In tropical-equatorial environments, where weather has a high degree of variability, a reliable weather forecast is an essential decision-making tool. To achieve reliability and accuracy it is important to study both the climatological trends and the elements affecting the meteorological variables of the areas under analysis. A precise weather forecast is not enough, however. The way in which the information is presented to users in weather bulletins is also important. The users need to comprehend the forecasted meteorological synopsis and to draw relevant conclusions. In this case study a Medium Range Forecast model was used (NOAA Air Resources Laboratory MRF) to predict, in statistical form, the likely weather conditions. The verification of the weather forecast scored good results, especially in the shorter period (7 days), showing that this specifically designed weather bulletin was a reliable decision tool. In order to demonstrate the key role of the meteorological information to the decision-maker, an economic estimation of the weather forecasts was carried out.

A new index to estimate aircraft icing in clouds from operational meteorological models has been developed by Swedish meteorologists. Although rather simple it takes into account, directly or indirectly, all the principal meteorological variables for icing. The index has been evaluated during three winter seasons and is now operational in the Swedish HIRLAM model. A graphical representation of the index is presented.

The solar ultraviolet radiation (UV) flux density at the earth's surface depends on the incoming solar energy and the transmission properties of the atmosphere. UV radiation is strongly absorbed by ozone in the spectral range 200-310 nm, while the attenuation is increasingly weaker at longer wavelengths. Following the discovery of the Antarctic ozone hole in 1985, the risk of a possible UV increase at ground level, due to the observed stratospheric ozone depletion, has heightened the interest within the scientific community given the potentially harmful effects on terrestrial and aquatic ecosystems. Spectroradiometers, broad-band meters and dosimeters may be used for measurements of solar UV. In addition, radiation transfer models can be used to quantify UV irradiances at various times and locations, provided that the extraterrestrial solar radiation and the state of the atmosphere are known. Information about UV radiation at the earth's surface is given by the ultraviolet index ‘UVI’, which is defined as the effective integrated irradiance (280-400 nm) weighted by the erythemal action spectrum. The UV Index is widely used by many international weather services as an indicator of UV levels at the earth's surface providing public awareness of the effects of prolonged exposure to the sun's rays.

The aim of this paper is to present a device capable of estimating the UV Index. This device is a compact disc, used as a sundial, and is based on modelled UV irradiances derived from the STAR radiative transfer model (System for Transfer of Atmospheric Radiation). The device was tested in an urban setting under clear sky conditions.

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%.

Using a statistical approach this paper focuses on an analysis of spatial air temperature variations and their relation to urban land use. The work is based on data collected during an 18-month period at 30 sites in Göteborg, Sweden. Measurements show intra-urban air temperature differences of up to 9 °C in the urban district, and the main purpose of this study is to determine if these variations are statistically significant. A stepwise multiple regression analysis confirmed that surface cover is important for governing air temperature differences in the area. Information on land use and surface cover was gathered from a continuously updated land use database, the Master Plan of Göteborg, and a separate site description analysis. The site description analysis includes a test of three methods using aerial or fish-eye photos for characterisation of surface cover in the urban district. The results show statistically significant temperature variations between different land use/land cover categories on a diurnal basis and for all weather conditions. The importance of the results for urban planning application is discussed.

The United States Defense Threat Reduction Agency (DTRA) DIPOLE EAST 169 experiment took place at White Sands Missile Range (WSMR), New Mexico on 23 September 1999. The objective of the experiment was to validate the DTRA Hazard Prediction and Assessment Capability (HPAC) suite of models in a weakly forced, light wind environment. Specifically, DTRA diagnostic and prognostic models were used to produce wind fields that were passed to DTRA's transport model. The resultant transport calculations were compared with the ground truth data. A release of SF6 was tracked by remote sensors to provide ground truth data. Test day weather conditions were gathered. Data from the WSMR Surface Automated Meteorological Site (SAMS) sites, a mobile profiling system, and from rawinsonde were collected and fed into the models.

A presentation of the general weather conditions that existed on the test day is first reviewed, followed by a discussion of the forecast profiles for the test site as well as a discussion of the weather condition that existed in the calculation domain as evidenced by the surface data. This provides the background conditions for the test such as the cloudy conditions that were experienced and the light winds throughout the domain. Included in this discussion is the presentation of the ground truth data collected by the Fourier Transfer Infrared Spectrometers (FTIRs) used to determine the location of the SF6 cloud centroid. The results of the transport model output based on input from the diagnostic and prognostic models are presented. Finally, the results of the actual SF6 cloud centroid are compared with the modelled results. The differences are discussed and potential causes for the differences considered. However, the overall model performance produced results that show that cloud tracking and calculations were within one grid unit of ground truth. Overall, the models produced reliable results for this limited study on a light wind case.

The interaction between atmospheric and construction conditions and the exothermic, temperature-dependent hydration reactions of the concrete's binding components may produce adverse conditions in curing concrete, thereby reducing the quality of that concrete. Accurate model forecasts of concrete temperatures and moisture would help engineers determine an optimal time to pour, an optimal mix design, and/or optimal curing practices. Existing models of curing concrete bridge decks and road surface prediction models lack realistic boundary conditions. The concrete models contain unnecessarily detailed hydration heat generation mechanisms for a simplified field forecast model. In this paper, a new energy balance model (SLABS), which can be easily adapted to predict road surface conditions, is described and applied to predict the temperatures and moisture of curing concrete bridge decks made with New York State Department of Transportation's Class HP concrete. Highest concrete temperatures occurred at high air temperatures, humidities and initial concrete temperatures and at low cloud cover fractions and wind speeds. Peak concrete temperatures can exceed 60°C. To minimise concrete temperatures and temperature gradient magnitudes, concrete should be placed during the late afternoon or early evening. As a field forecast model for which the meteorological inputs are taken from NGM MOS forecasts, the outputs of SLABS include the peak concrete temperature (to within 2°C of the observed in one application), peak temperature gradient, evaporation rate at the time of placement and several warning messages indicating adverse field conditions.

This paper is concerned with the transformation of an upper level trough into a shear line and a tropopause cyclone or cut-off low. During this event, the isentropic potential vorticity of air columns between fixed isentropes approaching the trough axis at its level of maximum intensity from the west is well conserved over a period of about 12 to 24 hours, but there is a significant repartitioning of the constituent parts, from a relatively large static stability to a relatively large absolute vorticity. This indicates stretching of vertical columns of air. The effect that is inducing the vortex stretching is identified within the context of quasi-geostrophic theory as ‘opposite vector-frontogenesis’. This implies that Q-vectors point in opposite directions, respectively, above and below the level of maximum trough-intensity. Isentropic analysis indicates that the criterion for unstable isentropic downgliding is fulfilled in the upper troposphere in a restricted area, covering several hundred thousand square kilometres, near the base of the trough when and where the cut-off low is formed. This area of unstable isentropic downgliding, to which we assign the term ‘baroclinic downburst’, and which maintains its identity during a period of time in the order of 24 hours, coincides approximately with the dry intrusion as identified on a ‘water vapour’ image. It is argued that unstable isentropic downgliding is in fact necessary in order to realise a cyclone-core characterised by relatively high static stability, relatively high absolute vorticity and relatively high potential vorticity, as is demanded by the invertibility principle for potential vorticity. The events described above are triggered by the interaction of two potential vorticity anomalies.