A ground-based network of Global Positioning System (GPS) receivers can be used to determine the excess propagation path of the GPS signals. Using surface pressure data it is possible to derive the excess path caused by water vapour. This quantity is often referred to as the wet delay. We have studied the relationship between the wet delay and the integrated precipitable water vapour (IPWV), based on more than 120 000 radiosonde profiles from 38 sites in Europe. We have studied four different models based on the surface temperature, the latitude of the site, and the time of the year in different combinations. The accuracies of the models have been evaluated and also compared with the accuracy resulting from the use of published model parameters derived from radiosonde data acquired in the United States. The models using the surface temperature data yielded the smallest relative root mean square errors of 1.14% and 1.15% respectively. We find that the accuracy can sometimes be further improved, typically down to 1.02%, by adjusting the values of the model parameters to the area or site of interest.

We study the contribution of surface data to convection nowcasting over regions of modest orography and under weak synoptic forcing. Hourly mesoscale analyses are performed using the CANARI optimal interpolation analysis scheme, which combines first-guess fields from the fine mesh (10 km) ALADIN model with hourly routine observations arising from a mesonet of automated ground stations. These analyses then allow the computation of diagnostic parameters that quantify convective instability, low-level lifting processes and moisture supply: these are the convective available potential energy (CAPE) and the moisture convergence (MOCON). A tuning of the analysis scheme is needed first for it to fit the meso-?-scale. Then, the skill of the computed diagnostics for convection nowcasting is evaluated by comparing their fields with radar reflectivities observed between one and four hours after the analysis time. This is done for four selected convective situations. With regard to thunderstorm triggering, results show that this usually happens over areas of persistently high values of CAPE which undergo convergence continuously from four to one hour before the event; on the other hand, areas of persistent divergence are never associated with convective developments. In addition, the proposed criteria allow a significant reduction in the areal extent of predicted thunderstorms (i.e. decreasing the false-alarm rate) compared with what can be currently done on an operational basis, while maintaining a low non-detection rate. As to convection monitoring, we find that the organization of convective systems into a reflectivity line is preceded by a similar organization in the MOCON field from one to three hours ahead.

During the past decade hydrologists have become increasingly aware of the problems of fluvial flood prediction during periods of intense convection, particularly in urbanised catchments whose rainfall-runoff responses tend to be rapid. Existing approaches to deterministic, short-range rainfall prediction are often deficient in their treatment of convective precipitation because they cannot resolve individual convective clouds or effectively model their evolution. In 1994 the UK Met. Office established a joint R&D programme with the Environment Agency (responsible for flood prediction in England and Wales) to explore the benefits of an Object-Oriented conceptual Model (OOM) of convection in the nowcasting of fluvial floods. This involved the development of an automated nowcasting system (GANDOLF) designed to run the OOM during episodes of air mass convection. This paper describes the structure and function of the GANDOLF system and compares the performance of the OOM with that of two other precipitation models routinely used by Thames Region of the Agency.

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 two latest changes introduced during 1998 into the Ensemble Prediction System (EPS) operational at the European Centre for Medium-Range Weather Forecasts (ECMWF) are described. The first change, the inclusion of instabilities growing during the data assimilation period in the generation of the EPS initial perturbations, increased the probability that the analysis lies inside the ensemble forecast range. The second change, the introduction of a simulation of random model errors due to parametrized physical processes, improved in particular the prediction of precipitation. The performance of the ECMWF Ensemble Prediction System from 1 May 1994 to 2 March 1999 is assessed using different statistical measures. Results indicate that the general performance of the EPS has been improving over the years. Finally, ongoing research projects on predictability issues developed either at ECMWF or at European research institutes in collaboration with ECMWF are discussed

The introduction of a new medium-range weather forecast presented an opportunity to establish a verification procedure for monitoring the reliability and accuracy of probability of precipitation (PoP) forecasts. The need for verification is discussed and a simple verification procedure described, using reliability diagrams as a visual guide to forecaster bias, Brier Scores to give levels of accuracy and Skill Scores to demonstrate skill levels when comparing forecast methods. The application of this verification procedure in a small meteorological office, with no dedicated research facilities, is presented and discussed.

The influence on the propagation of coastally trapped disturbances (CTD) of large gaps in the bounding coastal mountains is studied via application of the Colorado State University Regional Atmospheric Modeling System (RAMS). By large gap it is meant that the width is of order 25–50% of the Rossby radius. In order to isolate the effects of the gaps, idealised simulations are used. Thus, RAMS is configured with a straight north-south coastline with a ramp-shaped coastal mountain range in which a gap is inserted and a gravity current-like CTD generated through cooling the low level atmosphere at the southern end of the domain. Radiative, land surface and cloud parameterisations are turned off to avoid sea breeze generation and other thermal effects and to focus on the dynamical effect of the gaps. Four simulations are conducted: no gap in the coastal mountain barrier, an insertion of a narrow (80 km) gap, a wide (160 km) gap, and an idealised island in the barrier. It is found that the gaps influence CTD propagation by effectively stalling the propagation at their mouth for a period and subsequently weakening the gravity current-like features (vertical extent, strength and abruptness of the wind and stratification changes at the leading edge). The idealised island has a less obvious impact since it is bounded by relatively narrow straits and does not allow communication with the model interior like the gaps do. Although lack of data and the nature of the idealised simulation prevents a detailed assessment of the simulations against real cases, qualitative comparisons with observed events in western North America and south-eastern Australia are made. Application of these results to air quality and forecasting issues is discussed.

A method has been developed for retrieval of wind velocity profiles from satellite sounder radiances using a neural network technique. Each wind velocity represents a mean value for a layer approximately centred at that standard height or pressure level. The neural network input vector includes sounder radiances from the Geostationary Operational Environmental Satellite (GOES), plus ancillary information such as latitude and longitude. A set of ‘co-incident’ rawinsonde soundings provide the ‘truth’ data set. In the initial results root mean square errors for the standard levels from 850 to 300 hPa have about the same magnitude as those for current methods such as tracking of moisture features in GOES imagery. As with more traditional methods, the largest root mean square error occurs near the tropopause. Although the results to date appear highly promising, considerable further work is needed before we have an operationally capable technique.

An operational procedure for alerting the public of stressful weather (exceptionally hot and cold situations) in Hong Kong has been implemented since late 1997. From a 28-year (1968-1995) climatological database, criteria based on a weather stress index (WSI) was established to define stressful weather conditions. A special bulletin is issued to the public when exceptionally high (hot, i.e. ≥ 97.5%) or low (cold, i.e. ≤ 2.5%) WSI is forecast. The bulletin provides information on current or predicted stressful weather conditions as well as statements on precautionary action to take. A study was also carried out to examine the effects of WSI on mortality rates on days with extreme WSI. Preliminary results indicate that there was a relationship between low WSI and mortality rates in winter. In summer, however, the results were not conclusive.

The development of a hurricane-like vortex over the Mediterranean Sea was studied using (mainly) the UK Met. Office Unified Model. The Mediterranean cyclone formed in the morning of 15 January 1995 over the sea between Greece and Sicily. Strong convection was observed prior to its genesis. During the longest part of the cyclone's lifetime, strong surface fluxes and, as a result, deep convection existed in its vicinity. Its track was influenced by the surface fluxes and the flow in the wider region. The forecast of the mesoscale and limited-area models reproduced the general characteristics of the actual system as they appeared at the surface and upper-air charts and at the satellite imagery. The investigation of the cyclone's characteristics gave strong evidence (including an ‘eye’ and a warm core) to support the initial assertion that it was similar to tropical cyclones and some polar lows. Baroclinic instability does not seem particularly important, although the cyclone formed at the edge of a baroclinic zone. A numerical experiment showed the vortex did not develop in the absence of surface heat and moisture fluxes. Another experiment showed that sensible and latent heat fluxes were equally important in its development.

The aim of this work is to present a methodology to downscale weather forecasts (i.e. to give regional-scale forecasts based on synoptic-scale forecasts from a numerical model). This methodology consists of the regionalisation of a large zone into thermally homogeneous meteorological regions followed by a study of how different types of weather affect them. Once the meteorological behaviour of each region is determined, cokriging is used to predict its temperature knowing the current temperature and type of weather expected (i.e. based on forecasts from a numerical model). Thus, from a methodological point of view, it is shown that geostatistical prediction techniques can be used in the meteorological sciences by combining classical multivariate statistics and space-time prediction techniques. To illustrate this methodology, the maximum and minimum temperatures at 54 observatories in Catalonia in winter and synoptic data from Barcelona's airport have been used. The method proposed in this paper, which represents an improvement in temperature prediction compared to that given by classical statistical methods, allows the forecasting of temperature. This case study shows that forecasts can be easily downscaled and that, to produce daily forecasts, just a pocket calculator is needed. The results are presented for a one-day forecast, but this technique can be applied to forecasts over a four-day period.

A mesoscale model has been applied over the Greater Sydney region to an air pollution episode resulting from fire hazard reduction burns from 12 to 14 April 1997. The episode was characterised by two distinct events in a period of sustained light winds, which included humid sea breezes and land breezes. The first event occurred early in the period when the atmospheric circulation involved either weak sea breezes or no sea breeze at the coast. Described here are the meteorological conditions associated with the inter-regional transport of smoke from a prescribed, or hazard reduction, burn just north of Sydney, which concentrated smoke in the eastern part of the Sydney metropolitan area. The second event, which occurred late in the episode, was local in nature but was induced synoptically by the large-scale airflow by a change in wind direction that transported smoke and fog over a major highway. Also described are the meteorological conditions that contributed to the disruption of a major highway caused by a multiple vehicle pile-up just to the south-west of Sydney later in the period. Numerical model simulations using the University of New South Wales (UNSW) NWP model, which used archived real-time data, accurately predicted both the concentration of pollution in the eastern part of the Sydney metropolitan area in the first event and the hazardous smog formation and timing of the clearing south-west wind change in the second event.

Contribitions to the notes and news section are welcome. They should be sent to: Meteorological Applications, Met. Office College, Shinfield Park, Reading RG2 9AU, UK (Fax +44 (0) 1344 85 5410, E-mail: [email protected])

Camelot is a database system designed for output from the climate model run by the Hadley Centre of the UK Meteorological Office. It contains metadata for climate model output held in a tape archive and on the workstation system used for analysis. The system is required because of the large and rapidly growing volume of archive data and the great variety of uses of the climate model. Camelot enables the user to find out about what has been archived and to retrieve data selectively to the workstations with specifications given in terms of metadata rather than files, without recopying data already held online. The PV-WAVE analysis environment used on the workstation system also allows the user to access the data by reference to its metadata.

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.

Contribitions to the notes and news section are welcome. They should be sent to: Meteorological Applications, Met. Office College, Shinfield Park, Reading RG2 9AU, UK (Fax +44 (0) 1344 85 5410,E-mail: [email protected])

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.

Comments on 'Rainfall estimates of one-day PMP for India' by P R Rakhecha and C Clark (Meteorological Applications, 1999, 6: 343-350)

Contribitions to the notes and news section are welcome. They should be sent to: Meteorological Applications, Met. Office College, Shinfield Park, Reading RG2 9AU, UK (Fax +44 (0) 1344 85 5410, E-mail: [email protected],)