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).

A portable meteorology station capable of measuring various atmospheric parameters (mainly ambient temperature, relative humidity, atmospheric pressure, wind speed and direction) was designed and built. The physical quantities were converted to electrical signals using suitable sensors. These signals were then processed and transferred to digital values to be stored in suitable memories. A nuclear radiation alarm system was also built, on the main board, to monitor the nuclear radiation releases levels. The system consists of three main parts: control board, data acquisition board and signals conditioning board. The overall system is controlled by a BASIC-8052 micro-controller.

Météo-France is preparing the implementation of a revised graphical summary of the synoptic situation. This graphical product has been developed since 1993 as, as far as possible, an extension of the existing framework published by WMO. Graphics remain the best way to convey rapidly a lot of information. However, the existing graphical language for meteorology has seldom changed over the years, while our dynamical understanding has improved. Its current use is in fact poorer than the one allowed by the WMO reference definition. In an attempt to overcome these drawbacks, a richer graphical language is proposed. Its goal is to show and, often, to explain areas of synoptic weather activity in the initial spirit of the Norwegian method, since it has been so successful in shaping modern operational meteorology. However, it takes into account the present understanding of cyclogenesis and large-scale dynamics: the actual and potential baroclinic interactions replace the polar front concept. Dynamical meteorology also provides rather precise guidance for structuring and preparing the summary. The proposed code is described and explained. Examples are shown, with most cases taken from the recent FASTEX (Fronts and Atlantic Storm-Track Experiment) project.

Coastal ridging is an important component of the weather of south-eastern Australia during the summer. The mesoscale aspects of such an event are often not well forecast, and this can have serious implications for aviation, shipping, air quality and bushfire control. A previous study by Speer & Leslie (1997) identified three types of coastal ridging according to the associated synoptic conditions. In this study, a coastal ridging event, which appears to have aspects of all three types, is examined with the aid of mesoscale manual analyses, surface and radiosonde data and simulations with a mesoscale numerical model (Colorado State University Regional Atmospheric Modeling System).

It is found that the model is able to capture the salient features of the event but, on the east coast, tends to produce a weaker event than that observed. The model ridging also tends to lag the observed feature on the east coast by a few hours. It is suggested that these model deficiencies may relate to deficiencies in the lower atmosphere air mass characteristics, which, in turn, may relate to the surface parameterisations in the model.

A multiscale study of a cold-air comma cloud that produced an area of heavy rain and locally severe weather has been undertaken by synthesising data from a research microwave Doppler radar and VHF and UHF Doppler wind profilers, along with routinely available radar-network, satellite, in situ and mesoscale-model data. The rain area was generated in the exit region of an upper-level jet characterised by laminated velocity perturbations. Some of the perturbations were attributable to inertia-gravity wave activity. The rain area itself is shown to have been composed of a well-organised set of mesoscale rainbands each being due to a mixture of upright and slantwise convection. The existence of the multiple rainbands may have been related to the multi-layered atmospheric structure upwind. Each of the rainbands had cold-frontal and warm-frontal portions, so as to form a series of mini warm sectors stacked along the axis of the comma cloud at roughly 70 km intervals. The multiple rainbands were accompanied by multiple fingers of overrunning low-θw air from part of a dry intrusion originating from just below a major tropopause fold. The fold contained an intense potential-vorticity maximum which appeared to be the focus of the overall system. The operational mesoscale version of the Met. Office's Unified Model, with its 12 km grid, is shown to have resolved many but not all of the key features of the rainbands. It is suggested that further improvements in very-short-range forecasting of important local detail could be achieved by further increasing its resolution and assimilating more mesoscale observational data.

There is a need to assess the areal depth of the probable maximum precipitation (PMP) over specified catchment areas for the safe construction of dam spillways. The large number of dams in India, coupled with their risk of failure, makes this need imperative both for the maximum use of water resources and for public safety. On the basis of rainfall data for the heaviest storms that occurred in different parts of India during the period 1880–1983, improved estimates of one-, two-, and three-day point PMP for India have been made. In this paper the distribution of areal PMP over specified catchment sizes is provided for the first time. The areal reduction factors (ARF) were based on envelope curves of major storms to give the ARF for areas of 10–20,000 km2. These factors were found to vary from 1.0 to 0.41, though there was no real difference between durations of rainfall. These values of ARF were then multiplied by values of one- to three-day PMP. The resulting maps allow a broad description of the spatial distribution of areal PMP and also provide a rapid and consistent estimate of the probable maximum flood (PMF) from the PMP.

For 500 km2 the areal PMP varies from 40 to 120 cm for one-day duration; from 70 to 200 cm for two-day duration; and from 75 to 270 cm for three-day duration. The pattern of PMP is consistent with the geography and available moisture.

A wake vortex warning system (WVWS) has been developed for Frankfurt Airport. This airport has two parallel runways which are separated by 518 m, a distance too short to operate them independently because wake vortices may be advected to the adjacent runway. The objective of the WVWS is to enable operation with reduced separation between two aircraft approaching the parallel runways during appropriate wind conditions. The WVWS applies a statistical persistence model to predict the crosswind within a 20-minute period. One of the main problems identified in the old WVWS is discontinuity between successive forecasts. These forecast breakdowns were not acceptable to air traffic controllers. At least part of the problem was related to the fact that the forecast was solely based on the prediction of crosswind. A new method is developed on the basis of 523 days of sonic anemometer measurements at Frankfurt Airport. It is demonstrated that the prediction of the horizontal wind vector avoids these difficulties and significantly improves the system's performance.

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.

Three severe storms caused great damage in Europe in December 1999. The first storm hit Denmark and Germany on 3 and 4 December, and the other two storms crossed France and Germany on 26 and 28 December. In this study, the performance of the Ensemble Prediction System (EPS) at the European Centre for Medium-Range Weather Forecast (ECMWF) in predicting these intense storms is investigated. Results indicate that the EPS gave early indications of possible severe storm occurrence, and was especially useful when the deterministic TL319L60 forecasts issued on successive days were highly inconsistent. These results indicate that the EPS is a valuable tool for assessing quantitatively the risk of severe weather and issuing early warnings of possible disruptions.

The impact of an increase of the ensemble system horizontal resolution (TL255 integration from a TL511 analysis instead of the operational TL159 integration from a TL319 analysis) on the system performance is also investigated. Results show that the resolution increase enhances the ensemble performance in predicting the position and the intensity of intense storms.

Although numerical weather prediction (NWP) models provide an objective forecast, poor representation of local topography and other features in these models, necessitates statistical interpretation (SI) of NWP products in terms of local weather. The Perfect Prognostic Method (PPM) is one of the techniques for accomplishing this. At the National Center for Medium Range Weather Forecasting, PPM models for precipitation (quantitative, probability, yes/no) and maximum/minimum temperatures are developed for monsoon season by using analyses from the European Centre for Medium-Range Weather Forecasts. The SI forecast is then obtained by using these PPM models and output from the operational NWP model at the Center. Direct model output (DMO) obtained from the NWP model and the SI forecast are verified against the actual observations. The present study shows the verification scores obtained during the 1997 monsoon season for 10 locations in India. The results show that the SI forecast has good skill and is an improvement over DMO.

Satellite images occasionally show the existence of mesocyclones (MCs) close to the eastern coast of Greenland, especially in the region of Angmagssalik/Tasiilaq. The forcing mechanisms of such MCs are investigated by means of numerical simulations with a three-dimensional mesoscale model. The special characteristics of the East Greenland topography are shown to be a key factor for the development of the MCs. The channeling of the flow in large valleys along the East Greenland coast leads to a convergence, implying a vertical stretching of the flow through the valleys and the generation of cyclonic vorticity. This convergence can be strong during intense katabatic storms, so-called Piteraqs, which are a much-feared phenomenon in that area of Greenland. During these situations the synoptic environment enforces the katabatic flow. The results of the simulations suggest a close relationship between the occurrence of Piteraqs and the generation of mesoscale vortices close to the East Greenland coast. An improved understanding of these processes will help forecasters to advise aircraft and ship operations near the coast of East Greenland.

We describe a method to remotely sense precipitation and classify its intensity over water, coasts and land surfaces. This method is intended to be used in an operational nowcasting environment. It is based on data obtained from the Advanced Microwave Sounding Unit (AMSU) onboard NOAA-15. Each observation is assigned a probability of belonging to four classes: precipitation-free, risk of precipitation, precipitation between 0.5 and 5 mm/h, and precipitation higher than 5 mm/h. Since the method is designed to work over different surface types, it relies mainly on the scattering signal of precipitation-sized ice particles received at high frequencies. For the calibration and validation of the method we use an eight-month dataset of combined weather radar and AMSU data obtained over the Baltic area. We compare results for the AMSU-B channels at 89 GHz and 150 GHz and find that the high frequency channel at 150 GHz allows for a much better discrimination of different types of precipitation than the 89 GHz channel. While precipitation-free areas, as well as heavily precipitating areas (> 5 mm/h), can be identified to high accuracy, the intermediate classes are more ambiguous. This stems from the ambiguity of the passive microwave observations as well as from the non-perfect matching of the different data sources and sub-optimal radar adjustment. In addition to a statistical assessment of the method's accuracy, we present case studies to demonstrate its capabilities to classify different types of precipitation and to work over highly structured, inhomogeneous surfaces.

This paper deals with the utility of Monin-Obukhov similarity theory for wind speed simulation in stable conditions over flat terrain in northern Croatia. Wind speed profiles observed through a 150-m deep layer are compared with those obtained from two simple wind speed models. For this purpose, the similarity functions for two stable classes based on Monin-Obukhov theory are locally determined. They are tested in wind speed simulations using a model based on the Monin-Obukhov theory (MO) and a model which combines the surface-layer model of Monin-Obukhov theory and the Ekman-Taylor type model (MOE). Under less stable conditions the combined model (MOE) produces better skill, whereas the model based on the Monin-Obukhov theory (MO) performed better under more stable conditions.

There is an increasing need for more rigorous and more broadly based determination of the economic value of meteorological services as an aid to decision-making on the appropriate level of funding to be committed to their provision at the national level. This paper develops an overall framework for assessment of the economic value of meteorological services based on the recognition that most national meteorological infrastructure and services possess the non rival properties of public goods. Given this overall framework for determination of both total and marginal benefits, four main methodologies appropriate for use in valuation studies - market prices, normative or prescriptive decision-making models, descriptive behavioural response studies and contingent valuation studies - are outlined and their strengths and limitations described. Notwithstanding the methodological limitations and the need for a much more comprehensive set of studies for the various application sectors, it is clear that the actual and potential benefits to individuals, firms, industry sectors and national economies from state-of-the-art meteorological and related services are substantial and that, at this stage, they are inadequately recognised and insufficiently exploited in many countries.

The qualitative parameters describing present weather are particularly difficult to automate. The weather types which create most of these difficulties are known, but little attention has been given to investigating the reasons for disagreements between the primary reference, the professional observer and an automated instrument. This paper provides a method - multiple logistic regression - to compare the WMO present weather codes detected by a professional observer and an automated system. A new approach is introduced to explain the errors relative to the official weather variables. Many weather periods have been analysed, but here results are presented for a snow period with slight and very slight precipitation. The best predictive variables were the dew point temperature, wind direction, relative humidity and visibility. The catalyst for this study was the need for better quality control and for tools to enable the development and the manufacture of better instruments and systems to detect present weather.

A method of estimating the economic value of weather forecasts for decision-making is described. This method has recently been used for user-oriented verification of probability forecasts, but is here applied to aid forecast users in optimising their decision-making from probability forecasts. Value may be calculated in the same way for either probability forecasts or deterministic forecasts, and thus provides the user with a direct comparison of the value of each in terms of money saved, which is more relevant than most standard verification scores. The method is illustrated using site-specific probability forecasts generated from the ECMWF ensemble prediction system and deterministic forecasts from the ECMWF high-resolution global model. It is found that for most forecast events and for most users the probability forecasts have greater value than the deterministic forecasts from a higher resolution model.

In this study a non-hydrostatic version of the PSU/NCAR mesoscale model is used to simulate an active western disturbance (WD) that affected the Indian region in January 1997. The role of the planetary boundary layer (PBL) and convection parameterisation schemes in the development of the WD is investigated. Analysis and predictions for some fields, including sea level pressure, geopotential height, temperature, horizontal wind and precipitation are examined. Some statistical scores are also calculated and compared. It is found that the performance of the Hong-Pan (as implemented in the NCAR MRF model) and Betts-Miller (or Grell) schemes as PBL and convection parameterisation schemes respectively are best compared to the other schemes used in this study.

This paper is concerned with meteorological processes that may have an influence on the occurrence of episodes of high pollution involving chimney plumes. It reviews meteorological mechanisms that could lead to or maintain high concentrations at distances beyond the range normally considered in dispersion models (distances greater than 30 km). Fundamental parameters of the atmospheric boundary layer are shown to largely determine short-range and long-range dispersion, but their values are usually not well known in specific cases. A simple estimate is provided of the magnitude of the maximum hourly average concentrations during a long-range fumigation episode from a tall stack.

Different options for funding the provision of meteorological services and for charging for the information provided are described and evaluated. The basic infrastructure and general forecasts and warnings have public good properties of non rival consumption and high costs of exclusion. For these, direct government funding and free provision to all are favoured. Value added meteorological services for use by small groups of specialised users have mixed good properties, and in some cases private good properties. In this case, setting fees at marginal incremental costs for value added services is favoured for reasons of efficiency and practicality. The other options considered include government funding with zero price, and loading user fees for a contribution to funding the public good supply costs.

The amount of solar radiation absorbed, reflected or transmitted by vegetation depends on the radiation distribution and the canopy properties. This study of the optical properties of an oak forest under foliated and non-foliated conditions is attempted in order to better understand radiation use and exchange. Canopy absorptivity, reflectivity and transmittivity to global solar and photosynthetically active radiation (S and PAR, respectively) are considered. To estimate the forest optical properties, radiation measurements above and below the canopy were taken in the middle of a 0.97 km2 natural, slightly sloped, 50-year-old, oak forest in the Northern Peloponese, Greece.

It was found that the greatest part of the incident S is absorbed by the foliated or non-foliated canopy. In the first case, absorptivity is close to 0.7, with reflectivity and transmittivity sharing the remaining portion. Before leaf appearance, although reflectivity may be only a little lower, transmittivity is considerably higher, allowing more than 40% of the incoming radiation to reach the ground. PAR is absorbed to an even higher degree by the green elements of the canopy, allowing only a 5% radiation loss, via transmission, since canopy reflectivity is practically zero. The ground contribution to the canopy short-wave radiation balance is small during the dense foliation period, but it becomes considerable during the leafless period. Especially when considering the balance in the PAR region, the ground has no effect.