realise a cyclone-core characterised by relatively high static stability, relatively high absolute vorticity and relatively high potential vorticity, as is demanded by the invertibility principle for potential vorticity. The events described above are triggered by the interaction of two potential vorticity anomalies.

An interesting case of cyclogenesis over the western Mediterranean Sea, accompanied by extraordinary convective activity, is described in this paper. The event took place between 18 and 21 July 2001. Its synoptic features are examined on the basis of quasi-geostrophic theory using numerical analyses from the global model of Deutscher Wetterdienst GME and from the ECMWF. Particularly remarkable is the large amount of latent heat released in the condensation process, which produces significant modifications of the flow. These effects are evident both in the fields of relative vorticity as well as potential vorticity in the upper levels. The infrared Meteosat satellite pictures are used to follow the evolution of the upper-level cloudiness associated with the process and its connection with the values of vorticity and winds at 300 hPa. Radar pictures, reflectivity and radial velocity from Doppler surveillance radar in the Veneto region allowed us to outline the structure of the mesoscale convective system – a squall line that developed during the night of 19–20 July over Italy, in the Padana Valley. The features of this system were compared with a conceptual model proposed by Houze et al. (1989). The vertical wind shear, as inferred from the radial velocity pictures, was likely to play an important role in the development of the storm. Finally, an analysis of the vertical stability in the pre-storm environment was performed using sounding data from Udine, 20 July 2001, 00 UTC. The analysis shows the importance in the storm development of the potential instability released by ascent during the transit of the baroclinic system, a process that was investigated using the vertical profile of equivalent potential temperature and an estimate of ascending motion given by the ECMWF model.

In February and March 1996, the French NWP model Arpège was run to give operational forecasts five days ahead. These forecasts were a part of preparations for FASTEX (Fronts and Atlantic Storm-Track Experiment), whose field phase took place in 1997. The extensive collection and availability of model output during this period gave an excellent opportunity to study in real time the behaviour of different NWP models during the period of the year when strong winds and rapid changes in weather conditions are frequent. In this paper, we study a case where the ECMWF model predicted an incorrect pressure gradient at Northeast Iceland, while Arpège correctly simulated the gradient. The model errors are connected to anomalies in the mid-tropospheric vorticity field, which is in general quite different in the two models. The differences in the vorticity fields are discussed in relation to orographic effects.

There is a severe risk of road icing and slipperiness when rain or sleet falls on a frozen road surface. Traffic can be severely affected, with a higher frequency of road accidents. These conditions usually occur during warm frontal passages when cold weather and low road surface temperatures are replaced by warmer air and precipitation. Two case studies from southern Sweden show the spatial and temporal distribution of rain or sleet on a frozen road surface. The influence of regional-scale characteristics is demonstrated. Air temperature governs the spatial distribution when temperatures around -10 °C precede the event. Weather stations in an open environment experience slippery conditions first. If temperatures are just below zero before the front arrives, road surface temperature becomes the crucial factor. Weather stations in locations screened by forest or road cuts are the first to register this event. The methodology presented in this paper can be used to increase knowledge of the spatial and temporal patterns of road slipperiness in an area. As a result, forecasts and maintenance activities can be improved and there is a better understanding of the relationship between synoptic events and road slipperiness.

A cold air outbreak occurred in the third week of April 1999 in southern Brazil. A synoptic study of this rare event is undertaken. A surface high pressure cell in the Pacific Ocean off the coast of Chile slowly built up and moved eastward on 14 April. It started crossing the Andes, acquiring the characteristic shape of a bean on 15th, and separated into two cells later in the day. The cell on the lee side of the mountains moved gradually northwards over central South America affecting northern Argentina, Paraguay, Uruguay, Bolivia and southern Brazil. The temperature fell 15°C in the state of Mato Grosso (about 15°S). In southern Brazil temperatures below freezing were registered in some places on the 17th and 18th and frost occurred in many states. There was snowfall in the uplands of Santa Catarina on the 17th, and such an early snow had not been recorded for 30 years. Baroclinic synoptic wave intensification followed by surface low development in association with a cut-off low formation in the middle and upper troposphere were responsible for strong surface southerlies over Argentina on the 16th and 17th. Cold air advection by the southerlies in the eastern sector of the high-pressure centre was responsible for the northward projection of the high pressure cell east of the Andes. The whole event was well predicted by the Centro de Previsão de Tempo e Estudos Climáticos/Instituto Nacional de Pesquisas Espaciais (CPTEC/INPE) operational global and regional models with lead times of 120 and 60 hours respectively

Four different models for the prediction of frost events caused by air cooling due to thermal radiation are presented in this paper. Three of them are based on polynomial functions that simulate different types of temperature variations in comparison with an exponential one. The fourth, a post-processing method based on Kalman filters, is proposed for those cases where a systematic type of bias has emerged.

This paper describes a new model developed by the Met Office to predict night-time light levels. The Met Office Night Illumination Model (MONIM) predicts light levels both in the visible (photopic) range and in the waveband to which night vision goggles (NVGs) are sensitive. The model will be used operationally for support of night-time flying operations. The model is described in detail and its light-level forecasts are compared with observations.

Astronomical research at present requires that a telescope with an aperture diameter of between 50 and 100 metres be constructed within the next 10 years or so. This new generation of telescopes will be called OWL (Overwhelmingly Large), and it represents one order of magnitude increase in size over today's telescopes. Selection of an ideal site for this giant telescope is dependent on many climatological, meteorological and geomorphological parameters (Grenon 1990). Among these are cloud cover, atmospheric humidity, aerosol content, airflow direction and strength, air temperature, topography, and seismicity. Even relatively minor changes in weather patterns can have a significant effect on seeing conditions (Beniston et al. 2002).

The breakpoint format for rainfall data, by recording the rainrate and the times when the rainrate changes, consists of variable length periods during which rain of constant intensity fell. It is compact and retains a level of detail equivalent to that found in high temporal resolution data. However, breakpoint data are available only at the relatively few locations where automatic raingauges produce pluviographs. On the other hand, radar observations of rainfall provide precipitation data at a high spatial density but at poor time resolution. Advantage can be taken of this high spatial density to give more locations where breakpoints are available through interpolation between the radar images to provide precipitation data at sufficient temporal resolution for the extraction of breakpoints. The interpolation scheme, based on the PARAPLUIE method, detects the movement and evolution of precipitating echoes to provide additional interpolated radar images. Breakpoints, extracted at pixels of the images that contain raingauges, showed similar overall accumulations and characteristics when compared with those of the collocated raingauges. The similarity is particularly so for gauges within a 50 km radius of the radar where the spatial resolution of the radar and the temporal resolution of the raingauges are equivalent.

An expert system (MEDEX) for predicting the gale-force onset, continuation, and cessation of seven major wind types within the Mediterranean basin has been designed, developed, and tested. The six wind types consist of the bora (flowing through both the Adriatic and Aegean Seas), etesian, levante, mistral, sirocco and westerly (poniente and vendaval). Except for the sirocco, these winds result from synoptic situations that lead to topographical channelling. MEDEX is rule-based and incorporates fuzzy logic to handle both objective and subjective inputs, the latter being a unique application of fuzzy logic. MEDEX has approximately 330 fuzzy rules, covering the seven winds in both winter and summer seasons. While MEDEX has been designed as a nowcasting tool (0–12 h), it can be applied to any future time for which forecasting charts (consisting of surface pressure and 500 mb height fields) are available. Inputs consist of objective pressure gradients in addition to subjective interpretations of various synoptic features. These inputs, as well as the corresponding rules, were developed, tuned, and verified over a two-year period during which the weather conditions from 539 days were individually analysed. Ground truth verification was produced primarily from over-water Special Sensor Microwave/Imager (SSM/I) wind speed measurements but also included observations as available, model predictions, and official Navy wind warnings. Evaluations of MEDEX performance for both onset and cessation of winter and summer winds are presented. In addition, comparisons with forecast statistics for the Navy's regional weather centre in Rota, Spain show that MEDEX has comparable forecasting skill.

Over the last decade, research has greatly increased into the use of Geographical Information Systems (GIS) in a variety of applications that involve the processing of climatological and meteorological data. This upsurge of interest is related directly to the fall in price of ‘commercial off-the-shelf’ GIS products together with large advances in computer processing ability. Add to this the proliferation of the Internet and the result is a set of fast real-time bespoke solutions and visualisations for many national meterological services and end-users.

Hydrometric information is fundamental to the planning, operation and management of water resources and flood defence. The data network is greatly valued by those who use it, yet assignment of a monetary or economic value is problematic. Data managers are often frustrated by the need to continue to justify a network. The paper focuses on the uses of data within the Environment Agency of the UK, explores the difficulties in assigning a value to the data and attempts to review the value ascribed to such data by drawing on some UK and wider examples.