Radar reflectivity information is often displayed in two dimensions, making it difficult to extract the structural characteristics of convective storms. The maximum radar reflectivity and the vertical profile of liquid water distribution in a vertical column of a convective cell is used to determine a structural and intensity classification of the cell. The application of this Storm-Structure-Severity classification is demonstrated by investigating a tornado event on 15 November 1998 in Harrismith, South Africa, by using volume-scanned reflectivity data from the S-band MRL-5 radar. The regions of convective development, decay and mesocyclone evolution are identified with this classification. The method is able to extract the three-dimensional structural information of convective storms embedded in volume-scanned reflectivity data and present it in a simple display format.

The south-west monsoon (June–September) is the major rainy season over India. Information about the occurrence of precipitation and the expected quantity at a specific place is important in many sectors of human activity. In this study, objective methods are developed to forecast the probability of precipitation (POP) and provide the quantity of precipitation forecast (QPF) over Delhi. As the onset of the monsoon at Delhi is around 30 June, the models are developed for the months of July, August and September (JAS) using surface and upper-air data for the period 1985–90 and tested with data from JAS for 1994 and 1995. A multiple linear regression equation is developed to forecast the POP and multiple discriminant analysis is used to produce the QPF in terms of one of four groups (0.1–1.0; 1.1–10.0; 10.1–30.0; and ≥ 30.1 mm). The QPF model is used only if precipitation is expected to occur (the POP forecast is turned into a categorical forecast). The categorical forecasts based on the POP exhibit positive skill scores consistently with both the development and independent data sets. The model for QPF also performed satisfactorily

For over two decades operational rainfall estimations from geostationary satellites have represented an ambitious aspiration of scientists and an identified need of operational meteorologists. A wide variety of infrared and combined visible and infrared methods have been proposed for the identification of suitable relationships between satellite-observed cloud top radiative features and rainfall at the ground. Microwave-based retrievals, however, correlate rainfall and internal cloud microphysical features more successfully. The most significant limitation, however, is the indirect character of the retrieval that correlates microphysical and dynamical cloud characteristics with rain amounts at ground level. METEOSAT Second Generation signals a new era for geostationary satellites with its new 12 channel imager SEVIRI and 15 minute full-disk image repeat cycle. SEVIRI is expected to contribute significantly to a better characterisation of clouds and atmospheric stability by means of improved infrared calibration, radiometric performances, imaging frequency and multispectral image analysis. The significant increase of multispectral cloud observations is expected to provide new data for the improvement of rainfall estimations from geostationary orbit. The anticipated progress from enhanced imaging frequency and multispectral data for the definition of new techniques is discussed. Considerations for operational applications, chiefly for nowcasting, are also provided as they are the main goal of the satellite. Future developments and synergies with other geostationary and polar orbiting instruments, passive and active, are finally considered as the ultimate strategy for more accurate instantaneous rainfall estimations at all latitudes.

This is the second part (statistical approach) of a work concerning the relationship between heavy rain and cyclonic centres in the western Mediterranean. Using a statistical approach we seek to verify the indirect role of the cyclone centres in locating, triggering or focusing heavy rain: a cyclonic centre – even if neither strong nor deep – may contribute to the low-level flow organisation and so to the creation or intensification of a low-level warm and wet current that can feed and sustain convective rain in favourable environmental conditions. The foundation for the statistical approach is several Several databases maintained and updated by the Spanish Institute of Meteorology (INM). These databases of cyclonic centres, heavy rain and strong wind events, and mesoscale convective systems (MCSs), all covering the western Mediterranean or part thereof, can be cross-referenced by looking for simultaneity between heavy rain events (or MCSs) and cyclone centres in the vicinity. We have found that in most of the heavy rain events (around 90%) there is a cyclonic centre in the vicinity, usually located so that its presence favours the creation or intensification of a feeding flow of Mediterranean air towards the area affected by heavy rain. The same occurs with the MCSs.

During November and December 1998, ten cyclonic disturbances over the North Atlantic and Mediterranean area were studied. Features in potential vorticity (PV) fields were tracked in the analyses and short-range forecasts of the Spanish version of the HIRLAM model (HIRLAM-INM) and the ECMWF model. They were superimposed on the corresponding Meteosat water vapour (WV) images with the aim of ascertaining the qualitative relationship between Meteosat WV imagery and PV fields that might be used to assess NWP model behaviour. Two cases predicted incorrectly by HIRLAM-INM were studied in detail: a rapidly deepening depression over the Atlantic (26–27 November 1998) and a cut-off low process over the western Mediterranean (3–4 December 1998). For evaluating HIRLAM-INM model behaviour, absolute vorticity at 2 PVU surface, positive PV anomalies at 500 hPa as well as pseudo WV images were compared with Meteosat WV pictures. Being synthetic products of the numerical model, the pseudo WV images were used to indicate the strength of the relationship between the Meteosat WV imagery and the PV fields, and to indicate whether any mismatches correspond to real NWP model errors. It is demonstrated that the HIRLAM-INM phase error at 3–4 December 1998 was clearly seen in the comparison between Meteosat and pseudo WV images.

In this study, thermal mapping along the road and recordings from field stations in the Swedish Road Weather Information System (RWIS) are analysed. The purpose is to explore how long it takes for air and road surface temperatures to adjust from clear to overcast conditions after a warm front arrival, and to see if there are any variations in reaction time between the two temperatures. The study also focuses on whether different sitings affect the adjustment time when the warm front arrives and if the time of arrival is important for the resulting temperature variations. The results show that weather preceding the front, temperature difference when the front arrives, arrival time of the front, wind speed and the occurrence of precipitation all have an effect on the adjustment time. Air temperature reacts faster to a weather change than the road surface temperature. During the evening the road surface temperature can still be affected by previous shading for up to four hours after a front arrival. When the total cloud cover increases above 4 oktas before noon there are no or small lags in reaction time between air and road surface temperature, since there is insufficient time to establish shading effects.

In this paper the position and intensity of the long waves around South America are related to the precipitation in Argentina. For this purpose indices of zonal and meridional circulation as well as of the curvature of geopotential height at 1000 and 500 hPa were selected. The daily 12 UTC analyses from the European Centre for Medium Range Forecast for 1980 to 1988 were used, averaged over at least five days. A strong positive (negative) anomaly of the meridional index at 1000 and 500 hPa is associated with a negative (positive) anomaly of precipitation. Positive (negative) anomalies of the zonal index at both levels are related to negative (positive) deviations of rainfall. Positive (cyclonic) anomalies of the curvature at 500 and 1000 hPa correspond to enhanced precipitation over the central-eastern part of the country, while over the central-western part the correspondence is of opposite sign. Once methods for forecasting long wave behaviour are available, including for the region around South America, the results may be used to produce medium-range and long-range forecasts of precipitation anomalies over Argentina.

This study measured the amount of hoar-frost on the road surface with the help of copper plates. The amounts were compared to data collected at a station in the Road Weather information System (RWiS) together with additional measurements of the temperature and humidity profile above the road surface. The data were used to specify the weather parameters when hoar-frost forms on the road surface and relate the measured amount of hoar-frost and resulting friction to measured weather parameters. The aim was also to see whether there was an increased possibility of predicting the amount of hoar-frost with additional measurements of the temperature and humidity profiles. The results of this study confirm that the amount of hoar-frost deposited on the road surface increases with increasing average wind speed, increasing maximum difference between dew point (Td) and road surface temperature (Tr) and increased duration when Tr < Td, measured at the RWiS station. However, the difference between Td and Tr failed to indicate hoar-frost on several occasions due to the location and poor accuracy of the measuring equipment. Measurements of temperature and humidity in a profile up to 2.5 m above the road surface showed that the air layer up to 0.4 m reflected conditions at the road surface whereas the surrounding areas influenced the measurements at higher levels. A forward multiple regression performed on the data showed that measurements of temperature and humidity at lower levels improved predictions of deposition of hoar-frost on the road surface. Measurements of friction on the road surface when hoar-frost was present showed that friction decreased with increasing difference between Td and Tr and it also increased with the duration of Tr < Td. There was no correlation between the amount of hoar-frost and friction at the road surface. The friction is probably influenced not only by the amount of hoar-frost but also by the structure of the hoar-frost, which has not been determined in this study.

Two methods for estimating ten-day rainfall totals from Meteosat infra-red imagery were compared for the April-June 1996 ‘long rains’ of Kenya in an area covering the eastern highlands and the Tana and Athi river basins. One of these (the Bristol ‘B4’ method) was then used for rainfall estimation for the whole of Kenya, for November 1996 and the other, the TAMSAT ‘Cold Cloud Duration’ (CCD) method was used to estimate rainfall for the whole of Kenya for November 1997 to April 1998. April-June 1996 was an unusual season with very few large rainstorms. For this comparison period the B4 method gave better estimates of actual rainfall than the TAMSAT method because it used a variable cold cloud threshold temperature and ongoing calibration against rain gauge data. Comparison of ten-day CCD totals with rainfall for the 1997-1998 period indicated that using the TAMSAT method gave best rainfall estimates for the arid and semi-arid areas of eastern and northern Kenya and for months other than the main rainy season months of November and April. Both methods could be used successfully to identify periods with well below or well above average rainfall even over highland areas, and they are therefore useful for providing food security early warnings.

Clear air turbulence (CAT) at high altitude remains a hazard to aviation which can result in passenger injury and aircraft damage. Two limited surveys of CAT events over South Africa, 1993-1995 (inclusive) and 1998, are used to illustrate the most likely synoptic conditions under which CAT can be expected. A case study of CAT associated with an upper-air trough and a mountain wave is presented. The study also evaluates the effectiveness of the Ellrod Turbulence Index (ETI) derived from model data provided by the UK Met. Office. A forecast of ETI derived from the Global Spectral Model of the United States National Center for Environmental Prediction (NCEP) is also reviewed.