This brief paper further develops the objective front-plotting methodology described in Hewson (1996), to enable the tips of frontal wave cyclones to also be objectively identified. The method embraces a new definition of frontal waves, but is analogous to operational practice in that these waves are located where cold and warm fronts join. It is suggested that for the early stages of cyclonic development the new methodology will perform better than those previously published.

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.

Heavy rain events constitute a major operational problem in the Mediterranean area. The present work tries to contribute to a better understanding of this problem by considering the possible indirect influence of surface depressions in the triggering and location of the heavy rain. The idea is that any surface low contributes to the organisation of the low-level flow. This then affects the creation/intensification of a low-level feeding current of warm and wet air that can sustain heavy rain if there is convective instability. Two approaches have been used: (a) a numerical and synoptic approach, Part 1, and (b) a statistical approach, Part 2. In Part 1, the differences between a control run and a numerical experiment without the Iberian mountains for the 'MAP Piedmont flood event' are discussed. Among other effects, the Iberian orography seems to intensify a secondary low near Liguria as well as increase the mixing ratio convergence and the rainfall intensity near it. A few other cases of heavy rain are also briefly reviewed in order to identify the presence of a Mediterranean depression in the vicinity.

A physically-based 1-D heat and mass transfer model was tested to estimate the heat fluxes of an asphalt surface. The model was run for two summer months for a road test site in southwest Sweden. Parameters for thermal properties, surface runoff, radiation and turbulent transfer were obtained from a description of the road stratification and from the literature. Coefficients of determination (r2) 0.94, 0.93 and 0.97 were obtained when simulated results were compared with observations of net radiation, heat flow below the surface and surface temperature respectively, all with slope coefficients close to unity. In addition, simulation results elucidated the role of water vapour transport through the asphalt-soil profile and its effect on the latent heat flow from the surface. Problems were identified with closure of the heat balance in measurements based on discrepancy between simulated and observed sensible heat flux.

Two high resolution quantitative precipitation forecasts with different levels of realism are evaluated. Classical scores (bias, correlation and scores based on contingency tables) confirm that the two forecasts do not have the same quality. A multi-scale extension of these scores has then been made to produce a validation for hydrological purposes. Rainfall fields are integrated over surfaces of various scales. For better simulation, scores indicate an increase in the quality of the simulated precipitation for larger surfaces (typically more than 100 km2): the localisation errors are reduced by the aggregation. This helps to determine the usefulness of such forecasts for hydrological purposes.

An important phase in a nowcasting system is the diagnosis of the forecast variables. This work focuses on the diagnosis of precipitation. The Nimrod automatic nowcasting system at the Met Office (UK) has long used Meteosat visible and infrared data to supplement the data it receives from the UK weather radar network to produce rainfall analyses. With the advent of Meteosat Second Generation (MSG) attention has focused on how best to use the larger range of spectral information from MSG to improve the rainfall analyses. Earlier work at the Met Office had suggested artificial neural networks (ANNs) to be a useful tool for such applications. Pending the availability of data from MSG, ANNs were used to process data from appropriate visible and infrared channels on the MODIS instrument. Sixty daytime winter cases were collected, and Nimrod radar rainfall analyses provided ‘ground truth’ for both training and testing the ANNs. The optimal combination of MODIS channels was investigated and it was found that almost all the skill in rain/no rain discrimination was provided by the radiance values from six selected spectral channels. A notable result was that the 1.64 μm channel had no value as a discriminator when used alone, but produced a large increase in skill when used in conjunction with a visible channel. The ANN with MODIS data was found to outperform the corresponding Nimrod look-up table technique applied to Meteosat data. Application of the technique to SEVIRI data is proposed, as is extension to other seasons.

A brief description of the ECWMF Ensemble Prediction System is given, including the method for producing initial perturbations, products and verification, and planned developments.

Topographic exposure is a variable which represents the degree of shelter afforded to a location. A detailed knowledge of exposure is useful for a wide variety of applications, ranging from the dynamics of drifting snow to the optimal location of wind-farms. The degree of exposure can be calculated by considering the relative heights and distances of the horizon surrounding the point of study. Numerous approaches have been developed to measure this useful variable, from the early qualitative studies of wind-shaped trees to Pyatts's quantitative TOPEX assessment. However, these studies have been superseded by the use of computers and in particular the application of digital terrain models. These provide a fast office-based solution, virtually eliminating the need for fieldwork.

A multimodel short-range ensemble forecasting system created as part of a National Oceanic and Atmospheric Administration program on improved high temperature forecasting during the summer of 2003 is evaluated. Results from this short-range ensemble system indicate that using the past complete 12 days of forecasts to bias correct today's forecast yields ensemble mean forecasts of 2-m temperature, 2-m dewpoint temperature, and 10-m wind speed that are competitive with or better than those available from any of the model output statistics presently generated operationally in the United States. However, the bias-corrected ensemble system provides more than just the ensemble mean forecast. Probabilities produced by this system are skilful and reliable, and have been found to be valuable when evaluated in a cost-loss model. The ensemble appears to provide better guidance for more unlikely events, such as very warm temperatures, that likely have the greatest economic significance. Industries that are sensitive to the weather, such as power companies, transportation and agriculture, may benefit from the probability information provided. Thus, it is possible to develop post-processing for short-range ensemble forecasting systems that is competitive with or better than traditional post-processing techniques, thereby allowing the rapid production of useful and accurate guidance forecasts of many near surface variables.

Design floods for major dams throughout the world are based on the probable maximum flood (PMF) which results from the probable maximum precipitation (PMP). Estimates of PMP can be made using a number of methods, the most favoured being the hydrometeorological method involving maximisation and transposition of historic storms. In this paper the in situ maximisation and storm transposition approach has been used for estimating PMP for a one-day duration for different locations in India. Based on these PMP values a generalised map of one-day PMP was produced. It was found that one-day PMP over the Indian region varied from 70 cm to 170 cm, with high values in the mountainous areas of the Western Ghats and in some locations in the north-eastern parts. These results are considerably higher than those obtained using statistical analysis of daily rainfall records using Hershfield's method and they have serious implications for dam safety over much of India.

A continuously calibrated infrared (IR) geostationary satellite rainfall estimation technique (CCB4) is introduced, in the context of the Nile River Forecast System, an operational system for hydrological modelling and forecasting. The CCB4 incorporates near-real-time rain gauge data to continuously calibrate optimum IR rain/no-rain thresholds and daily rain rates on a daily time step. The ability of the CCB4 and two comparative techniques to estimate daily rainfall at the regional and pixel scales is assessed, using Meteosat IR imagery and gauge data from six wet season months covering three years. The CCB4 shows improved skill in identifying rain days and estimating daily rain amounts at a range of spatial scales, from regional to pixel scales. At the pixel scale, however, improved root mean square errors remain relatively high, ranging between 66% and 84% of the mean unconditional rain rate.

This paper describes the evaluation of three diagnostic wind models by direct comparison with wind field data. The models are the California Meteorological Model (CALMET), the Mass Consistent model (MCSCIPUF) associated with the Second Order Closure Integrated Puff (SCIPUFF) transport/dispersion model, and the Stationary Wind Field and Turbulence (SWIFT) model. The evaluation follows previous works by Chang, Franzese & Hanna, who compared the same three models, and by Bradley & Mazzola who evaluated SWIFT coupled with SCIPUFF. As with SWIFT, MCSCIPUF is incorporated in the Hazard Prediction and Assessment Capability (HPAC), while CALMET is linked with the California Puff model (CALPUFF), another transport and dispersion model. The Dipole Pride 26 (DP26) experiments, performed at the US Department of Energy (DOE) Nevada Test Site, are used as the source of the wind data. They provide a comprehensive set of meteorological data with wide-ranging atmospheric stability conditions over a complex terrain. Model calculations were compared with measured data in two phases. The first phase uses complete sets of data from eight locations (the 8M phase) as model inputs, and thus tests the ability of the models to reproduce input conditions. In the second phase, five of the measured wind sites are withheld from input, and instead used for validation of model calculations (the 3M phase). In the first phase, the errors were found (with some exceptions) to be quite small. In the second phase, mean absolute errors were found to be of the order of 1 ms−1 and 30°, with only small differences among models in terms of performance.

An operational lee wave forecasting code is described and tested on a number of actual cases. Although the solution procedure involves a well-known numerical algorithm, its implementation for operational purposes needs to be intelligent and flexible. The technique involves an eigenvalue problem based on the vertical structure equation for stationary, internal gravity waves. Since the orientation of the trapped lee waves is not known a priori, the code solves the eigenvalue problem for a range of wind directions centred on the low-level wind direction and makes a decision as to which direction will have the largest amplitude and most highly trapped lee wave. The amplitude of the lee wave motion depends on the intensity of orographic forcing in a spectral band centred on the resonant wavelength. For complex terrain this is difficult to quantify. Here, it is assumed that the Fourier spectrum of the orography follows a power law in wavenumber and that the wave amplitude is attributed with a ‘wave launching-height’ condition. One of the test cases considered is the notorious ‘Sheffield gale’ of 16 February 1962, and in that case the lee wave code does predict an unusually large-amplitude wave.

There are many cases where observed orographic enhancement has been plausibly attributed to the seeder feeder mechanism. To date, however, there have been few examples of radar data that clearly illustrate the concept of process, in the manner that the data presented here do. In this case study, vertical pointing radar data collected on the windward side of New Zealand's Southern Alps show two separated precipitating systems, with shallow rainfall enhanced by snowfall from aloft. Simple analysis of the synoptic conditions and rainfall data showed that this interaction did result in orographic enhancement. Further evidence of a seeder feeder event is given by simultaneous scanning radar data, which show a marked increase in raining area at the onset of the snowfall.

Forecasting the occurrence and strength of the nocturnal jet is important to aviation, especially to balloonists and small aircraft. Jets also have implications for pollution transportation and the clearance of low cloud and fog. A one-dimensional model showed that the main mechanism for the development of the nocturnal jet was a hybrid between an inertial oscillation in a layer within and above the jet core and a quasi-steady jet within the inversion. The model, supported by observations, suggests that a nocturnal inversion is required to produce a jet and that the strength of the jet maximum is approximately 1.3 times the geostrophic wind. A wind profile minimum is also predicted to occur when the inertial oscillation reaches a minimum in the stress-free layer and this was observed using a tethered balloon flown at Cardington.

The effect of screening on road surface temperatures is analysed using data from the Swedish road weather information system (RWIS) together with data from thermal mapping. The study deals with the analysis of temperature variations caused by shading of the road surface during clear daytime conditions and focuses on the relation between solar elevation and magnitude of road surface temperature differences between screened and sun-exposed areas. Also included is an analysis of how the temperature differences during the day affect the establishment of temperature variations after sunset and the time it takes after sunset for temperature differences caused by shading to decline. The results show that the magnitude of road surface temperature differences between sun exposed and screened sites (RSTdiff) that develop during clear day conditions can be attributed to solar elevation (β) and can be expressed by the equation: RSTdiff=−2.7+0.46(β). A relationship between the daily maximum temperature difference andthe preservation of the screening effect after sunset is observed. The effect after sunset can be described by: RSTdiff=A−Bh+Ch2−Dh3−Eh4+Fh5, where h is the time in hours after sunset and A to F are constants related to the time of the year at the actual site.

Cut-off lows that develop off the east coast of Australia are a major cause of elevated coastal sea levels in this region. Their duration often exceeds a day and the combination of elevated sea levels with the high rainfall that commonly accompanies these events means that coastal flooding can be a major hazard. The processes contributing to higher-than-normal sea levels are a combination of storm surge and breaking wave setup. Three events are modelled using an atmospheric model, storm surge model and wave setup model. The storm surges resulting from the depressions are found to explain between one-third and one-half of the measured sea-level residuals at the coast and are shown to develop in a regime of coast-parallel winds that produce onshore Ekman transport. The timing of the storm surges are well captured in two of the events in which the atmospheric depressions are of broad spatial scale and well simulated by the atmospheric model. However, a poor simulation by the atmospheric model of the location of the third low, which is of smaller spatial scale, results in an inaccurate timing of the onset of the surge. The wave setup model produces sea levels at the coast that are around 11% of the incident rms wave heights. The total modelled sea-level residual, obtained by adding the modelled wave setup to the storm surge, is qualitatively similar to the measured sea-level residual. However, the modelled values are higher, especially at Sydney where the tide gauge is situated in a sheltered harbour location. In such sheltered locations, it is expected that some attenuation of the wave setup would occur and suggests that sea levels on the open coast could be considerably higher during such events.

The influence of a strong low-level temperature inversion on the occurrence of lee waves, rotors and hydraulic jumps has been investigated using high resolution numerical model simulations. The aim of the work is to develop tools for forecasting hazardous winds downstream of mountains. Two-dimensional simulations were conducted for a range of inversion heights and strengths and a fixed hill shape; lee waves, rotors and hydraulic jumps were found to occur. The flow type depends largely on the ratio of mountain height to inversion height and the upstream Froude number. A flow regime diagram based on these two parameters has been constructed and suggests that rotors could be forecast using upstream profiles, which are generally readily available from numerical weather prediction models.

The applicability of the regime diagram for two-dimensional flow to flows over real terrain has been tested using three-dimensional simulations of flows over East Falkland, South Atlantic, under a range of upstream conditions. The flow type is found to be determined largely by the upstream profiles of wind and temperature, and the maximum height of orography directly upstream, indicating that the flow regime diagram can be used to predict flow type downstream of such terrain. Various three-dimensional flow phenomena occur, such as flow channelling through gaps, and could be taken into account to improve the information available from the regime diagram.

The Met. Office currently operates a high resolution mesoscale model that provides guidance on the likely formation of radiation fog on the spatial scale of 30-40 km. A project has been under way to build a Site-Specific Forecast Model (SSFM) using a one-dimensional version of the Met. Office Unified Model as a basis, with suitable modifications to its surface treatment driven by output from three-dimensional NWP models. The SSFM has been applied to the forecasting of radiation fog as part of the European Commission sponsored project 4MIDaBLE (4Dimensional Met. Information DataBase Linked across Europe). The first part of this paper describes the model formulation and discusses its behaviour simulating radiation fog in idealised cases and its sensitivity to geostrophic wind speed and initial humidity. In addition to the expected sensitivity to wind and humidity, it has been found that the treatment of surface exchange processes can have a small but significant impact on the formation of fog, its formation and clearance times and its depth.

Accurate prediction of thunderstorms during the pre-monsoon season (April-June) in India is essential for human activities such as construction, aviation and agriculture. Two objective forecasting methods are developed using data from May and June for 1985–89. The developed methods are tested with independent data sets of the recent years, namely May and June for the years 1994 and 1995. The first method is based on a graphical technique. Fifteen different types of stability index are used in combinations of different pairs. It is found that Showalter index versus Totals total index and Jefferson's modified index versus George index can cluster cases of occurrence of thunderstorms mixed with a few cases of non-occurrence along a zone. The zones are demarcated and further sub-zones are created for clarity. The probability of occurrence/non-occurrence of thunderstorms in each sub-zone is then calculated. The second approach uses a multiple regression method to predict the occurrence/non-occurrence of thunderstorms. A total of 274 potential predictors are subjected to stepwise screening and nine significant predictors are selected to formulate a multiple regression equation that gives the forecast in probabilistic terms. Out of the two methods tested, it is found that the multiple regression method gives consistently better results with developmental as well as independent data sets; it is a potential method for operational use.