Estimates of the downward global solar and long-wave radiations are commonly made using simple models. We have tested the estimates produced by a number of these simple models against the values predicted by the radiative transfer model used in a climate model in order to determine their suitability for global applications. For clear sky, two simple models were comparable, but under cloudy conditions a combination of a clear-sky model based on the Angstrom-Prescott equation (which deals with the downwelling solar radiation) with a cloud transmissivity utilising total cloud fraction proved best. The lowest root mean square errors were 27 W m−2 for clear-sky global solar radiation and 90 W m−2 for cloudy conditions. For downward long-wave radiation in clear-sky conditions, the model of Garratt (1992) performed best with a root mean square error of 24 W m−2. However, in cloudy conditions the model of Idso & Jackson (1969) performed best with a root mean square error of 22 W m−2, and, as it performs nearly as well as that of Garratt (1992) in clear-sky conditions, it is probably the best choice.

The annual amount of salt used to prevent slippery conditions on roads was estimated with the help of winter monthly mean air temperatures. Air temperature is measured reliably at almost all meteorological stations and the use of air temperature for the estimation of road maintenance conditions is a tempting alternative compared with the use of more complicated indices calculated using meteorological observations available at only a small number of stations. It was found that in Finnish climatological conditions, warmer than normal weather in November and March and colder than normal weather in December, January and February reduces the need for salting. Warm mid-winter months mean slippery conditions and, consequently, an abundant use of salt. The temperature explained about 60% of the annual variation of salt use and thus gave surprisingly good estimates for the salting amounts.

On 24 May 1993, part of the Iberian Peninsula was affected by convective activity concentrated on the eastern side of a low-level thermal boundary which was orientated north–south. A series of storms evolved into a squall line striking at the province of Guadalajara (Central Spain). The southernmost cell of this mesoscale convective system generated a tornado that hit the city of Sigüenza between 1930 and 1940 UTC. This paper has two objectives. The first aim is to investigate the ingredients (buoyancy, wind shear, vertical distribution of moisture content, and synoptic and mesoscale lifting mechanisms) and the atmospheric agents (mid-upper level trough, low level-boundary, convergence zones, etc.) which favour the production of severe storms. The second one is to analyse information provided by operational remote-sensing systems (satellite, radar and lightning) in order to test and evaluate some conceptual models of severe storms in an operational environment. For the first objective, the ingredient-based methodology will be shown to be a valuable operational approach allowing forecasters to focus on the possibility of a severe event using the appropriate diagnostic or prognosis tools. For the second objective, special attention is given to the suitability of conceptual models associated with convective phenomena and the potential value of such models (lightning charge related to wind shear and radar-based multicell conceptual models) for understanding the observed processes. From an operational viewpoint some problems may arise because of the technical characteristics of the equipment, particularly related to radars (e.g. scanning strategy, distance between radar and targets, and radar signal processing and displaying). Possible guidelines for using volumetric radar data (non Doppler) for deep convection monitoring are suggested for rugged and complex topographical regions.

A new method (XRWIS) to predict the minimum road surface temperature for the winter maintenance of roads, using geomatics, has been tested in Poland. A geographical database was constructed for a 200 km test route from Krakow to the Slovakian border. A GPS survey to measure sky-view factor was carried out as part of the COST 719 project. A computer energy balance model ‘IceMiser’ was run retrospectively to predict road surface temperature every 20 minutes for every 20 m along the road using hourly weather data recorded at two adjacent climate stations: at low elevation in Krakow and at high elevation in Zakopane, in March 2003. A GIS was used to visualise the predicted road surface temperatures. The IceMiser model was verified by comparing the predicted road surface temperatures with measured road surface temperatures at a number of road weather outstations along the route. The results for four road weather outstations are discussed. The best results are for Libertow (264 m) and Myslenice (304 m) close to Krakow, whereas the results for Skawa (518 m) and Piatkowa Gora (649 m) at high elevation are not as good, probably due to their distance from the Zakopane climate station.

A divergence-based procedure for diagnosing development, based on a two-layer model of the atmosphere, is discussed. It is pointed out that thinking in terms of vorticity advection and thermal advection is directly equivalent to considering divergence due to advective and isallobaric components of the ageostrophic wind. It is shown that such an approach is similar in basis to the quasi-geostrophic omega equation, but it is argued that it is more suitable for subjective application. However, simply estimating vertical velocity or instantaneous pressure tendency from synoptic charts is of questionable accuracy and little operational use, especially since such diagnostics are much more reliably obtained from NWP models. It is necessary to think in terms of the development and movement of features in the flow of the upper as well as the lower troposphere, and their vertical interactions.

The term ‘medium range’ is taken to refer to forecasts for lead times ranging from about 2 or 3 days ahead up to about 10 days ahead. A wide variety of numerical model products are available to the forecaster nowadays, and one of the most important of these is the ECMWF Ensemble Prediction System (EPS). This paper shows how forecasters at the Met Office use these products, in particular the EPS, in an operational environment in the production of medium-range forecasts for a variety of customers, and illustrates some of the techniques involved. Particular reference is made to the PREVIN post-processing system for the EPS which is described in the companion paper by Legg et al. (2002). Forecast products illustrated take the form of synoptic charts (produced primarily via Field Modification software), text guidance and other graphical formats. The probabilistic approach to forecasting is discussed with reference to various examples, in particular the application of the EPS in providing early warnings of severe weather for which risk assessment is increasingly important. A central theme of this paper is the vital role played by forecasters in interpreting the output from the models in terms of the likely weather elements, and using the EPS to help assess confidence levels for a particular forecast as well as possible alternative synoptic evolutions. Verification statistics are presented which demonstrate how the EPS helps the forecaster to add value to the wide range of individual deterministic model products and that furthermore, the forecaster can improve upon many probabilistic products derived directly from the ensemble.

The motivation for this study stems from the fact that there are no uniform criteria to identify the onset of the summer monsoon at a particular location. Furthermore, proper understanding of the events that culminate in the onset of the monsoon is crucial to allow its prediction over various time scales. An attempt is made to elucidate the characteristics of the onset and retreat of the monsoon using a global data assimilation and forecast system. For this purpose, the time series of net tropospheric large-scale budgets of kinetic energy, heat and moisture are examined over the Arabian Sea, Bay of Bengal and some land locations in India. The study makes use of daily data comprising operational analysis (0000 UTC) and forecasts (day1 through day5) produced by the National Centre for Medium Range Weather Forecasting, India, for the summer season of 1994.

The sequence of events associated with the advent of the monsoon is noticeably different at various locations. The horizontal fluxes of heat and moisture produce a divergence regime prior to the evolution and change to convergence during the onset of monsoon over the Arabian Sea. Similarly, intense diabatic cooling is noticed prior to the onset of mansoon and changes to heating subsequently. On the other hand, heat and moisture fluxes remain in the convergence regime well before the arrival of the monsoon over the Bay of Bengal. In turn, diabatic heating is noticed prior to the onset here. Onset characteristics at Bombay and Nagpur are similar to those of the Arabian Sea. However, features at Calcutta are identical to those over the Bay of Bengal. Further, the budgets of kinetic energy, heat and moisture depict monotonic decrease at various land locations corresponding to the retreat. Interestingly, the signature of retreat is very similar at the various locations considered here. These changes are observed in the analysis and further corroborated by the model forecasts. Despite the systematic biases, the model captures the essential signature of the onset at various forecast ranges.

Using a variety of trend extraction techniques applied to monthly observations from 1766 to 2002 on the EWP dataset, this study confirms that there is evidence of a trend towards wetter winters (November to March), particularly in December and January, and drier summers, although this feature appears to be concentrated in June and July. Trend precipitation in autumn (August to November) is more difficult to pin down, with alternative methods producing quite different trend patterns, thus highlighting how difficult it can be to extract trends from noisy data. In general, the results appear to be reasonably consistent with those of Alexander & Jones (2001) using rather different techniques, thus lending weight to the views expressed by them.

This paper illustrates a key lesson related to most uses of long-range climate forecast information, namely that effective weather-related decision-making requires understanding and integration of weather information with other, often complex factors. Northern Illinois University's heating plant manager and staff meteorologist, along with a group of meteorology students, worked together to assess different types of available information that could be used in an autumn natural gas purchasing decision. Weather information assessed included the impact of ENSO events on winters in northern Illinois and the Climate Prediction Center's (CPC) long-range climate outlooks. Non-weather factors, such as the cost and available supplies of natural gas prior to the heating season, contribute to the complexity of the natural gas purchase decision. A decision tree was developed and it incorporated three parts: (a) natural gas supply levels, (b) the CPC long-lead climate outlooks for the region, and (c) an ENSO model developed for DeKalb. The results were used to decide in autumn whether to lock in a price or ride the market each winter. The decision tree was tested for the period 1995-99, and returned a cost-effective decision in three of the four winters.

An objective Visibility Analysis/Forecast System has been developed as part of Nimrod, the United Kingdom Meteorological Office (UKMO) very-short-range forecasting system. The analysis blends satellite imagery and surface observations with a one-hour forecast using a variational algorithm. An extrapolation forecast is produced by adding UKMO Mesoscale Model (MM) trends to the analysis values. This is merged with persistence (the analysis) and MM values, to produce the final forecast. Results from eight case studies are presented, showing the potential of the system, which demonstrates significant improvements on the MM predictions (the currently available objective forecast method).

The moist convective instability over the Arabian Sea during the Arabian Sea Monsoon Experiment 2002 is studied using high resolution radiosonde data over the Arabian Sea off the west coast of India and inland at Goa (15°21′N, 73°51′E). The vertical structure of the lower troposphere is discussed in terms of the profiles of conserved variables and the time series of convective available potential energy (CAPE), mixed-layer depth, lifting condensation level and level of free convection. Analysis shows that the Arabian Sea within 200 km of the west coast of India is characteristically a low-CAPE region irrespective of whether it is a good or bad monsoon season. Little or no variability in the mean (0–500 hPa) lapse rate during July and August indicates the quasi-equilibrium state of the tropical atmosphere during the summer monsoon with an approximate balance between the convective and large-scale forces. Large-scale forcing for the ascent of surface air seems to be of prime importance for triggering deep convection and rainfall over the East Arabian Sea. Case studies of precipitating convection over inland at Goa during the monsoon indicate a moistening and warming of the lower troposphere following the convection.

The coastal climates of France have been studied using temperature and precipitation data from 1961 to 1990. It is found that the coastlines of the French Atlantic (and adjacent seas) and Mediterranean show many similar climatic features, but the coastal orientation and nearby topography cause some local climatic peculiarities. The relatively flat topography near the coasts of the North Sea, English Channel and the Atlantic result in a fairly homogeneous coastal climate. In these coastal regions there are days of frost each year, but the precipitation amount is usually not high, especially in summer on the Atlantic coast. On the other hand, the mountainous areas of Provence and the Riviera, which are adjacent to the Mediterranean, cause a juxtaposition of varied coastal climates. Large temperature and rainfall differences are observed, especially between the capes and the heads of gulfs, but also between the narrow and wide gulfs. During the warm months, some sea breezes cause coastal thermal peculiarities, which depend on the orientation of the prevailing flow.

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.

For forecasting strong dust storms in the central and western areas of the Inner Mongolia Autonomous Region an objective discrimination approach has been developed. The 37 strong dust-storm events that occurred in this area during the last 40 years have been classified into four circulation patterns. Using the method suggested by Saaty (1996), a priority relationship matrix was set up and the discrimination function was then established. Depending on the function values, a judgement is then made about whether the real-time situation belongs to any of the four patterns and, if so, to which one. If there is a match, then this typical event can be used as a basis for forecasting a strong dust storm. The successful application of this approach is demonstrated using results from 1998 and 2000.

A comparison of atmospheric transport and dispersion calculations over complex terrain was investigated using a mobile profiling system (MPS) versus standard meteorological balloons. Meteorological and sulfur hexafluoride (SF6) concentration data were collected and used to evaluate the performance of a transport and diffusion model coupled with a mass consistency wind field model. Meteorological data were collected throughout April 1995, and parts of August 1995. Both meteorological and concentration data were measured in December 1995. Once the models were validated, the comparison of performance with different upper-air data were accomplished. The models used included the SCIPUFF (Second-order Closure Integrated Puff) transport and diffusion model and the MINERVE mass consistency wind model. Evaluation of the models was focused primarily on their effectiveness as a short-term (one to four hours) predictive tool. These studies showed how the combination of weather and transport models could be used to help direct emergency response following a hazardous material release. The models were used in tandem to direct the deployment of mobile sensors intended to intercept and measure tracer clouds. The MINERVE model was validated for the specific terrain of interest using April 1995 data. The capability of SCIPUFF driven by realistic three-dimensional wind fields generated by MINERVE is demonstrated using data collected in December 1995.

The existing models for predicting road surface temperature are often based on surface energy balance which includes contributions from sensible and latent heat exchanges between the atmosphere and road surface. The similarity theory is usually applied to compute the atmospheric turbulent heat fluxes. However, the theory only applies in horizontally homogeneous situations, which is certainly not the case for road conditions. An investigation is carried out to see how much measured profiles of mean wind and temperature from a road meteorological station deviate from the profiles given by similarity theory. The difference between the data and the theory is revealed by comparing the observed gradients with the theoretical ones. The sign of the vertical gradients of all data and the relationships between two vertical gradients at different levels under near-neutral conditions are examined. It is found that the measured profiles are all systematically different from the theoretical ones. The deviation is discussed within the context of the possible existence of an internal boundary layer. Moreover, detailed temperature profiles were measured to study the development of temperatures in the first 2.5 m. The results show that the sign of the temperature gradient reverses in a layer between 0.8 and 1.4 m and that a large change in temperature occurs in this layer; these effects were probably caused by local advection.

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.

Ice accumulation on overhead railway wires can damage railway devices and disrupt railway services. In order to provide timely short-period forecasts of ice on the wires, an automated wire ice-prediction model is developed in this study. It is a statistical model with much simplified mechanisms of icing, and provides forecasts of wire surface temperature and wire surface state (icy or not), along with the forecasts of air temperature, dew point and wind speed, for three hours ahead. The model was validated against actual sensor measurements at the Dalton test site in north-east England during the winter of 1998-99. The results of the validation show that the model forecasts of wire surface temperature, air temperature, dew point and wind speed are slightly biased. The standard deviation of wire surface temperature forecast is 1.43°C. The accuracy of ice prediction when ice was reported is 81.8% and the accuracy of no-ice forecast when ice was not observed is 97.5%.

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.

A comparison between potential vorticity (PV) fields and water vapour imagery shows a close relationship in the region of the polar front. Examples are given showing the nature of this relationship and how a mis-match between the PV and the imagery can be used to detect errors in model analyses and forecasts. The nature of the relationship varies according to the synoptic situation and some of the possible pitfalls encountered by not applying additional checks using model humidity or ‘pseudo imagery’ fields are illustrated.