The use of the SSM/I sea ice concentration data together with a sea surface temperature (SST) analysis in a numerical weather prediction (NWP) model is described. The ice concentration data provide a useful representation of the SST freezing-point isotherm while the SST field allows a quality control of the sea ice data. A land-sea mask, an orography field and a climatological SST field are also used to improve the analysis of surface temperature in some inland seas and lakes. A monthly mean sea ice concentration climatology has been created from 13 years of the ECMWF re-analysis ice concentration dataset which was also derived from satellite multi-channel microwave radiometer data. Data assimilation experiments have shown a small positive impact in the southern hemisphere model forecast scores when this new surface field is used in the NWP model.

The British Antarctic Survey operates both aircraft and ships in the Antarctic. High quality weather forecasts can have a large impact on safe operations at high southern latitudes. This paper assesses the benefits the weather forecasts have had on safety and on costs in the Antarctic during the 1995/96 season. The forecasts of cloud amount, cloud base and weather have a greater accuracy than persistence although the accuracy is lower than might be expected for an airfield in the United Kingdom. The major advantage in having good forecasts available at Rothera would seem to be an increased safety. Although cost benefits exist they are more difficult to assess.

A very short range forecasting system has been developed which integrates nowcasting techniques with Numerical Weather Prediction (NWP) model products to provide forecasts over the UK and surrounding waters up to six hours ahead. There are three main components, producing analyses and forecasts of precipitation, cloud and visibility, respectively. The precipitation rate analysis uses processed radar and satellite data, together with surface reports and NWP fields. The forecast is based on an object advection technique, modified for growth and decay using model products. Related variables, such as precipitation type, are also diagnosed using the NWP fields. The cloud analysis is based largely on satellite imagery and surface reports, the forecast being carried out in a similar way to precipitation rate. The visibility analysis combines surface reports with NWP model fields and satellite imagery: Meteosat during the day and NOAA-AVHRR at night. The forecast is an extrapolation using trends from the NWP model, and relaxing towards the model values themselves. Results show a substantial improvement over both persistence and raw NWP model products.

The main developments in aviation forecasting over the 50 years up to 1985 are summarised. The importance of subsequent increasing Regional and World Area Forecasting commitments at the UK Meteorological Office is stressed, along with a demonstration of the more recent advances in forecasting low-level and middle/upper-level significant weather, respectively. Meteorological hazards requiring warnings (e.g. SIGMETS) and current aviation forecast output are considered, along with a glance into the future, cautioning that the trend towards fully automated products may be hindered by slower improvements in some aspects of model technology such as derived-field forecasts of fog and low cloud.

On 11–12 November 1987, a frontal system associated with an intense narrow cold frontal rainband was observed over the Channel between England and France. This case constitutes intensive observational period 2 (IOP2) of the FRONTS 87 experiment. It was an ideal example of upper-level and low-level jet coupling with substantial ascending motions and precipitation. The frontal system is analysed, in this paper, using the mesoscale non-hydrostatic model CSU/RAMS. The model uses a two-way interactive nested grid structure and full cloud microphysics parameterisation at various levels of complexity. The model results are compared with previous numerical investigations of this case as well as with observations obtained from the experimental network deployed during the campaign. The model succeeded in representing the frontal structure at meso- and convective scales, taking into account the large scale forcing; thus it is an important tool for the study of secondary cyclogenesis observed during the FASTEX campaign.

A straightforward model for the spatial calculation of the time of fog clearance is presented which is based on thermodynamic equations, spatial data sets such as NOAA-AVHRR satellite data, Digital Elevation Model, and horizontal and vertical meteorological observations. The model has been tested on two days with extended fog layers within the study area. The local validation of the model reveals an accuracy in fog clearance of 4 minutes by comparing the model result with meteorological observations. A spatial validation by means of a reference NOAA overpass indicates an under-estimation of the fog-covered area by the model of 2.9% at the time of the reference image. Five minutes before reference time, the spatial correspondence of the modelled and the reference fog coverage increases to 98.9%. The temporal deviation of uncoinciding pixels between reference and modelled fog image at reference time is less than ±30 minutes in 68% of the total number of fog-covered pixels and therefore matches the accuracy of local tephigram methods. However, for individual pixels a time error of up to ±60 minutes occurs for the time of fog clearance. Time errors are probably mainly due to an inaccurate estimation of fog thickness.

This report is a summary of the methods and findings of a research project that evaluated the social and economic benefits of meteorological services in Australia. The meteorological services evaluated were the basic public weather services and several specialist user-pay and commercial services used by business firms in several sectors of the national economy. Overall, the results indicated that the quality of the selected services was high. In addition, the benefits of these services were extensive, resulting in high benefit-cost ratios.

Smoke pollution from controlled burning of forested areas can be amajor problem for populated areas, even those at some distance fromthe site of the burning. In particular, Perth, the largest city inWestern Australia, is affected by smoke from controlled forest burnsto the south of the city when meteorological conditions advect smokeover the city. Such controlled burns are extensively carried out onan annual basis in spring, early summer and late autumn by theWestern Australian Department of Conservation and Land Management(CALM) in the southwest forests, in order to reduce flammable fuelsand mitigate the undesirable social, economic, environmental andhuman problems caused by destructive wildfires. In this article,results are presented from a mesoscale model prediction of smoketrajectories from a controlled burn event that took place inNovember, 1995. In this example, two meteorological factors, astrong sea-breeze front and a mesoscale low pressure system,unexpectedly turned the smoke back from its initial trajectory overthe ocean to directly over Perth itself. The result was very highsmoke concentration levels over the city and associated healthconcerns to residents as well as disruption to transport includingthe closing of Perth airport. The mesoscale guidance was veryaccurate and can provide valuable guidance when run routinely.

During the last two weeks of March 1980 an intense heat wave developed in central and northern Argentina, with its greatest intensity over the city of Buenos Aires. The sequence of temperature anomalies showed the importance of this heat wave. During the period of maximum intensity the daily minimum temperatures were over 27 °C and the apparent temperatures over 30 °C. It was the most intense heat wave of the last two decades and the only one that has occurred in March. The intensity of the event is shown by the high values of several biometeorological indices that indicate dangerous levels of thermal stress on people. The persistence of an anticyclonic circulation at all tropospheric levels and the extension of the barotropic zone to the south led to the maintenance of high values of temperature and humidity. The advective effect added to the intense solar radiation was the most important cause of the high temperatures.

An expert system (MEDEX) for predicting the gale-force onset, continuation, and cessation of seven major wind types within the Mediterranean basin has been designed, developed, and tested. The six wind types consist of the bora (flowing through both the Adriatic and Aegean Seas), etesian, levante, mistral, sirocco and westerly (poniente and vendaval). Except for the sirocco, these winds result from synoptic situations that lead to topographical channelling. MEDEX is rule-based and incorporates fuzzy logic to handle both objective and subjective inputs, the latter being a unique application of fuzzy logic. MEDEX has approximately 330 fuzzy rules, covering the seven winds in both winter and summer seasons. While MEDEX has been designed as a nowcasting tool (0–12 h), it can be applied to any future time for which forecasting charts (consisting of surface pressure and 500 mb height fields) are available. Inputs consist of objective pressure gradients in addition to subjective interpretations of various synoptic features. These inputs, as well as the corresponding rules, were developed, tuned, and verified over a two-year period during which the weather conditions from 539 days were individually analysed. Ground truth verification was produced primarily from over-water Special Sensor Microwave/Imager (SSM/I) wind speed measurements but also included observations as available, model predictions, and official Navy wind warnings. Evaluations of MEDEX performance for both onset and cessation of winter and summer winds are presented. In addition, comparisons with forecast statistics for the Navy's regional weather centre in Rota, Spain show that MEDEX has comparable forecasting skill.

A forecasting system was set up to infer the impact of mountain waves for producing mesoscale temperature anomalies in the stratosphere. It was successfully applied during a recent aircraft campaign which operated out of Rovaniemi, northern Finland, between 18 December 1996 and 15 January 1997. The forecast information led to the successful detection of mountain-induced polar stratospheric clouds on 9 January 1997. For this event mesoscale forecasts, hindcast runs including grid nesting and comparisons with lidar observations and radiosonde measurements are presented.

Meteorological aspects of a fatal aircraft accident which resulted from collision with a fog-shrouded 132 Kv suspension transmission tower (an electricity pylon) on approach to Coventry Airport are discussed.

The University of New South Wales (UNSW) high-resolution numerical weather prediction model (HIRES) has been used to forecast wind direction and speed at a height of 10 m over the Sydney to Hobart race yacht area for the past three years. The model is run routinely, three times daily, at the New South Wales Regional Office of the Australian Bureau of Meteorology in Sydney. One of these model runs is out to seven days ahead and its boundary conditions are provided by the Bureau of Meteorology's global model. For the 1996 Sydney to Hobart yacht race the model was run at 25 km resolution out to five days ahead and was subjected to detailed verification by one of the authors (KLB), who participated in the race aboard the yacht AMP Wild Oats and carried out an observational program during the race. The model winds were verified on a six-hourly basis utilising instrumentation on the yacht. The yacht carried wind sensors, which are situated on top of the yacht's mast at a height of 17.5 m above the water. The authors were interested in both the wind directional trends and the wind speeds forecast by the model. It was found that the model gave overall guidance of good quality but was particularly accurate early in the race when a major wind change known locally as a Southerly Buster occurred just after the start of the race. Later in the race from about the fourth day, the quality of the forecasts decreased in accuracy. An improved version of the model, run at higher resolution (10 km), will be verified again in the next race, in December 1997.

Synoptic surface pressure analyses for the Southern Ocean have been raised to a new level of realism by satellite imagery and NWP models, but the rare ship observations remain important analysis ingredients. This is illustrated here with observed and analysis pressures for an icebreaker journey from Hobart (Tasmania) to Dumont d'Urville and McMurdo (Antarctica).

In order to test the current recommendation for reservoir safety that a snowmelt of 42 mm/day can be used throughout the UK, calculations of hourly snowmelt rates were made for hourly weather reporting stations in the UK. These data were analysed to give annual maximum melt totals for periods of 3, 6, 9, 12, 18, 24, 48, 72, 120 and 168 hours. Supplementary data from high altitude climate stations allowed further 24-hour melts to be calculated. The annual maxima were entered into an extreme value type one (Gumbel) analysis and estimates of the melt with return periods of 5, 20 and 50 years were made for the durations of 3, 6, etc. hours. An estimate of the return period of a 24-hour melt of 42 mm was made for the sites. These return periods were below 10 years in some high altitude Pennine and Scottish sites, but in excess of 1000 years at low altitude in England. The 24-hour melt with 5 year return period at the sites was related to altitude, northing, mean January temperature and windspeed in linear regressions which permit a calculation of snowmelt at any point in the UK.

To provide ice warnings for road maintenance, a forecasting model has been developed that computes the state of the road several hours in advance. The model, which is called HS4Cast, runs autonomously and does not need human input. The current version of the forecast model features a fuzzy logic engine that determines the best forecast by evaluating certain patterns in the measured parameters. Using this forecast engine, predictions up to 24 hours in advance are possible with reasonable accuracy. If available, additional forecasts from a weather centre may be imported into the model so that the effects of precipitation are taken into account, this further reduces forecast errors. Statistics summarising the model's performance are presented using data from several measurement sites along an Austrian motorway, where HS4Cast has been in use since about 1990.

This paper addresses the long-standing problem of plotting atmospheric fronts objectively. A thermodynamic definition of a front is proposed. Time-independent diagnostic quantities are then devised according to this definition. These can be computed from any gridded dataset. Fronts are then plotted, without human intervention, by utilising standard graphics package facilities, such as contouring, to represent the diagnostics. The methodology includes applying threshold criteria to erase automatically fronts which are thermally weak. By tuning these criteria a good match with subjective fronts is obtained. In addition, the objective surface fronts very often coincide with troughs in surface pressure, implying that a thermodynamic definition is sufficient. The objective front-plotting is further developed to recognise split fronts, and ana- and kata-front characteristics. Many examples are presented; these illustrate how objective fronts are potentially a very powerful tool for both forecasting and research. At the Joint Centre for Mesoscale Meteorology plots and animations showing objective fronts have been produced on a daily basis for many months now.

The need for a continued reassessment of reservoir safety in relation to heavy rainfall and floods is of paramount importance. The latest advice given by the Institution of Civil Engineers states that for much of Somerset in southwest England, the Flood Studies Report underestimates the frequency of heavy rainfall and that local data should be adopted. However, this advice has not always been followed. Furthermore, the advice does not extend to other areas in southwest England. Therefore, it is essential to assess in detail the reliability of the four methods which can be used in order to estimate storm rarity at present in southwest England. These are the Bilham method, the Flood Studies Report (FSR) method, the FORGE method, and the Four Parameter Model. Both the Flood Studies Report and the FORGE method give higher values of storm rarity than either the Bilham or Four Parameter Model. The Bilham method is not sensitive to geographical variation in rainfall frequency. Comparison of all four methods with observed storm events and storm depths showed that the Four Parameter Model gave the most realistic results, followed by the Bilham method. For a given storm rarity the Four Parameter Model tends to give the highest storm depth, with the differences increasing with storm rarity. These new results suggest that in southwest England many existing dams and other drainage works have been underdesigned.

A mesoscale numerical weather prediction model and its associated diagnostics are evaluated to gauge their ability to forecast convection and the convective environment for ten case-study days. The diagnostic indices and parameters evaluated are those used to assess atmospheric instability, mesoscale forcing, inhibition and low-level moisture supply. In addition, bulk Richardson number and helicity diagnostics are examined for convection organisation. Convection over mountainous regions is not considered. The case-study days are grouped according to their synoptic type in order to evaluate the usefulness of the forecast diagnostics. In the evaluation it is found that by combining the CAPE (convective available potential energy) and low-level relative humidity diagnostics one can delineate quite large areas where convective triggering is possible and outside which convection will not develop. Organisation of the mesoscale convective systems is also considered. It is found that forecast strong low-level shears tend to be well correlated with observed linear convective systems. A set of stability indices are evaluated quantitatively and qualitatively by comparing the forecast stability indices with observations at a sounding location. CAPE and a simple index based on a moist temperature difference between two layers (Adedokun2) are identified as the most useful forecast stability indices for all of the cases.

Predicting rainfall along the New South Wales (NSW) coast is a major forecasting problem because of sharp gradients in rainfall amounts with the heaviest falls on the coastal fringe decreasing rapidly inland. On some occasions the rainfall pattern is less spatially coherent and consists of isolated maxima. Both rainfall patterns are associated with mesoscale coastal ridging. The first rainfall pattern arises from coastal ridging occurring in combination with an offshore trough. In the first case study presented here, a typical ridge-trough system was aligned parallel to the coast, and located just offshore, with the observed rainfall heaviest at the coast, decreasing rapidly from over 60 mm to near zero 30 km inland. The model captured well the southward temporal evolution of the maximum relative humidity values and rainfall. The second rainfall pattern occurs when shallow coastal ridging interacts with downdrafts from thunderstorm activity over the ranges to the west. The second case study was one in which convergence and condensation generated a quasi-stationary line of thunderstorms, resulting in flash flooding. The model precipitation rates and accumulations matched very closely those of the squall line, as revealed by radar precipitation intensities and the observed rainfall.